natural uranium and strontium isotope tracers of water sources and surface water groundwater interactions in arid wetlands pahranagat valley nevada usa summary near surface physical and chemical process can strongly affect dissolved ion concentrations and stable isotope compositions of water in wetland settings especially under arid climate conditions in contrast heavy radiogenic isotopes of strontium 87sr 86sr and uranium 234u 238u remain largely unaffected and can be used to help identify unique signatures from different sources and quantify end member mixing that would otherwise be difficult to determine the utility of combined sr and u isotopes are demonstrated in this study of wetland habitats on the pahranagat national wildlife refuge which depend on supply from large volume springs north of the refuge and from small volume springs and seeps within the refuge water budgets from these sources have not been quantified previously evaporation transpiration seasonally variable surface flow and water management practices complicate the use of conventional methods for determining source contributions and mixing relations in contrast 87sr 86sr and 234u 238u remain unfractionated under these conditions and compositions at a given site remain constant differences in sr and u isotopic signatures between individual sites can be related by simple two or three component mixing models results indicate that surface flow constituting the refuge s irrigation source consists of a 65 25 10 mixture of water from two distinct regionally sourced carbonate aquifer springs and groundwater from locally sourced volcanic aquifers within the refuge contributions from the irrigation source and local groundwater are readily determined and depend on proximity to those sources as well as water management practices

generation of annual rossby waves in the north pacific abstract a new mechanism is proposed for the generation of the annual period baroclinic rossby waves which have been observed in the central north pacific by kang and magaard it is shown that annual north south fluctuations in the eastern boundary current off vancouver island can efficiently generate first baroclinic mode rossby waves throughout the central north pacific in particular to the southwest of vancouver island the direction wavelength and speed of phase propagation associated with the far field asymptotic wave solution agree favorably with observations also along and fixed latitude contained within the observed latitude band 30 40 n the amplitudes of the vertical displacement at 300 m and surface horizontal current speed both monotonically increase to the west in agreement with the observed trends for these quantities however the existence of this monotonicity in the solution appears to be quite sensitive to the north south spatial structure of the fluctuating coastal current for

patterns of deposition from experimental turbidity currents with reversing buoyancy turbidity currents are turbulent sediment laden gravity currents which can be generated in relatively shallow shelf settings and travel downslope before spreading out across deep water abyssal plains because of the natural stratification of the oceans and or fresh water river inputs to the source area the interstitial fluid within which the particles are suspended will often be less dense than the deep water ambient fluid consequently a turbidity current may initially be denser than the ambient sea water and propagate as a ground hugging flow but later reverse in buoyancy as its bulk density decreases through sedimentation to become lower than that of the ambient sea water when this occurs all or part of the turbidity current lofts to form a buoyant sediment laden cloud from which further deposition occurs deposition from such lofting turbidity currents containing a mixture of fine and coarse sediment suspended in light interstitial fluid is explored through analogue laboratory experiments complemented by theoretical analysis using a box and cloud model particular attention is paid to the overall deposit geometry and to the distributions of fine and coarse material within the deposit a range of beds can be deposited by bimodal lofting turbidity currents lofting may encourage the formation of tabular beds with a rapid pinch out rather than the gradually tapering beds more typical of waning turbidity currents lofting may also decouple the fates of the finer and coarser sediment depending on the initial flow composition the coarse fraction can be deposited prior to or during buoyancy reversal while the fine fraction can be swept upwards and away by the lofting cloud an important feature of the results is the non uniqueness of the deposit architecture different initial current compositions can generate deposits with very similar bed profiles and grading characteristics highlighting the difficulty of reconstructing the nature of the parent flow from field data it is proposed that deposit emplacement by lofting turbidity currents is common in the geological record and may explain a range of features observed in deep water massive sands thinly bedded turbidite sequences and linked debrites depending on the parent flow and its subsequent development for example a lofting flow may lead to a well sorted largely ungraded or weakly graded bed if the fines are transported away by the cloud however a poorly sorted largely ungraded region may form if during buoyancy reversal high local concentrations and associated hindered settling effects develop at the base of the cloud

paleomagnetic data bearing on style of miocene deformation in the lake mead area southern nevada abstract paleomagnetic and structural data from intermediate to mafic composition lava flows and related dikes in all major blocks of the late miocene hamblin cleopatra volcano which was structurally dismembered during the development of the lake mead fault system lmfs provide limits on the magnitude and sense of tilting and vertical axis rotation of crust during extension of this part of the basin and range province sinistral separation along the fault system dissected the volcano into three major blocks the eastern cleopatra lobe of the volcano is structurally the most intact section of the volcano normal and reverse polarity data from paleomagnetic sites collected along traverses in the cleopatra lobe yield an in situ grand mean of declination d 339 inclination i 54 95 3 1 k 27 2 n 81 sites the rocks of the central core of the volcano yield an in situ grand mean of d 3 i 59 95 6 8 k 42 5 n 11 sites six normal five reverse polarity sites collected within the western hamblin lobe of the volcano are exclusively of reverse polarity and yield an overall in situ mean of d 168 i 58 95 6 5 k 28 9 n 18 sites interpretation of the paleomagnetic data in the context of the structural history of the volcano and surrounding area considers the possibility of two different types of structural corrections a stratigraphic tilt correction involves restoring flows to the horizontal using the present strike this correction assumes no initial possibly radial dip of flows of the volcano and is considered invalid a structural tilt correction to the data assumes that dikes of the radiating swarm associated with the volcano were originally vertical and results in block mean directions of d 9 i 53 95 3 1 k 27 2 and d 58 i 78 95 6 8 k 42 5 for the cleopatra lobe and the central intrusive core respectively the data from the cleopatra lobe are slightly discordant in a clockwise sense from expected middle to late miocene field directions the data from the volcano are not consistent with a proposed structural model of uniform moderate magnitude statistically significant counter clockwise vertical axis rotation of fault bounded blocks during overall sinsitral displacement along the lmfs we also analyzed dikes of the northernmost part of the miocene wilson ridge hypabyssal igneous complex strata of the triassic chinle formation and basalt flows of the miocene west end wash callville mesa volcanic centers dikes in the wilson ridge pluton and the triassic strata yield magnetizations with directions suggestive of statistically significant clockwise vertical axis rotations consistent with local large magnitude shear of crustal fragments near some of the faults of the lmfs late cenozoic deformation of the hamblin cleopatra volcano area appears to have been non uniform in scale and magnitude and no single structural model involving strictly strike slip faulting can account for the observed paleomagnetic data

the interaction of ash flows with ridges using both laboratory experiments and theoretical models we examine the different flow regimes that may develop when an ash flow encounters a ridge for very small ridges all the flow may pass over the ridge for intermediate size ridges the flow may be partially blocked with a fraction of the flow reflected upstream as a travelling bore in this case the remainder of the flow which does pass over the ridge is hydraulically controlled at the ridge crest finally if the ridge is sufficiently high then the flow will be totally blocked new laboratory experiments show that the sedimentation patterns associated with these flow regimes may be very different most importantly flows that involve partial blocking and the formation of upstream propagating bores display enhanced sedimentation upstream of the ridge analogous to valley ponded and caldera fill deposits in contrast under some circumstances if the flow is able to scale a ridge the deposit may be relatively unaffected by the presence of the ridge the minimum ridge height that leads to total blocking of the flow increases with mass eruption rate and has a complex variation with distance from the source in a one dimensional channel the minimum ridge height that causes blocking increases with distance downstream this is because the flow becomes less dense through sedimentation of particles and entrainment of air and so requires less energy to scale a ridge of a particular height in axisymmetric flow the minimum ridge height initially decreases with distance downstream as the flow spreads radially but subsequently increases as the flow becomes less dense through sedimentation and entrainment a new quantitative model of dilute ash flows propagating over ridges indicates that flows with mass fluxes in excess of 108 109 kg s can partially scale barriers as high as 1000 m at distances of tens of kilometres from the source whereas smaller flows are likely to be totally blocked by such an obstacle our results shed new insight on the possible long range transport mechanism of several large flows including the ata fisher and aniakchak pyroclastic flows

an experimental investigation of density stratified inertial gravity currents turbidity currents and pyroclastic density currents may originate as stratified flows or develop stratification during propagation analogue density stratified laboratory currents are described using layers of salt solutions with different concentrations and depths to create the initial vertical stratification the evolving structure of the flow depends on the distribution of the driving buoyancy between the layers b proportional to the layer volumes and densities and their density ratio q when the lower layer contains more salt than the upper layer and so has a greater proportion of the driving buoyancy b 0 ae5 this layer travels to the nose of the current by mixing into the back of the head along the body wake density interface to produce a homogeneous flow q fi 1 or overtaking leading to streamwise stratification q fi 0 timescales describing the mixing between the layers and the streamwise separation of the layers are used to understand these flow behaviours and are in accordance with the experimental observations distance time measurements of the flow front show that strongly stratified flows initially travel faster than weakly stratified flows but during their later stages they travel more slowly in natural flows that are stratified in concentration and grain size internal features such as stepwise grading gradual upward fining and reverse grading could be produced depending on b and q stratification may also be expected to affect interactions with topography and overall fan architecture

mass fluxes in the canary basin abstract ocean studies in the 1970s provided an improved knowledge of the coastal upwelling region off nw africa while in the 1980s and 1990s they led to a good description of the open ocean flow patterns in the canary basin it was not until the late 1990s that major research addressed the open coastal ocean coupled response here we examine the mean and seasonal circulation patterns in the canary basin with data from four hydrographic cruises carried out in the region between cape ghir madeira island and the canary islands we apply an inverse box model to an ocean divided into 14 layers with several layers representing each water mass or stratum to obtain mass fluxes consistent with the thermal wind equation an optimum flow description is obtained using conservation of mass salt and heat anomaly biologically corrected oxygen and silicate and allowing for ekman transport in the surface layer and dianeutral mixing between adjacent layers the deep waters show no predominant flow direction while the intermediate waters display localized southward flowing mediterranean water far from shore and northward flowing antarctic intermediate water near the continental slope specially in the passage between the eastern canary islands and the african slope the mean upper thermocline canary current composed of north atlantic central water flows south with an open ocean branch transporting about 3 1 sv 1 sv 10 6 m 3 s 1 10 9 kg s 1 and an upwelling related branch near the continental slope carrying 1 0 3 sv the seasonal transport by the open ocean branch intensifies and moves offshore from spring to fall 2 8 1 2 sv in spring 2 9 1 1 sv in summer and 4 5 1 2 sv in fall while it carries its lowest southward mass flux in winter 1 7 1 0 sv possibly as a result of a migration offshore the sampled region upwelling related southward flow is present in spring and summer 1 9 0 1 sv and 2 4 0 1 sv respectively while in fall and winter it merges with the offshore southward branch this westward migration allows a northward mass flux between the canary islands and the african coast 1 8 0 1 sv that by winter reaches cape ghir 0 5 0 2 sv seasonal air sea heat fluxes fit well with the climatological values the net phosphate transport in the surface layer indicates that primary production was negligible in this region during january 1997 and april 1998 though in this last month production was probably starting as a result of significant nutrient supply

hyperpycnal plume formation from riverine outflows with small sediment concentrations a series of laboratory experiments has been conducted in order to elucidate the sediment induced mixing processes accompanying riverine outflows specifically the discharge of a warm fresh particle laden fluid over a relatively dense cool brine in a parameter regime analogous to recently acquired field measurements hypopycnal surface plumes were subject to a convective instability driven by some combination of heat diffusing out of the warm fresh sediment laden plume and particle settling within it convection was robust in the presence or absence of intense turbulence at sediment concentrations as low as 1 kg m 3 and took the form of millimetre scale sediment laden fingers descending from the base of the surface plume a consequence of the convective instability of the original hypopycnal plume is the generation of a hyperpycnal bottom riding flow the experiments presented here indicate that natural river outflows may thus generate hyperpycnal plumes when sediment concentrations are 40 times less than those required to render the outflow heavy relative to the oceanic ambient the resulting hyperpycnal plumes may play an important role in transporting substantial quantities of sediment to the continental slope and beyond

the northeast pacific ocean response to the 1982 1983 el nino the ocean s response to atmospheric forcing and to forcing along the coast by kelvin waves is examined in light of nearshore seasonal variability and the major 1982 1983 el nino in addition to driving the northeast pacific gyre and local currents the wind modifies the depth to the main thermocline changing the apparent westward phase speed of rossby waves excited by passing kelvin waves the amplitude of the passing kelvin wave determines whether the rossby wave depresses or raises the thermocline while propagating westward along the northeast pacific coast from oregon to washington model results suggest that the wind raises the main thermocline during the 1982 1983 el nino generally the coastal response off oregon and washington appears to be strongly driven by the large scale atmosphere while along the california coast and south to the equator the dominant el nino signal has an oceanic origin that signal with periods between 2 and 5 years originates in the western equatorial pacific ocean and is tracked eastward through the equatorial waveguide and then poleward along the coast to 50 n using data from two reduced gravity primitive equation models coupled at 18 n at shorter seasonal time scales the acceleration of the wind appears to be an important mechanism in driving coastal currents

large 13c enrichment in primary carbonates from andean altiplano lakes northwest argentina abstract we report here extreme 13 c enrichments up to 13 pdb in primary calcite and aragonite precipitates in saline well oxygenated waters from high altitude lakes in the southern andean altiplano northwestern argentina biological effects as well as variations in carbon source inputs and in the exchange rate with atmospheric co 2 are commonly considered the main controls on the carbon isotope values of authigenic lacustrine carbonate we present sedimentological and geochemical evidence that favors physical processes evaporation effects and co 2 degassing as major controls on 13 c enrichment we propose that large enrichments may result from the non equilibrium gas transfer isotope fractionation during co 2 degassing from thermal springs and evaporation effects in arid environments the dilution effect by large quantities of 14 c free co 2 hinders accurate 14 c chronology of these lake records based on lacustrine organic matter and aquatic plants our results indicate that geothermal and volcanic co 2 sources in lake basins located in volcanic settings and physical fractionation may have a greater significance than commonly accepted to explain lacustrine carbon isotope records

hyperpycnal and buoyant plumes from a sediment laden river the interaction of river plumes and sediment transport is examined using a three dimensional numerical model which couples the hydrodynamics and sediment transport only large rivers in midlatitudes are considered so that the effect of coriolis deflection becomes important winds and tides are excluded in an initially quiescent coastal sea sediment freshwater mixture is released from the inner reaches of a river a high ratio of sediment to buoyancy input is chosen so that the initial sediment freshwater mixture is denser than the receiving seawater as the dense underflow enters the adjacent sea release of sediment to the sea floor changes the plume from hyperpycnal to buoyant the forced convection of freshwater from depths triggers an expansive buoyant plume expanding in the direction of kelvin wave propagation the resulting buoyant plume is far more expansive than a typical buoyant plume in clear seawater produced by a comparable buoyancy input circulation associated with the plume is anticyclonic throughout the water column lacking a cyclonic gyre underneath the anticyclone as one would expect from a typical buoyant plume in clear seawater a heuristic argument follows to explain the lack of baroclinic flow structure in plumes forced by the sediment freshwater mixture

long period transport changes in the eastern north atlantic and their simulation by propagating waves an analysis is presented of geostrophic volume transport across a zonal line along 28 degrees n in the eastern atlantic the data are from an array of five moorings with 200 km spacing carrying temperature sensors and one current meter each for 1 or 2 years transport changes in the main thermocline relative to a fixed depth level are obtained by the use of temperature salinity relationships the transport variability is simulated by two propagating waves with first order baroclinic mode structure solutions exist with annual and semi annual periods and zonal wavelengths of 100 200 km and 300 km respectively assuming quasi geostrophic dynamics and using results on the reynolds stress the dominating waves of annual and semi annual period are found to propagate to the southwest with 45 degrees 60 degrees and 25 degrees to the south off the westward direction respectively wave solutions with a 90 day period and a zonal wavelength of about 300 km are interpreted as an effect of barotropic waves arising due to horizontal temperature inhomogeneity the propagation is about 25 degrees off the westward direction in general good approximations are obtained with the propagating wave simulations in the western and central part of the array while large differences occur between observation and simulation close to the canary archipelago possible causes for these differences are discussed

a mechanism to explain rift basin subsidence and stratigraphic patterns through fault array evolution rift basin stratigraphy commonly records an early stage of slow subsidence followed by an abrupt increase in subsidence rate the physical basis for this transition is not well understood although an increase in extension rate is commonly implied here a numerical fault growth model is used to investigate the influence of segment linkage on fault displacement rate patterns along an evolving normal fault array the linkage process we describe is controlled by a stress feedback mechanism which leads to enhanced growth of optimally positioned faults model results indicate that even with constant extension rates slow displacement rates prevail during an initial phase of distributed extension followed by an increase in displacement rates as strain becomes localized on linked fault arrays this is due to the dynamics of fault interactions rather than mechanical weakening comparison of model simulations with rift basin subsidence and stratigraphic patterns in the gulf of suez and north sea suggests that the occurrence and timing of rapid basin deepening can be explained by the mechanics of fault zone evolution without invoking a change in regional extension rates

unroofing incision and uplift history of the southwestern colorado plateau from apatite u th he thermochronometry the source of buoyancy for the uplift of cratonic plateaus is a fundamental question in continental dynamics the 1 9 km uplift of the colorado plateau since the late cretaceous is a prime example of this problem we used apatite u th he thermochronometry 230 analyses 36 samples to provide the fi rst single system regional scale proxy for the unroofi ng history of the southwestern quadrant of the plateau the results confi rm overall southwest to northeast unroofi ng from plateau margin to plateau interior a single phase of unroofi ng along the plateau margin in late cretaceous to early tertiary sevierlaramide time contrasts with multiphase unroofi ng of the southwestern plateau interior in early and mid to late tertiary time the early cretaceous was characterized by northeastward tilting and regional erosion followed by aggradation of 1500 m of upper cretaceous sediments along the eroded plateau margin sevier laramide denudation affected the entire southwestern plateau was concentrated along the plateau margin and migrated from northwest to southeast following a period of relative stability of the landscape from ca 50 30 ma signifi cant unroofi ng of the southwestern plateau interior occurred between ca 28 and 16 ma additional denudation north of the grand canyon took place in latest tertiary time mid tertiary dates from the grand canyon basement at the bottom of the upper granite gorge limit signifi cant incision of the modern grand canyon below the kaibab surface to 1500 m difference in vertical structural position if these models are correct they indicate that a proto grand canyon of kilometer scale depth had incised post paleozoic strata by the early eocene evidence for kilometerscale mid tertiary relief in northeast fl owing drainages along the plateau margin as well as the mid tertiary episode of plateau interior unroofi ng imply that the southwestern plateau interior had attained substantial elevation by at least 25 20 ma if not much earlier these observations are inconsistent with any model calling for exclusively late tertiary uplift of the southwestern plateau sevier laramide plateau surface uplift and incision thus result from one or more processes that enhanced the buoyancy of the plateau lithosphere expanding the cordillera s orogenic highlands into its lowstanding cratonic foreland the onset of the laramide slab s demise at ca 40 ma and the major pulse of extension in the basin and range from ca 16 10 ma appear to have had little infl uence on the denudation history of the southwestern plateau in contrast the post laramide unroofi ng episodes may be explained by drainage adjustments induced by rift related lowering of regions adjacent to the plateau without the need to otherwise modify the plateau lithosphere our data do not preclude a large component of post early eocene elevation gain or the geodynamic mechanisms it may imply but they do point toward laramide age buoyancy sources as the initial cause of signifi cant surface uplift ending more than 500 m y of residence near sea level

traveling wave like mesoscale perturbations in the north pacific current abstract individual seasonal mean maps of temperature at 300 m in the north pacific current cast of 180 from 1976 to 1980 were constructed from transpac xbt data the long term annual mean map is relatively smooth with some weak quasi stationary meander activity most of the total variance was due to large scale interannual variability i e 60 loss to the mesoscale perturbations i e 30 and least to the annual cycle i e 10 however individual mesoscale perturbations were significant clearly wave like with a wavelength scale of 500 1000 km and a period scale of 1 2 years and generally coherent in phase over 10 of latitude these wave like mesoscale perturbations emanated from the eastern boundary and propagated westward as coherent features at the phase speed of linear non dispersive baroclinic long waves the latitudinal reduction in phase speed from 2 7 cm s 1 at 35 n to 1 4 cm s 1 at 45 n was consistent with baroclinic long wave theory an increase in time scale of these wave l

mean temperature salinity salinity depth and temperature depth curves for the north atlantic and the north pacific abstract all available hydrographic data are used to compute average temperature salinity salinity depth and temperature depth curves for 5 squares in the north atlantic and north pacific 10 s to 60 n both objective and subjective methods are used to filter out obviously erroneous data summary maps of all 5 square mean curves and their corresponding standard deviations are used to select regions of spatial homogeneity over which the curves are averaged scatter diagrams of each parameter pair are used to identify 5 squares containing a variety of water masses the area average curves and their standard deviations are presented individually along with tables of the parameter values these curves in conjunction with the 5 square summary maps provide a comprehensive picture of the distributions of these properties in the upper and mid depth portions of these oceans this well resolved picture of the water mass structure both confirms and adds new insights into the traditional concepts of these property distributions the application of the mean temperature salinity and salinity depth curves to the inference of salinity from observed temperature profiles is examined shallow 0 500 db dynamic heights calculated from observations of temperature and salinity true dynamic height are compared with dynamic heights computed from observed temperature profiles and salinities inferred from the mean property curves maps of the rms differences between true and inferred dynamic heights by 5 square reveal those areas where salinities inferred from either temperature salinity or salinity depth curves yield reliable dynamic heights

flexural rotation of normal faults a conceptual model is proposed for the generation of low angle normal faults in metamorphic core complexes the model is based on three assumptions 1 the isostatic response to normal fault motion is of regional extent 2 when a fault segment is significantly rotated from the optimum angle of slip relative to the crustal stress field it is replaced by a new planar fault oriented in the optimum direction and 3 the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust the stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations active low angle normal faults are difficult to reconcile with rock mechanics theories earthquake focal mechanism studies and geochronologic results indicating rapid cooling of core complex rocks the model does not require active fault slip on low angle faults the flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation the anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust this can explain the observed short wavelength of topographic response to normal fault loads the model results in 1 a nearly flat lying abandoned normal fault or detachment below slices of upper plate rocks and sedimentary infill 2 a strong contrast in metamorphic grade across the abandoned detachment 3 rapid movement of lower plate rocks from midcrustal depths to shallower depths these results are qualitatively in agreement with geologic observations for core complexes

currents fronts and eddy fluxes in the canary basin hydrographic data from two cruises in the canary basin meteor 57 july 1981 poseidon 86 april 1982 are analysed with respect to current distribution and lateral heat flux in the azores madeira region the first part of the data base consists of long transects of xbt and g e k measurements between cape finisterre north west spain and the northern canary basin where several year long current meter records exist further information is obtained by thermosalinograph surface data and by expendable current profilers xcp geostrophic currents are derived from xbt profiles using the tight temperature salinity relationship in the depth range of the warmwassersphare the results compare well with the g e k and xcp current observations the second part consists of ctd data from an eddy resolving box shaped ctd survey 500 500 km2 centered at the mooring location 33 n 22 w the observations are supplemented by satellite buoy trajectories horizontal parameter distribution is shown in terms of objectively contoured maps bands of spatially enhanced energetic structures seen in the long transects are further resolved by the box survey as a deep jet like current system cross the canary basin in a west east direction associated with this azores current is a frontal zone with near surface temperature and salinity steps of order 2 k and 0 3 practical salinity units the dynamic topography field can be decomposed into a linear background field a rossby wave and a mesoscale eddy field we find that major contributions to the meridional eddy heat flux are confined to the vicinity of the azores current frontal zone it is shown that the principal balance in the temperature equation is between heating by the mean horizontal advection terms and cooling by the eddy flux divergence

on the frequency distribution of turbidite thickness the frequency distribution of turbidite thickness records information on flow hydrodynamics initial sediment volumes and source migration and is an important component of petroleum reservoir models however the nature of this thickness distribution is currently uncertain with log normal or negative exponential frequency distributions and power law cumulative frequency distributions having been proposed by different authors a detailed analysis of the miocene marnoso arenacea formation of the italian apennines shows that turbidite bed thickness and sand interval thickness within each bed have a frequency distribution comprising the sum of a series of log normal frequency distributions these strata were deposited predominantly in a basin plain setting and bed amalgamation is relatively rare beds or sand intervals truncated by erosion were excluded from this analysis each log normal frequency distribution characterizes bed or sand interval thickness for a given basal grain size or basal bouma division measurements from the silurian aberystwyth grits in wales the cretaceous great valley sequence in california and the permian karoo basin in south africa show that this conclusion holds for sequences of disparate age and variable location the median thickness of these log normal distributions is positively correlated with basal grain size the power law exponent relating the basal grain size and median thickness is different for turbidites with a basal a or b division and those with only c d and e divisions these two types of turbidite have been termed thin bedded and thick bedded by previous workers a change in the power law exponent is proposed to be related to i a transition from viscous to inertial settling of sediment grains and ii hindered settling at high sediment concentrations the bimodal thickness distribution of thin bedded and thick bedded turbidites noted by previous workers is explained as the result of a change in the power law exponent this analysis supports the view that a and b divisions were deposited from high concentration flow components and that distinct grain size modes undergo different depositional processes summation of log normal frequency distributions for thin and thick bedded turbidites produces a cumulative frequency distribution of thickness with a segmented power law trend thus the occurrence of both log normal and segmented power law frequency distributions can be explained in a holistic fashion power law frequency distributions of turbidite thickness have previously been linked to power law distributions of earthquake magnitude or volumes of submarine slope failure the log normal distribution for a given grain size class observed in this study suggests an alternative view that turbidite thickness is determined by the multiplicative addition of several randomly distributed parameters in addition to the settling velocity of the grain sizes present

a numerical model study of the effects of interannual time scale wave propagation on the predictability of the atlantic meridional overturning circulation we investigate processes leading to uncertainty in forecasts of the atlantic meridional overturning circulation amoc a climate model is used to supply initial conditions for ensemble simulations in which members initially have identical ocean states but perturbed atmosphere states baroclinic transports diverge on interannual timescales even though the ocean is not eddy permitting interannual fluctuations of the model amoc in the subtropical gyre are caused by westward propagating rossby waves divergence of the predicted amoc with time occurs because the waves develop different phases in different ensemble members predominantly due to differences in eastern boundary windstress curl these windstress fluctuations communicate with interior ocean transports via modifications to the vertical velocity and the vortex stretching term dw dz consequently errors propagate westwards resulting in longer predictability times in the interior ocean compared with the eastern boundary another source of divergence is transport anomalies propagating along the gulf stream and other boundary currents the propagation mechanism seems to be predominantly advection by mean currents and we show that the arrival of westward propagating waves can trigger development of these anomalies the mean state of the amoc has a small effect on interannual predictability in the subtropical gyre most likely because eastern boundary windstress curl predictability is not strongly dependent on the state of the amoc in the subtropics eastern boundary windstress curl was more predictable at 45 degree sign n when the amoc was in a strongly decreasing state but unlike at 30 degree sign n no mechanism was found linking windstress curl fluctuations with deep transports

the initial giant umbrella cloud of the may 18th 1980 explosive eruption of mount st helens abstract the initial eruption column of may 18th 1980 reached nearly 30 km altitude and released 10 17 joules of thermal energy into the atmosphere in only a few minutes ascent of the cloud resulted in forced intrusion of a giant umbrella shaped cloud between altitudes of 10 and 20 km at radial horizontal velocities initially in excess of 50 m s the mushroom cloud expanded 15 km upwind forming a stagnation point where the radial expansion velocity and wind velocity were equal the cloud was initiated when the pyroclastic blast flow became buoyant the flow reduced its density as it moved away from the volcano by decompression by sedimentation and by mixing with and heating the surrounding air observations indicate that much of the flow covering an area of 600 km 2 became buoyant within 1 5 minutes and abruptly ascended to form the giant cloud calculations are presented for the amount of air that must have been entrained into the flow to make it buoyant assuming an initial temperature of 450 c and a magmatic origin for the explosion these calculations indicate that the flow became buoyant when its temperature was approximately 150 c and the flow consisted of a mixture of 3 25 10 11 kg of pyroclasts and 5 0 10 11 kg of air if sedimentation is considered these figures reduce to 1 1 10 11 kg of pyroclasts and 1 0 10 11 kg of air

predicting the geometry of channelized deep sea turbidites analytical relationships between the dynamic properties of a deposit forming turbidity current propagating across a sea floor with small slope and the geometry of the resulting deposit have been confirmed experimentally for a wide range of particle sizes initial concentrations and volumes of the driving suspension these simple expressions provide a basis for inferring the dynamics of natural flows from their deposits we surmise that the turbidity current responsible for depositing the black shell turbidite in the northwestern atlantic ocean had an initial sediment concentration on the order of 100 g of silt per litre and a volume on the order of 1000 km 3

sediment sorting in the deposits of turbidity currents created by experimental modeling of explosive subaqueous eruptions abstract deposits of turbidity currents induced by subaqueous volcanic eruptions are increasingly recognized in settings ranging from lakes to the deep sea these eruption fed density currents have high current temperatures and low density particles which affect current dynamics and deposit features field data indicate that deposit thickness and grain size decrease downflow at a changing rate this trend is similar for deposits of noneruptive turbidity currents but with volcanic activity the deposits often exhibit a break in slope of the profile of deposit thickness and grain size distribution here we present laboratory results on density currents that address the effects of variations in grain density and water temperature on flow properties and depositional processes parallel runs were performed under identical flow conditions using quartz sand and pyroclastic sediment each grain type was used in one series of runs in which a hot density current enters a cold environment and one series in which

stable isotope characterization of the miocene lacustrine systems of los monegros ebro basin spain palaeogeographic and palaeoclimatic implications the lacustrine carbonate facies from three allostratigraphic units n1 n2 and n3 of miocene age in the los monegros region ebro basin are characterized isotopically the dominant facies are marls laminated limestones stromatolitic limestones massive limestones and bioturbated limestones these are associated with lacustrine sulphate deposits and distal alluvial facies palaeohydrological reconstructions of the study area are supported by the isotopic and the mineralogical composition of the carbonates massive and bioturbated limestones occupy a very similar compositional domain and have the lowest isotopic compositions of all the facies 9 18opdb 4 5 and 6 4 13cpdb 0 4 reflecting a short residence time of the water and variable commonly high influence of biogenic co2 laminated limestones and stromatolites define a comparatively enriched domain with 18o values ranging from 6 to 0 and 13c values ranging from 3 5 to 0 these values indicate stronger evaporation and enhanced 12c assimilation due to intense biological activity during periods of longer water residence time marls have isotopic values intermediate between these two domains and correspond to periods of lacustrine dilution dolomite bearing samples mainly laminated and stromatolitic facies show a 18odol cal 7 5 with respect to calcite from the same facies while 13c values show very little enrichment up to 1 the positive correlation r 0 85 between 18o and dolomite contents suggests that dolomite resulted from the progressive evaporative concentration of a single water mass and not from the mixing of waters of different compositions the dolomite is thought to be primary or at most the product of very early diagenetic processes the isotopic composition of the facies and their spatial and temporal variations depend on the depositional environment and were constrained by changing regional paleogeography isotopic trends from units n1 to n3 upper agenian vallesian period display a regular depletion in heavy isotopes that indicates a climatic change towards wetter conditions

bed thickness and grain size of turbidites in measured sequences of limestone and greywacke turbidites the bed thickness is found to vary proportionally with the fall velocity of the maximum grain size found at the base of the bed a simple theoretical model based on the decay of isotropic turbulence suggests that bed thickness should be a function not only of this fall velocity but also of bottom slope flow depth and the concentration and grain size distribution of the sediment in the turbidity current the field data do show some influence of these additional factors nevertheless for many natural sequences of turbidites the flows must have carried very poorly sorted sediments and the inferred flow volumes and densities must cluster tightly about modal values thus grain size remains the primary variable and the modal regression curve of bed thickness on maximum grain size is well defined and resembles a fall velocity curve relatively steep basin floors near to source can theoretically cause these modal regressions for distal and very proximal parts of a turbidite to diverge introducing a crudely parabolic appearance in the form of the total regression curve the form of this parabolic curve predicts the deposition of thin but relatively coarse proximal beds such beds do occur they are different from the thin but relatively fine proximal beds that have been interpreted as the result of a fractionation of a turbidity current during levee forming processes

high resolution numerical simulations of resuspending gravity currents conditions for self sustainment received 21 february 2005 revised 30 june 2005 accepted 29 august 2005 published 22 december 2005 1 we introduce a computational model for high resolution simulations of particle laden gravity currents the features of the computational model are described in detail and validation data are discussed physical results are presented that focus on the influence of particle entrainment from the underlying bed as turbulent motions detach particles from the bottom surface resuspended particles entrained over the entire length of the current are transferred to the current s head causing it to become denser and potentially accelerating the front of the current the conditions under which turbidity currents may become self sustaining through particle entrainment are investigated as a function of slope angle current and particle size and particle concentration the effect of computational domain size and initial aspect ratio of the current on the evolution of the current are also considered applications to flows traveling over a surface of varying slope angle such as turbidity currents spreading down the continental slope are modeled via a spatially varying gravity vector particular attention is given to the resulting particle deposits and erosion patterns

experimental turbidity currents entering density stratified water analogues for turbidites in mediterranean hypersaline basins experimental turbidity currents entering two layer density stratified water behave differently from similar currents flowing over the same topography into non stratified water experiments were designed as analogues for flows entering mediterranean hypersaline pools in both the hypersaline pools and the experiments the water density changes abruptly across a pycnocline turbidity currents generated on a platform at the level of the pycnocline behaved in one of three ways as they flowed from the platform into deeper stratified water 1 when the bulk density of the current was less than the dense water layer the current spread at the pycnocline the head of the current advanced rapidly when it lost contact with the bed grains settling out of the current fell through the dense water layer forming an extensive deposit in nature this behaviour will lead to turbidites with sharp but non erosive bases strongly developed grading and no traction features 2 when the bulk density of the current was greater than the dense water layer the current continued as an underflow plunging into the deeper water sedimentation lowered the bulk density of the current and the low density interstitial fluid caused the head to loft low density interstitial fluid convected from the body of the current lofting particles into the water column these particles were hydraulically sorted during upward transport and subsequent settling to the floor the resulting turbidites had a more limited extent than the deposits of either non lofting underflows or interflows by inference from the experiments natural deposits of this type may have local proximal erosion and traction features at the base and strongly graded tops 3 in some of the currents with high bulk density the rising turbid water reached the pycnocline and spread at that level as a secondary interflow the tail of the turbidity current which was less dense than the head and body of the current flowed above the pycnocline adding momentum to the secondary interflow the thin non erosive graded deposit from the secondary interflow may extend beyond the deposits of the primary underflow in all three cases but more pronounced in cases 2 and 3 the interaction of the current with the pycnocline displaced that surface and generated a wave that was reflected back and forth from each end of the pool the waves remobilized sediment on the ramp

origin and trajectory of the frenchman mountain block an extensional allochthon in the basin and range province southern nevada abstract the central basin and range province has provided a natural laboratory for the study of continental extension since the pioneering studies of chester r longwell it is here that large magnitude crustal extension along systems of normal faults was first documented by r ernest anderson estimates of crustal extension in the lake mead extensional domain eastern central basin and range province have been based largely on restoration of displaced blocks most notably the frenchman mountain block the original position of frenchman mountain an allochthonous block that has been translated more than 60 km westward during regional extension has been controversial for more than 2 decades we use correlation of mass wasting deposits with source areas paleozoic stratigraphic characteristics and subunconformity pinchout relations to evaluate previously published models for the original position of the frenchman mountain block and use new evidence to propose a more tightly constrained preextension posi

role of tectonic denudation in warping and uplift of low angle normal faults large displacement on a low angle normal fault results in isostatic uplift of the lower plate in response to tectonic denudation simple models of the denudation process predict warping of the lower plate into a broad antiform or antiform synform pair with axes perpendicular to the direction of extension the amount of warping is strongly influenced by initial fault geometry surface topography the amount of extension and the distribution of extension within the upper plate late stage processes that augment antiform growth include one sided denudation of the antiform reverse faulting due to concave upward flexure and wholesale detachment and reverse drag folding of the antiform antiformal uplifts now exhumed by erosion form mountain ranges in the southwestern united states where conditions were favorable for warping during mid tertiary crustal extension four major structural domains are apparent in transects across these uplifts and adjacent areas 1 unextended area 2 synformal upper plate 3 antiformal uplift and 4 wedge shaped upper plate each domain is separated from adjacent domains by the traces of a master low angle normal fault

source and dispersal of jokulhlaup sediments discharged to the sea following the 1996 vatnajokull eruption the october 1996 gjalp eruption beneath vatnajokull glacier led to one of the largest jokulhlaups glacial floods in iceland in the twentieth century a catastrophic discharge of meltwater and sediment swept across the skeidararsandur flood plain to the sea tephra from the eruption consists of vesicular sideromelane shards with a basaltic andesite composition 53 sio 2 3 mgo 0 8 k 2 o after the flood sediment samples were collected from the flood plain and off the southeast coast of iceland where a major sediment plume had been created by the discharge compositions of glass shards from flood plain and seafloor deposits do not match those of the gjalp magma flood plain samples consist primarily of blocky to poorly vesicular sideromelane clasts with compositions that are characteristic of grimsvotn volcanic products 50 sio 2 5 5 mgo 0 4 k 2 o marine samples collected near the jokulhlaup outflow into the sea also consist primarily of blocky to poorly vesicular sideromelane clasts with compositions that are for the most part similar to products of the grimsvotn volcanic center distal marine samples have more vesicular sideromelane clasts with compositions that are similar to products of the katla volcanic center e g 48 sio 2 4 5 mgo 0 8 k 2 o significant deposition to the seafloor was apparently limited to an area just offshore of the skeidararsandur there is no indication that juvenile volcanic material from the gjalp eruption was carried by the 1996 jokulhlaup onto the flood plain or into the ocean instead the jokulhlaup carried primarily older volcaniclastic material eroded by the flood

late oligocene early miocene landscape evolution of the lake mead region during the transition from sevier contraction to basin and range extension late oligocene to miocene sedimentary strata of the lake mead nevada region hold evidence that can test models of central basin and range extension and tectonic evolution and illuminate paleodrainage systems related to the carving of the grand canyon in this region sevier contraction and a period of relative tectonic quiescence predate miocene extension and magmatism the herein revised rainbow gardens formation formerly the oldest member of the horse spring formation is found throughout the lake mead area and spans the transition from quiescence to the start of extension new detailed stratigraphic work within the rainbow gardens formation including paleocurrent analyses stable isotope geochemical data new 40 ar 39 ar dates tephrochronologic data and ongoing structural studies was used to build a detailed chronostratigraphic framework and reconstruct the rainbow gardens basin through time our results show that the basin formed by a combination of inherited topography and tectonic events outside the lake mead region prior to ca 25 ma sedimentation rates sedimentary facies patterns and the lack of evidence for syndepositional faulting indicate that the basin did not form during extensional faulting although recent work suggests that this basin was modified by uplift to the south between 18 and 20 ma initially the basin recorded fluvial sedimentation within the center of an e ne trending inherited valley with two facing gravel dominated bajadas on either side as the valley filled rivers from the caliente volcanic field to the north delivered an influx of volcaniclastic material into a broad area of marshes and ephemeral lakes in the final stages of sedimentation just prior to the onset of extension at ca 17 ma the basin filled with limestone in a series of shallow lakes understanding the development and evolution of this basin is crucial for testing models of extension as recorded by the younger horse spring formation in addition the detailed sedimentology and stratigraphy place constraints on models of the formation of the grand canyon and suggest that a paleo colorado river did not deposit sediment in the lake mead area between 25 and 17 ma this work highlights the importance of detailed basin analyses which are crucial companions to structural tectonic and geomorphologic studies aimed at understanding the complex evolution of lithospheric and landscape changes across a large region

sediment laden gravity currents with reversing buoyancy there are many natural occurrences of sediment laden gravity currents in which the density of the interstitial fluid is less than that of the ambient fluid although the bulk density of the current is greater such currents are driven by the excess density of suspended particles however after sufficient particles have sedimented the current will become buoyant cease its lateral motion and ascend to form a plume examples of such currents include brackish underflows in deltas turbidity currents and pyroclastic flows experimental studies are described which show that due to sedimentation sediment laden gravity currents decelerate more rapidly than saline currents of the same density there is little difference in the experiments between a sediment laden current with neutrally buoyant interstitial fluid and one with buoyant interstial fluid until sufficient sediment has been lost to cause the latter kind of current to lift off a marked deceleration is then observed and a plume is generated with lift off occurring along the length of the current the resulting buoyant plume then generates a gravity current below the upper surface of the fluid in the tank the deposit from a current with buoyant fluid shows a fairly abrupt decrease in thickness beyond the lift off distance and has a flatter profile than that from a simple sediment current a theoretical model is presented which is based on the two layer shallow water equations and incorporates a model of the sedimentation in which particles are assumed to be uniformly suspended by the turbulence of the current the model shows good agreement with the observed lengths of the experimental currents as a function of time and predicts the lift off distance reasonably well these processes have implications for the behaviour of turbidity currents the interpretation of turbidites mixing processes in the oceans and the lift off of pyroclastic flows

on the potential energy of baroclinic rossby waves in the north pacific abstract estimates of baroclinic rossby wave potential energy spectra for various parts of the north pacific were calculated from published material containing information about this energy in many different formats definitions and units the standardized results lead to frequency spectra of potential energy ranging from 0 1 to 0 45 cpy cycles per year in 34 5 squares between 20 and 30 n to energy spectra ranging from 0 28 to 2 11 cpy in eight subsections of the great circle route from honolulu to san francisco and to a map of annual rossby wave energy between 30 and 40 n the most remarkable finding is a consistent sequence of interannual rossby wave spectra with a peak at 0 15 cpy i e at a wave period of 6 7 years covering the area 20 25 n and 175 130 w the most complete information is available for a 5 square east of the hawaiian islands there the spectrum shows besides the 0 15 cpy peak another broad peak ranging from about 0 4 to 1 4 cycles per year

excision and the original low dip of the miocene pliocene bannock detachment system se idaho northern cousin of the sevier desert detachment geologic mapping and structural analysis in the bannock range se idaho indicate that the bulk of cenozoic extension in southeast idaho was accommodated by slip on the low angle bannock detachment system the bannock detachment system consists of low angle normal faults with a likely n s extent of 130 km it was active from ca 10 3 ma to ca 3 4 ma and accommodated 15 km of top to the wsw extension the master low angle detachment fault cuts steeper older normal faults and extensional folds in its hanging wall crosscutting relationships and prior work show that the hanging wall to the detachment system began as a cohesive block that later broke up along normal faults that either sole into or are cut by the master detachment fault after breakup began the master detachment fault was folded isostatically by a nnw trending extensional anticline the oxford ridge anticline the folding produced a new listric low angle normal fault east of the oxford ridge anticline to replace the back tilted portion of the detachment fault this new low angle normal fault excised part of the hanging wall cut hanging wall structures and served as a secondary breakaway for the detachment west of the oxford ridge anticline the master detachment fault initiated and slipped at a low angle is the youngest low angle normal fault of the system and has not been rotated to a low dip angle through time the exhumed bannock detachment system although somewhat smaller shorter lived and more disrupted by younger basin and range normal faults is similar to the buried sevier desert detachment system beneath central utah both top to the west systems formed near the transition between the sevier thrust belt and its hinterland collapsed broad thrust related culminations eroded to cambrian rocks are paired with partly coeval top to east metamorphic core complexes 100 160 km to the west lie east of major thrust ramps formed and slipped at low angles in the most extended part of the system and exhibit a translation and breakup phase of deformation both are late cenozoic in age and have voluminous lacustrine synrift deposits that record saline and alkaline conditions during early extension similar processes likely produced both detachment systems

generation and propagation of annual rossby waves in the north atlantic abstract results of a nonlinear quasi geostrophic mode model on the plane with topography and coastal geometry of the north atlantic in the area 10 50 n 10 80 w are presented two modes are used in this study the baroclinic one is based on the mean density profile from the levitus dated at 30 n mean and annual variations of the wind stress curl which drive the model were obtained from the bunker climatic atlas annual rossby waves emanating off the eastern coast are strongly influenced by the mid atlantic ridge west of the mar a relative minimum of energy is observed the waves are influenced and modified by variable bottom topography in the western basin two regions are identified where rossby waves are generated the gulf stream and its extension towards the east and the southern flank of the subtropical gyre compared to the north pacific annual rossby waves seem to be less energetic in the north atlantic

seasonal variability of the canary current a numerical study 1 a high resolution numerical model study of the canary basin in the northeast subtropical atlantic ocean is presented a long term climatological solution from the regional oceanic modeling system roms reveals mesoscale variability associated with the azores and canary current systems the northwest african coastal upwelling and the canary island archipelago the primary result concerns the canary current canc which in the solution transports 3 sv southward in line with observations the simulated canc has a well defined path with pronounced seasonal variability this variability is shown to be mediated by the westward passage of two large annually excited counterrotating anomalous structures that originate at the african coast the anomalies have a sea surface expression permitting their validation using altimetry and travel at the phase speed of baroclinic planetary rossby waves the role of nearshore wind stress curl variability as a generating mechanism for the anomalies is confirmed through a sensitivity experiment forced by low resolution winds the resulting circulation is weak in comparison to the base run but the propagating anomalies are still discernible so we cannot discount a further role in their generation being played by annual reversals of the large scale boundary flow that are known to occur along the african margin an additional sensitivity experiment where the azores current is removed by closing the strait of gibraltar presents the same anomalies and canc behavior as the base run suggesting that the canc is rather insensitive to upstream variability from the azores current

traction carpet stratification in turbidites fact or fiction abstract the spaced stratification that characterizes some thick deep water sandstones consists of a stack of layers 5 10 cm thick that are inversely graded at their base and structureless at their top these layers have been attributed to deposition from a succession of collapsed inertia layers traction carpets prior to collapse each layer was believed to have been maintained by shear from the overriding turbidity current this depositional model is based on the mistaken assumption that the distribution of shear stress in such a traction carpet is indistinguishable from the distribution of shear stress in a grain flow on a steep slope instead shear stress in an externally sheared traction carpet should be approximately constant from the top to the base of the layer and deposition sho ld be a gradual process leaving no record of sequentially active inertia layers inertia layers that do form under large turbidity currents are probably never as thick as 5 10 cm based on available data from shear cell experiments instead inertia layers are probably about 1 cm thick similar to the thickness of the inversely graded basal part of each layer in field examples of spaced stratification the rest of each inversely graded stratum probably forms due to rapid fallout from suspension the basal inversely graded 1 cm is believed to record strong shear that may accompany impingement on the bed of large turbulent eddies hence spaced stratification may not be a structure produced beneath steady flows but may record strong y fluctuating hydrodynamic conditions and vigorous burst sweep cycles in large turbidity currents

experiments on gas ash separation processes in volcanic umbrella plumes abstract we present a series of analogue laboratory experiments which simulate the separation of ash and gas and the formation of secondary intrusions from finite volcanic umbrella plumes we examined the lateral spreading of mixtures of freshwater and particles released into a laboratory tank containing a uniformly stratified aqueous solution for times smaller than the sedimentation time of particles through the intrusion the current remains coherent and intrudes laterally as some of the particles settle into the underlying ambient fluid a layer of particle depleted fluid develops below the upper surface of the current and the density of the residual fluid is reduced over longer times the intrusion ceases to be coherent with small fingers of relatively buoyant particle depleted fluid rising from the upper part of the intrusion into the overlying fluid meanwhile the lateral motion of the injected solution induces a return flow in the ambient fluid which sweeps some of the particles sedimenting from the lower surface of the intrusion inwards as a result relatively dense particle laden fluid collects below the intrusion and then sinks into the underlying fluid eventually this fluid reaches a new neutral buoyancy height where it intrudes to form a second laterally spreading current below the original intrusion the process then repeats to form further weaker intrusions below these results of the separation of the ash and volcanic gas in an umbrella plume are consistent with field observations at sakurajima volcano where positively charged plumes thought to consist of volcanic gas have been observed above negatively charged plumes of ash this work also suggests that volcanic aerosols may form up to a kilometer above the original injection height of the ash in a strong wind shear this could result in very different trajectories of the ash and gas and so be important for evaluating the impact of ash plumes on both aviation safety and volcanic aerosol formation the separation of the ash from the umbrella plume after a time of order 10 3 10 4 s corroborates recent models of the may 18 1980 eruption at mount st helens that after a few hours the motion of the ash was simply controlled by wind dispersal and gravitational settling the complex motion of the sedimenting ash may also explain some of the observations of multiple intrusions in short lived umbrella plumes such as observed at redoubt alaska in april 1990

the canary basin contribution to the seasonal cycle of the atlantic meridional overturning circulation at 26 n this study examines the seasonal cycle of the atlantic meridional overturning circulation amoc and its eastern boundary contributions the cycle has a magnitude of 6 sv as measured by the rapid mocha wbts project array at 26 n which is driven largely by the eastern boundary the eastern boundary variations are explored in the context of the regional circulation around the canary islands there is a 3 month lag between maximum wind forcing and the largest eastern boundary transports which is explained in terms of a model for rossby wave generated at the eastern boundary two dynamic processes take place through the lanzarote passage lp in fall the recirculation of the canary current and the northward flow of the intermediate poleward undercurrent in contrast during the remaining seasons the transport through the lp is southward due to the canary upwelling current these processes are linked to the seasonal cycle of the amoc

fine grained sediment lofting from meltwater generated turbidity currents during heinrich events turbidity currents generated from sediment carrying freshwater discharges into the sea contain a fluid that is less dense than ambient seawater from experiments it is known that such currents will eventually lift from their substrate either in part or as a whole through buoyancy reversal this ascent will happen when their density is lowered below that of seawater through settling of suspended sediment from the top or deposition from the bottom of the flows evidence for large scale lofting of suspended sediment from the top of giant sand and gravel carrying turbidity currents in the labrador sea comes from two independent lines of observations 1 the first is a distinct sedimentary facies consisting of stacked centimeter thick graded mud layers that contain grains of ice rafted debris ird supported by the mud deposition of these unusual layers requires a gradedlayer forming process that is slow enough to allow the incorporation of ird this is not possible with normal mud carrying turbidity currents 2 the second observation is the presence of a huge abyssal sand and gravel plain in the central labrador sea that received its sediment from bed load rich meltwater discharges from the hudson strait outlet of the pleistocene laurentide ice sheet these discharges turned into turbidity currents that released rising columns of freshwater that carried fine grained suspended sediment and spread out at a water level where their density equaled that of ambient seawater deposition from these slow turbid interflows would allow the incorporation of ird in the accumulating graded mud deposits the ird spiked graded mud facies is restricted to heinrich layers within 300 km radius of the hudson strait ice stream terminus tying the sand carrying turbidity currents via fine grained sediment lofting to heinrich events estimated total discharge volumes of individual currents are on the order of 10 3 km 3 supporting the notion that heinrich ice rafting events were times of maximum meltwater generation

reversing buoyancy in turbidity currents developing a hypothesis for flow transformation and for deposit facies and architecture a turbidity current that contains fresher or otherwise less dense water than its surroundings may initially be denser than the ambient and propagate as a bottom hugging flow but later reverse in buoyancy as its bulk density decreases through sedimentation to become lower than that of the ambient seawater it is proposed that this reversal in buoyancy may be a significant mechanism controlling the structure and facies of turbiditic deposits buoyancy reversal followed by lofting may directly affect the relative distribution of fine and coarse material in the deposit while buoyancy reversal itself may mediate the transformation between dilute and highly concentrated suspension flows particularly in distal regions and thus lead to the formation of complex turbiditic beds in particular the generation of distal co genetic debrites may be expected similar transformations occur within dilute pyroclastic density currents where a mobile basal concentrated flow termed a surge derived pyroclastic flow develops through rapid sedimentation from the suspended load of the overlying surge the physical mechanisms involved in these processes are discussed leading to the proposal of some associated facies models these are compared with field data from the northern apennines with some striking similarities being noted as well as some differences on the basis of this discussion some directions are suggested for future experimental and modelling work on the topic

structural and geochemical constraints on the reassembly of disrupted mid miocene volcanoes in the lake mead eldorado valley area of southern nevada in the lake mead eldorado valley lmev area of southern nevada mid tertiary volcanic and plutonic rocks in the river mccullough and eldorado mountains lie in the upper plate of a regional detachment structure the detachment structure and strike slip faults of the lake mead fault zone are temporally and kinematically related strike slip systems and normal faults eldorado valley fault serve as boundaries between regions of variable extension in the upper plate of this detachment geochemical correlation and geometric reconstructions suggest that prior to extension the lmev area was characterized by three stratovolcano complexes each above or adjacent to a chemically correlative pluton geochemical correlation techniques are useful tools that may have general application in reconstructing structurally disrupted volcanic plutonic terranes

persistent westward flow in the azores current as seen from altimetry and hydrography temporal and spatial variability of the eastward flowing jet like azores current is investigated by applying a new method for combining altimetry with hydrography in order to produce absolute surface geostrophic currents challenor et al 1996 the altimeter surface measurements contain information about both the geoid and the geostrophic ocean circulation the geoid signal dominates and is generally not sufficiently well known at the small length scales required to identify the mesoscale ocean signal in this study hydrography is used to distinguish the mean ocean signal from the geoid along an ers 1 satellite track south of the azores during both 3 day repeat phases and thereby determine cross stream absolute dynamic topography of the azores current at high temporal resolution hydrographic data collected near 28 w in march 1992 is combined with ers 1 altimeter data from january march 1992 and january to early april 1994 time series of 3 day repeat profiles of absolute surface geostrophic velocity perpendicular to the altimeter cruise track are examined the azores current absolute velocity profile along this track is rather coherent the current variability is less than the mean surprisingly a persistent westward flow of 25 cm s 1 is observed near 35 n 28 w this flow appears to be a retroflection from the main eastward flowing current an isopycnal analysis reveals the westward flow to be cooler by 0 5 c and fresher by 0 15 practical salinity units than the main body of the azores current the flow appears to persist throughout each of the above 3 month periods in late winter early spring but the question of whether this is a seasonal feature or more permanent remains open it is suggested that the westward flow is topographically controlled

sources of interannual baroclinic waves in the eastern subtropical north pacific abstract subsurface temperature and surface salinity observations along the great circle transect between san francisco and honolulu are used to investigate the source of annual baroclinic long waves in the eastern subtropical north pacific ocean long term mean 15 day values 1966 74 of depth of the 9 c isotherm indicative of thermocline depth and of surface salinity are each fit by least squares method with a time varying cosine yielding phase and amplitude of the mean annual cycle at standard locations every 92 6 km 50 mi along the transect local wind stress curl is expected to influence the observed depth of the 9 c isotherm through the mechanism of ekman pumping therefore its annual cycle is computed in a manner like that of the depth of the 9 c isotherm and its effect removed from the latter the residual annual cycle has phase and amplitude profiles along the transect that closely resemble that of model annual baroclinic long waves emanating from a line source of large amplitude wind st

channels echo character mapping and tectonics from 3 5 khz profiles distal bengal fan abstract the distal parts of the bengal fan are spectacularly affected by tectonic deformation related to the diffuse plate boundary between the indian and australian plates here we use 3 5 khz and 12 khz echosounder profiles seismic reflection profiles and piston core results to examine sedimentary processes and their relationships to tectonism within an area 78 82 e 0 6 s just to the south of ocean drilling program leg 116 sites echo character mapping was completed using echosounder data in conjunction with results from piston coring and a total of five different echo types have been recognised four of these fall into the echo character classification scheme developed by damuth 1980a whilst the fifth is believed to represent hemiturbidite deposits several types of submarine channel were also identified from echosounder data and a correlation between echo type and channel location can be seen their abundance erosional and or depositional nature together with a complex meandering and bifurcation pattern across a wide region of average gradient around 1 km are all features characteristic of a broad channel termination zone on a large elongate fan it is clear therefore that the bengal fan extends beyond 6 s active faulting in the area has led to the development of an irregular topography of low rounded hillocks that interfere with incoming turbidity currents this has resulted in ponding between highs rather than lobe construction thinning and pinching out of turbidites against the flanks of local relief flow lofting and hemiturbidite drape and common small scale slumping in some cases uplifted channel segments can be seen abandoned and partially filled

significance of fine grained sediment lofting from melt water generated turbidity currents for the timing of glaciomarine sediment transport into the deep sea abstract sediment lofting from turbidity currents is a process that occurs in density currents generated from fresh water discharges into the sea and other gravity currents that can produce reversed buoyancy settling of suspended sediment from the top or deposition from the bottom of the flows may lower the density below that of ambient seawater causing the currents to lift up from their substrate either in part or as a whole through buoyancy reversal as is well known from experiments fine grained sediment lofting on a large scale during times of heinrich events has been inferred for ice proximal regions of the northwest labrador sea adjacent to the margin of the late pleistocene laurentide ice sheet lis the evidence comes from a distinct depositional facies of stacked graded mud layers containing ice rafted debris ird the significance of the lofted fine grained depositional facies arises from its genetic link to large sand and gravel transporting turbidity currents that were generated by melt water discharges from the hudson strait outlet of the lis and are the likely source of the lofted sediment the observed lofted depositional facies is exclusively found in heinrich layers through this stratigraphic relationship the lofted facies ties the main pulses of sediment supply in the labrador basin to heinrich events heinrich events thus appear to be the times of the main depositional activity in the labrador sea a hitherto unknown aspect of these phenomenal events besides their known role as intense iceberg discharges

on the origin of the azores current the azores current south of the azores archipelago is part of the subtropical north atlantic gyre using an international hydrographic data set we analyze mean and seasonal geostrophic transport fields in the upper 800 m of the ocean in order to determine the origin of the azores current in the western basin and seasonal changes in the related flow geostrophic currents are obtained by using the method applied by stramma 1984 in the eastern basin the azores current is found to originate in the area of the southwest newfoundland rise figure 10 in winter an almost uniform current connects this region of origin with the azores current while a branching into two current bands is observed in summer with the southern band forming a marked cyclonic loop within the upper 800 m all of the transport in the northern band and about 70 of the transport in the southern band recirculates in the eastern basin additionally expendable bathythermograph data from the azores current region indicate an increase of eddy potential energy from winter to summer

rossby wave analysis of subsurface temperature fluctuations along the honolulu san francisco great circle abstract seven years of xbt observations collected between honolulu and san francisco reveal subsurface temperature fluctuations with strong in phase vertical coherence and fair horizontal coherence over 1000 km distance their annual and semi annual harmonics exhibit randomness of phase suggesting that the fluctuations are modeled better as a stochastic process than as a deterministic signal cross spectral fitting of a stochastic first baroclinic mode rossby wave model to the fluctuations demonstrates the rossby waves could be producing 50 60 of the observed subsurface covariance at periods exceeding nine months phase is seen to propagate northwestward and energy westward and southwestward the corresponding sea surface velocities and sea level fluctuations are of the order 1 cm s 1 and 1 5 cm

physical oceanography of the azores front abstract there is a considerable body of evidence for the existence of a permanent front sw of the azores surveys in the summer of 1981 show the structure to penetrate to a depth of 2 000 m and suggest that its meandering is related to the alignment of the atlantis gt meteor seamount chain the total volume transport above 2 000 m is between 10 and 12 106 m3 s 1 towards the east the main water mass contrasts across the front are confined to the upper 400 m of the water column only a small change in mediterranean water strength was seen across the front satellite tracked drogued buoys seeded into the front showed surface speeds of up to 40 cm sec 1 and revealed the formation of a closed cold core eddy some 100 115 km in diameter which propagated westward at 2 2 km day 1 over a 200 day period sea surface temperature contrasts across the frontal boundary are small and together with frequent cloud cover prevent the detection of the front and associated eddies by satellite infra red imagery the front apparently marks the eastward extension of the southern branch of the gulf stream recirculation and the northern limit of the 18 c water mass eddies formed at the front could propagate westward into the interior of the sargasso sea but have not yet been observed to cross the mid atlantic ridge

simple estimates for the magnitude of bedload transport under a turbid surge characteristic length and timescales for a turbid surge are used to estimate bedload transport by the surge deriving estimates for the conditions under which deposited material will be mobilized as bedload and of the relative importance of bedload in determining the overall deposit geometry a critique is provided of the common modelling assumptions which underlie these estimates and of how their consistency can be checked for large turbidity currents such as those which emplaced the marnoso arenacea turbidites in northern italy model predictions of overall geometry are not easy to reconcile with the field data some possible reasons for this are discussed the estimates obtained from these characteristic scales are consistent with the widespread presence in turbidites of sedimentary structures which indicate bedload transport some of these structures from the marnoso arenacea formation are reviewed briefly however the estimates suggest that bedload transport is not a major factor responsible for the geometry of the large turbidites in this formation which exhibit a broad thickness maximum in their proximal region that contrasts with the downstream thinning geometry of smaller beds this effect suggests that the explanation for this theoretically unexpected geometry should be sought in other physical mechanisms

emplacement of the taupo ignimbrite by a dilute turbulent flow an ignimbrite is a pumice rich deposit that records the passage of a ground hugging ash flow a pyroclastic flow generated by the collapse of a volcanic eruption column1 geologists study such deposits to reconstruct the parent eruptions and to predict the consequences of future eruptions the 1 800 yr old taupo ignimbrite of new zealand has been interpreted to represent en masse emplacement by an avalanche like flow with a volumetric solids concentration in excess of 30 ref 2 the evidence for this is equivocal however and here we propose the alternative view that the deposit was emplaced by a relatively dilute and turbulent density current3 we present an isothermal hydraulic model the results of which taken together with existing observations suggest that the total flow rate of solids and gas was about 40km3s 1 for around 15 minutes this intense flux resulted in a flow which had a near vent solids concentration of 0 3 by volume was about 1 km thick and travelled outward from the vent with a typical speed of 200 ms 1 in view of the good agreement between predicted and observed radial trends in the taupo deposit we suggest that the origin of other ignimbrites with similar characteristics should be reconsidered

meandering of the subtropical front south east of the azores a subtropical front was observed in the area south and southeast of the azores during cruises of fs meteor and fs poseidon in early 1982 the front has a basically west east extension with considerable meandering observed meso scale eddies are found on both sides the overall flow pattern corresponds to earlier results on geopotential differences in the upper northeast atlantic but the baroclinic transport of the order of 107 m3 s 1 is found to be concentrated in a 60 km wide jet we suggest here that the current band is part of the gyre circulation resulting from a branching of the north atlantic current