new technique for access borehole drilling in shelf glaciers using lightweight drills this paper describes a new environmentally friendly drilling technique for making shortand long term access boreholes in shelf glaciers using lightweight drills the new drilling technique was successfully developed for installation of small diameter sensors under the ross ice shelf through 193m thick ice at windless bight mcmurdo ice shelf antarctica the two access boreholes were drilled and sensors installed in 110 working hours the total weight of the drilling equipment including the power system and fuel is 400 kg installation of small diameter sensors was possible for 1 8 6 hours after penetration through the glacier into the sea water beneath the new drilling technique does not require drilling fluid and therefore has minimal environmental impact it should permit access through ice shelf ice up to 350m thick or glaciers on grounded ice or subglacial lakes if there is no water permeable interface at the base modifications presented in this work of the drilling equipment and protocol will allow for 1 21 working hours for penetration through 200m of ice 2 installation of sensors up to 120mm in diameter and 3 drilling long term open boreholes through 400m thick ice in 100 working hours

periplatform drift the combined result of contour current and off bank transport along carbonate platforms hydroacoustic and sedimentological data from the western leeward flank of the great bahama bank document the interplay of off bank sediment export along slope transport and erosion which together shape facies and thickness distribution of slope carbonates the integrated data set depicts the combined product of these processes and allows formulation of a comprehensive model of a periplatform drift that significantly amends established models of carbonate platform slope facies distribution and geometry the basinward thinning wedge of the periplatform drift at the foot of the bank escarpment displays along slope and downslope variations in sedimentary architecture sediments are muddy carbonate sands that coarsen basinward the drift wedge has a pervasive cover of cyclic steps in zones of lower contour current speed depth related facies belts develop whereas strike discontinuous sediment lobes scarps and gullies characterize areas with higher current speed this understanding of the impact of currents on carbonate slope sedimentation has wider implications for seismic and sequence stratigraphic interpretation of carbonate platforms and for applied aspects such as hydrocarbon exploration

evolution of the semidiurnal m2 internal tide on the continental slope of the northern south china sea kinetic energy spectra from a site on the continental slope in the south china sea reveal that significant peaks appear at some nonlinear interaction frequencies namely m 3 m 1 m 2 and fm 1 m 1 f where f is the inertial frequency m 1 is the diurnal internal wave and m 2 is the lunar semidiurnal internal tide a possible generation mechanism of m 3 is explored analysis of bicoherence and shear spectra suggests that strong m 3 is indirectly associated with parametric subharmonic instability psi of m 2 in another word under the effect of psi the energy of m 2 is first transferred to m 1 then via other nonlinear coupling some nonlinear waves e g fm 1 m 3 are generated moreover m 1 is also present at another site near the bottom of the continental slope the shear spectra from these two sites show for the first time that m 1 can be significantly distinguished from lunar diurnal o 1 and lunisolar diurnal k 1

late quaternary reef growth and sea level in the maldives indian ocean abstract based on rotary drilling and radiometric and u series dating we present the first comprehensive data on holocene reef anatomy and sea level rise as well as nature and age of underlying pleistocene limestone in the maldives holocene reefs in rasdhoo atoll central maldives are composed of four facies including 1 robust branching coral facies 2 coralline algal facies 3 domal coral facies and 4 detrital sand and rubble facies branching coral and coralline algal facies predominate the marginal reefs and domal corals and detrital facies preferentially occur in a lagoon reef in addition microbialite crusts are found in lower core sections of marginal reefs microbialites formed during the early holocene in reef cavities holocene reef thickness ranges from 14 5 m to 22 m reef growth started as early as 8 5 kyr bp marginal reefs accreted in the keep up mode with rates of 15 m kyr rate of sea level rise significantly slowed down from 7 6 kyr bp and subsequently gradually rose with rates

internal waves across the pacific 1 the long range propagation of the semidiurnal internal tide northward from the hawaiian ridge and its susceptibility to parametric subharmonic instability psi at the critical latitude c 28 8 n were examined in spring 2006 with intensive shipboard and moored observations spanning 25 37 n along a tidal beam velocity and shear at c were dominated by intense vertically standing inertially rotating bands of several hundred meters vertical wavelength these occurred in bursts following spring tide contrasting sharply with the downward propagating wind generated features seen at other latitudes these marginally stable layers which have inverse 16 meter richardson number ri16 1 0 7 are interpreted as the inertial waves resulting from psi of the internal tide elevated near inertial energy and parameterized diapycnal diffusivity and reduced asymmetry in upgoing downgoing energy were also observed at and equatorward of c yet simultaneous moored measurements of semidiurnal energy flux and 1 km deep velocity sections measured from the ship indicate that the internal tide propagates at least to 37 n with no detectable energy loss or phase discontinuity at c our observations indicate that psi occurs in the ocean with sufficient intensity to substantially alter the inertial shear field at and equatorward of c but that it does not appreciably disrupt the propagation of the tide at our location

facies and geometry of drowning steps in a miocene carbonate platform maldives abstract the study of the miocene carbonate platform of the maldives allows understanding the controlling factors triggering the stepwise drowning of carbonate platforms this research presents high resolution seismic profiles and sediment cores retrieved from sites u1465 u1469 and u1470 drilled during international ocean discovery program iodp expedition 359 a microfacies analysis of the platform deposits was conducted and correlated with a sequence stratigraphic analysis of seismic reflection data the neogene evolution of the maldives encompasses a series of drowning steps two of these steps are compared with regard to the carbonate factory type to unravel if fossil assemblage surviving the first drowning step are different from younger assemblages in both cases the shallow water carbonate facies are formed by coral coralline algal boundstone and foralgal floatstone that change basinwards into slope facies dominated by large benthic foraminifera grainstone to packstone locally floatstone and rudstone the red algae and large benthic foraminifera assemblages are similar before and after the first drowning step after this first drowning the carbonate platform developed under enhanced bottom current conditions as deduced by the coeval drift deposits the carbonate platform and the drift deposits were balanced for circa 2 myr until a global sea level fall at 10 6 ma and later reflooding of the platform which destroyed the equilibrium between both depositional systems favouring the current controlled deposits the drowning of the miocene carbonate platform of the maldives was the result of the combined effect of the fluctuating sea level during the neogene and the intensification of oceanic currents due to the invigoration of the south asian monsoon during the middle miocene

long term sea level fluctuations driven by ocean basin dynamics earth9s long term sea level history is characterized by widespread continental flooding in the cretaceous period 145 to 65 million years ago followed by gradual regression of inland seas however published estimates of the late cretaceous sea level high differ by half an order of magnitude from 40 to 250 meters above the present level the low estimate is based on the stratigraphy of the new jersey margin by assimilating marine geophysical data into reconstructions of ancient ocean basins we model a late cretaceous sea level that is 170 85 to 270 meters higher than it is today we use a mantle convection model to suggest that new jersey subsided by 105 to 180 meters in the past 70 million years because of north america9s westward passage over the subducted farallon plate this mechanism reconciles new jersey margin based sea level estimates with ocean basin reconstructions

contour current accumulation off the vema channel mouth southern brazil basin pattern of a contourite fan abstract in the southern part of the brazil basin 3 5 khz echosounding reveals the existence of a cone shaped sedimentary accumulation 4480 m depth this sediment body is interpreted as deposited by the antarctic bottom water currents aabw at the downstream exit of deep valleys cutting through the rio grande rise it is located between two major channels that prolonged northeastward the valleys into the brazil basin the vema channel in the southeast and the sao paulo abyssal gap in the northwest as a consequence most of the aabw flow is channelized in two branches deposition of the current transported particles occurs below the shear zone which takes place between the two branches the quaternary sediments consist of silty clayey muddy contourites associated with manganiferous deposits the accumulation is built by lateral migration of secondary channels and by lateral construction of lobes between the two major channels which are both well stabilized on account of a morphotectonic control on their location the entire sediment body progrades upward and downstream toward the northeast on the basis of morphological seismic and lithological arguments we would like to propose the name of contourite fan for such a contour current controlled accumulation

glacial versus interglacial sedimentation rates and turbidite frequency in the bahamas the southern tongue of the ocean is a 1300 m deep flat floored basin in the bahamas that receives large amounts of sediment from the carbonate platforms surrounding it on three sides we have examined five 8 13 m long piston cores and determined bulk sedimentation rates turbidite frequency and turbidite accumulation rates for the past two glacial and interglacial periods the mean of bulk sedimentation rates is four to six times higher in interglacial periods average accumulation rates of recognizable turbidites are higher by a factor of 21 to 45 and interglacial turbidite frequency is higher by a factor of 6 to 14 sediment composition indicates that increased interglacial rates are due to higher accumulation of platform derived material additional data from other bahamian basins as well as published material from the caribbean strongly suggest that highstand shedding is a general trend in pure carbonate depositional systems carbonate platforms without a siliciclastic component export more material during highstands of sea level when the platform tops are flooded and produce sediment the response of carbonate platforms to quaternary sea level cycles is opposed to that of siliciclastic ocean margins where sediment is stored on the inner shelf during highstands and passed on to continental rises and abyssal plains during lowstands of sea level

seismic stratigraphy and depositional signatures of the maldive carbonate system indian ocean abstract as documented by industrial seismic reflection profiles and well data the cenozoic stratigraphic evolution of the maldive carbonate system shows three distinct intervals characterized by their own specific depositional signatures late palaeocene volcanics generated by the reunion hotspot filled deeper pull apart structures and created a flat topography that promoted the onset of the maldive carbonate system in the early eocene some of the initial structural depressions remained as deep internal seaways surrounded by reefal margins until the late oligocene the eocene early oligocene stratigraphic signature is essentially aggradational and corresponds to a transgressive cycle associated with a second order sea level rise in the mid oligocene a major sea level fall exposed the neritic carbonate system and restricted sedimentation to the former deep internal basins during the next transgression a neritic carbonate system re established itself on the newly flooded platform and was finally drowned near the oligocene miocene boundary the main effect of this second order transgressive cycle was the transformation of the central troughs into one single deep basin in the neogene the palaeo maldive inner sea when a reef framework was established in the early miocene near the present location of the platform edge systematic bank margin progradation toward the central inner sea basin became enhanced this neogene progradational phase is explained as the response to a long term sea level fall associated with a second order regressive cycle finally the late pliocene pleistocene depositional signature is mainly aggradational and corresponds to the regional modification from flat topped carbonate banks to atoll physiographies the antecedent karst theory suggests that increased karstification was triggered by high frequency eustatic and climatic changes in the last 3 0 ma

carbonate delta drift a new sediment drift type based on high resolution reflection seismic and core data from iodp expedition 359 we present a new channel related drift type attached to a carbonate platform slope which we termed delta drift like a river delta it is comprised of several stacked lobes and connected to a point source the delta drifts were deposited at the exit of two gateways that connect the inner sea of the maldives carbonate platform with the open ocean the channels served as conduits focusing and accelerating the water flow entrained material was deposited at their mouth where the flows relaxed the lobe shaped calcareous sediment drifts must have formed under persistent water through flow sediment supply was relatively high and continuous resulting in an average sedimentation rate of 17 cm ka 1 the two delta drifts occupy 342 and 384 km2 respectively with a depositional relief of approximately 500 m they have a sigmoidal clinoform reflection pattern with a particular convex upward bending of the foresets in the maldives the drift onset marks the transition from a sea level controlled to a progressively current dominated depositional regime this major event occurred in the serravallian about 13 ma ago leading to the partial drowning of the carbonate platform and the creation of shallow seaways the initial bank enclosed topography resembles an empty bucket geometry which is rapidly filled by the drift sediments that aggrade and prograde into the basin thereby the depositional environment of the delta drifts changes from deep water 500 to shallow water conditions at their topsets indicated by the overall coarsening upward trend in grain size and the presence of shallow water large benthic foraminifers at their top

monsoon induced partial carbonate platform drowning maldives indian ocean multibeam maps and high resolution seismic images from the maldives reveal that a late miocene to early pliocene partial drowning of the platform was linked to strong sea bottom currents in the upper miocene to holocene currents shaped the drowned banks the current moats along the bank edges and the submarine dune fields bottom currents in the maldives are driven by the monsoon it is proposed that the onset and the intensification of the monsoon during the neogene provoked platform drowning through injection of nutrients into surface waters since the late miocene topographically triggered nutrient upwelling and vigorous currents switched the maldives atolls into an aggradational to backstepping mode which is a growth pattern usually attributed to episodes of rising sea level

influence of air temperature on a glacier s active layer temperature seasonal temperature variations occur in the glacier layer about 15 20 m below the surface while at greater depths the glacier temperature depends on the long term surface conditions it is generally accepted that for glaciers without surface melting the temperature at 10 m depth t10 i s close to the mean annual air temperature at standard screen level ta i e t10 ta we found that this relationship is not valid for ta above 178c and below 558c the goal of our investigation is to find a better temperature transfer function ttf between ta and temperature at the boundary of the active layer in accumulation areas of polar and tropical glaciers low precision t10 temperatures from boreholes obtained at 41 sites are compared with air temperatures ta measured in the vicinity of these sites for at least a 1 year period we determine that when ta falls into the temperature range 60 to 78c empirical values can be approximated as t10 1 2ta 6 7 analysis of these data suggests that high t10 occurs in the areas of the glacier that collect meltwater

contourite porosity grain size and reservoir characteristics abstract contourites are now recognised as having a significant potential as hydrocarbon reservoirs in the subsurface and several fields have been interpreted as comprising bottom current reworked turbidite sands however very little has been published on the porosity characteristics of contourites this study documents porosity data from iodp expedition 339 sites in the gulf of cadiz we use grain size analyses porosity depth plots and exponential models to yield a better understanding of grain size characteristics and facies porosity characteristics and the reservoir potential of contourites in the subsurface new grain size data for over 350 samples from the cadiz contourites is presented building on earlier work these data confirm the distinctive trends in textural properties linked to depositional processes under the action of bottom currents the finest muddy contourites porosity depth relationships from four cadiz sites show a moderately high initial porosity for both sand and mud facies 50 60 and a systematic decrease with depth to around 35 40 near 500 m burial depth according to the exponential models of porosity with depth contourite porosity should exceed 10 at 2500 m burial depth we compare the data from the gulf of cadiz contourite depositional system with those of the eirik drift newfoundland drift gardar drift and canterbury slope drifts similar depth trends are observed and all show anomalies linked to interbedded sandy and muddy facies composition carbonate vs siliciclastic and the presence of hiatuses in the sediment record these data provide good insight into the likely reservoir characteristics of contourites for both conventional and unconventional reservoirs they are comparable with those of existing contourite fields most of which are mixed turbidite contourite systems

monsoonally influenced circulation around coral reef islands and seasonal dynamics of reef island shorelines synchronous measurements of reef flat waves and nearshore currents were made around the perimeter of three coral reef islands in the central maldives nearshore current velocities were shown to be tidally modulated in most instances with maximum velocities associated with lower tidal stages peak currents were consistently observed on the windward and lateral flanks of island shorelines 0 3 0 4 ms 1 the relatively low elevation of reef surfaces and medium to low energy incident wave energy hs 0 2 0 3 m accounts for low tidal stage peaks in velocity as this corresponds to the period when wave setup induced current processes are maximised significantly all nearshore currents were unidirectional and alongshore the generation of unidirectional alongshore flow patterns results from island configuration which deflects flow and variations in reef geometry that control spatial variations in wave setup around the reef platform edge and at the island shoreline which can drive alongshore currents the presence of a nearshore moat that channelizes flow in the nearshore and variations in reef geometry where topographic low points act as preferential drainage points results show that changes in monsoon conditions that modulate wind and wave patterns from the west to northeast can force substantial changes in reef platform and nearshore current flows but that the degree of change is dependent on reef platform shape and position in the atoll the circular study island was found to exhibit a near complete reversal in unidirectional flow from northeast flow in the westerly monsoon to southwest flow in the northeast monsoon however the elongate and triangular shaped reef platforms exhibited more localised changes in flow patterns significantly those shorelines that exhibit the greatest changes in shoreline currents correspond to the sections of coast that exhibit the largest morphological adjustments between seasons results provide significant insights for the morphodynamics of reef platform islands in particular the nearshore process regime is dominated by alongshore processes furthermore reef platform shape and the magnitude of change in direction of wave approach to reef platforms provide critical indicators of the morphodynamic behaviour of reef platform islands

anti torque systems of electromechanical cable suspended drills and test results abstract to prevent spinning of the upper non rotated part of the electromechanical drill an anti torque system has to be included in the downhole unit at the same time the anti torque must allowthe drill to move up and down the borehole during drilling and tripping operations usually the anti torque system has a blade form of various designs that engages with the borehole wall and counteractsthe torque from the stator of the driving motor this paper presents a review of the different anti torquesystems and test results with selected designs leaf spring skate and u shaped anti torque systems experiments showed that the skate anti torque system can provide the maximal holding torque between67 and 267 n m 1 depending on the skates outer diameter and ice temperature while the leaf springanti torque system can provide only 2 5 40 n m 1 in case of straight contact between the ice and theleaf springs the total resistance force to axial movement of the skate anti torque system lies in therange 209 454 n if the system is vibrating for the leaf spring anti torque system the total axialresistance force is far less 19 243 n keywords

moored observation of abyssal flow and temperature near a hydrothermal vent on the southwest indian ridge four moorings were deployed near dragon flag an active hydrothermal vent in the valley of the southwest indian ridge the goal was to examine the variability of currents and temperature which will guide the trajectory of spreading plumes the mean current was cross isobath and the circulation was characterized by a submesoscale circulation observed currents also showed fluctuations with periods of 1 15 days the inferred phase speed and wavelength for the wave with a period of 4 4 day are 10 4 km d 1 and 45 8km respectively which are consistent with the topographic rossby wave theory the persistent warming tendency with corresponding variation of salinity based on background s properties may be caused by background circulation and divergence of the water column the warming or cooling episodes were most likely as signatures of isopycnal surface depression or uplifting induced by the moving of mesoscale eddies well resolved rotary spectra exhibited important nonlinear interactions between inertial and semidiurnal tide in the velocity and temperature records amplification of near inertial currents in the near bottom is also exposed these discoveries provided new evidence for the nonlinear interaction and trapped near inertial waves by the ridge which occurred in the deep ocean of the southern hemisphere such nonlinear interaction may represent a significant energy loss pathway for the internal waves and part of the decay of such motion would likely result in increased mixing to maintain the abyssal stratification enhanced near inertial motions can play a major role for the local advection of hydrothermal plumes

processes influencing the non linear interaction between inertial oscillations near inertial internal waves and internal tides 1 a numerical model of the hebrides shelf edge represented by a cross section is used to examine the non linear interactions producing energy at the fm2 frequency calculations show that in the near coastal ocean this is primarily due to coupling between wind induced inertial oscillations although near inertial internal waves are present and the m2 internal tide the major non linear interaction and hence largest fm2 currents occurs in the region of the thermocline and is associated with shear in the inertial oscillation and the vertical velocity due to the internal tide this non linear process represents an important contribution to the energy cascade from the wind and tide into higher frequency waves and eventually mixing the fm2 current is shown to be a maximum in the shelf edge region and hence measurements in this area will be particularly valuable in determining the extent to which models can represent non linear processes

spatial variability of turbulent mixing in the abyssal ocean ocean microstructure data show that turbulent mixing in the deep brazil basin of the south atlantic ocean is weak at all depths above smooth abyssal plains and the south american continental rise the diapycnal diffusivity there was estimated to be less than or approximately equal to 0 1 x 10 4 meters squared per second in contrast mixing rates are large throughout the water column above the rough mid atlantic ridge and the diffusivity deduced for the bottom most 150 meters exceeds 5 x 10 4 meters squared per second such patterns in vertical mixing imply that abyssal circulations have complex spatial structures that are linked to the underlying bathymetry

micromorphology of a debris flow deposit evidence of basal shearing hydrofracturing liquefaction and rotational deformation during emplacement abstract micromorphological analysis has been applied to a very coarse debris flow deposit and associated sand dominated glaciofluvial sediments exposed in westend wood quarry carstairs central scotland the microstructures present folds faults plasmic fabrics within this diamicton and underlying predominantly sandy sediments are interpreted in terms of sedimentary processes active during emplacement of the debris flow these processes include the formation of a basal shear zone the deformation and mixing of fine grained sediments incorporated into the base of overriding debris flow features associated with liquefaction sediment remobilisation and water escape are also present and developed in response to increasing overburden pressure during debris flow emplacement these microstructures have also been recorded elsewhere within subglacial diamictons and an evaluation is made between microstructures in palaeo debris flow deposits and those found in modern debris flows

bottom trapped subinertial motions over midocean ridges in a stratified rotating fluid abstract linear analytical solutions for bottom trapped subinertial oscillatory flow over simple ridge topographies in a stratified two layer rotating fluid are presented results are compared to moored current meter observations of bottom intensified motions over the endeavour segment of juan de fuca ridge in the northeast pacific the solutions reproduce many of the observed features including preferential amplification of the clockwise rotary component of velocity over the ridge and increased velocity amplification with proximity to the ridge crest for a given internal deformation radius the degree of current amplification increases with increased bottom slope ridge height and oscillation frequency amplification decreases with increased width of the ridge relative to the deformation radius

contour current imprints and contourite drifts in the bahamian archipelago new data collected along the slopes of little and great bahama bank and the abyssal plain of the bahama escarpment provides new insights about contour current related erosive structures and associated deposits the bahamian slope shows abundant evidence of bottom current activity such as furrows comet like structures sediment waves and drifts at a seismic scale large erosion surfaces and main periods of drift growth resulted from current acceleration related to plate tectonic processes and progressive opening and closure of gateways and long term palaeoclimate evolution at present day erosion features and contourite drifts are either related to relatively shallow currents 2500 m water depth it appears that the carbonate nature of the drifts does not impact the drift morphology at the resolution addressed in the present study classical drift morphologies defined in siliciclastic environments are found such as mounded plastered and separated drifts in core contourite sequences show a bi gradational trend that resembles classical contourite sequences in siliciclastic deposits showing a direct relationship with a change in current velocity at the sea floor however in a carbonate system the peak in grain size is associated with increased winnowing rather than increased sediment supply as in siliciclastic environments in addition the carbonate contourite sequence is usually thinner than in siliciclastics because of lower sediment supply rates little bahama bank and great bahama bank contourites contain open ocean input and slope derived debris from glacial episodes inner platform platform edge and open ocean pelagic input characterize the classical periplatform ooze during interglacials in all studied examples the drift composition depends on the sea floor topography surrounding the drift location and the type of sediment supply carbonate particles are derived from either the slope or the platform in slope and toe of slope drifts very deep contourites have distant siliciclastic sources of sediment supply the recent discovery of the importance of a large downslope gravitary system along bahamian slopes suggests frequent interactions between downslope and along slope contour currents processes the interlayering of mass flow deposits and contourites at a seismic scale or the presence of surface structures associated with both contour currents and mass flow processes shows that both processes act at the same location finally contour currents have an important impact on the repartition of deep water coral mounds currents can actively interact with mounds as a nutrient and oxygen supplier or have a passive interaction with mounds solely being obstacles orienting erosion and deposition

a tidal influenced hydrothermal system temporarily cooled by a tropical storm abstract the lutao hydrothermal field is an intertidal arc volcanic system located offshore southeast taiwan hosting a zhudanqu zdq vent and a huwaichi hwc spring with strongly contrasting fluid chemistry low mg moderately enriched cl and h with respect to seawater indicate that the zdq endmember was derived from the brine phase that was formed during low degree subcritical phase separation in contrast the endmember for the hwc vent fluids is related to the vapor phase temperature and pressure of the phase separation were estimated as 150 c and 7 bar respectively the water rock ratio was roughly calculated as about 2 the lutao hydrothermal system was slightly affected by semi diurnal tides by some combination of tidal loading and tidal currents the time delay between tides and the response of the hydrothermal system was about 3 h while freshwater was almost absent in the hwc vent fluids at normal conditions the typhoon fung wong on sep 21st 2014 led to intrusions of freshwater into the vent fluids with a percentage of 16 both the zdq and the hwc endmember compositions showed some changes after the typhoon event suggesting a cooling of the reaction zone after the typhoon passed by the hydrothermal system began to recover evidenced by increasing percentages of the hwc endmember and decreasing freshwater contributions the flux of the hwc endmember was estimated as 460 560 l h 1 based on these observations this study for the first time reports a shallow depth tidal influenced hydrothermal system that was temporarily cooled by a tropical storm

flow and mixing in the rift valley of the mid atlantic ridge high levels of diapycnal mixing and geothermal heating near midocean ridges contribute to the buoyancy fluxes that are required to close the global circulation in topographically confined areas such as the deep median valleys of slow spreading ridges these fluxes strongly influence the local hydrography and dynamics data from a segment scale hydrographic survey of the rift valley of the mid atlantic ridge and from an array of current meters deployed there during an entire year are analyzed in order to characterize the dominant hydrographic patterns and dynamical processes comparison with historic hydrographic data indicates that the temporal variability during the last few decades has been small compared to the observed segment scale gradients the rift valley circulation is characterized by inflow from the eastern ridge flank and persistent unidirectional alongsegment flow into a cul de sac therefore most of the water flowing along the rift valley upwells within the segment with a mean vertical velocity 1025 ms 21 the observed streamwise hydrographic gradients indicate that diapycnal mixing dominates the rift valley buoyancy fluxes by more than an order of magnitude in spite of the presence of a large hydrothermal vent field supplying several gigawatts of heat to the water column hydrographic budgets in the rift valley yield diffusivity values of order 5 3 1023 m2 s 21 consistent with estimates derived from statically unstable overturns the largest of which were observed downstream of topographic obstacles in the path of the along segment flow this suggests vertical shear associated with cross sill flows as the dominant contributor to the mechanical mixing in the rift valley

origin of seismic reflections in a carbonate gas field lower miocene offshore myanmar abstract the geological origin of seismic reflections within a carbonate gas reservoir has been determined on the basis of an integrative analysis of cores well logs pre production and 4d time lapse seismic data the gas reservoir is subdivided into meter to decameter thick diagenetic units which are characterized by distinct acoustic properties low impedance units are dominant and consist of highly microporous and vuggy foraminiferal coralline algal floatstones intervals whose porosity has been significantly enhanced in burial diagenetic environments high to moderate acoustic impedance units thickness 1 20 m are related to various depositional and diagenetic fabrics 1 early lithified coral floatstones 2 foraminiferal rudstone whose intergranular space is occluded by shallow burial sparry calcite and 3 carbonate breccia with neomorphosed corals occurring below an intra formational subaerial exposure surface within the gas zone seismic reflection horizons may form at the boundary between two diagenetic units with contrasted acoustic impedance or may result from the interference between reflection at base and top of such intervals a stratigraphy cross cutting reflection horizon has been shown to form at a major and laterally continuous shift in porosity which is related to a differential diagenetic evolution between the gas and the water zone during or after hydrocarbon emplacement finally the analysis of the geological origin of seismic reflections has been proved to help predicting and mapping the major heterogeneities with significant impact on flow during production

observations of parametric subharmonic instability of the diurnal internal tide in the south china sea 1 shipboard observations are presented that suggest the occurrence of parametric subharmonic instability psi of diurnal k1 and o1 internal tides at critical latitudes of 14 52 n and 13 44 n respectively in a transect spanning 12 5 18 n depth mean shear squared shows sharp peaks at 14 52 n elevated relative to that at 15 n by a factor of ten and at 13 44 n by a factor of 7 wind speed measured from the ship and quikscat scatterometer during and before the transect was 10 m s 1 at these latitudes eight hour time series about 1 6 of an inertial period of shear and isopycnal depth at 14 52 n are sufficient to associate the elevated shear with alternating clockwise rotating layers analogous to those observed at the m2 critical latitude of 28 8 n

characteristics of 4 day oscillations trapped by the juan de fuca ridge current meter observations from different studies along the crest of the juan de fuca ridge have been combined to show new features of the relatively large broad spectral peak centered near a period of about 4 days the coherence range of the 4 day oscillations has been extended to the extreme ends of the ridge and the trapped waves appear to propagate northward along the entire length of the ridge at about 1 m s spectral amplitudes are about double on the south end compared to the north end and a recent model for bottom trapped subinertial waves over topography shows this may be due to variations in ridge width allen and thomson 1993 new observations show coherence between temperatures and currents near the south cleft vent field and suggest 4 day oscillations can play a major role in the local advection of hydrothermal plumes these observations combined with earlier results demonstrate that 4 day oscillations are a significant ridge trapped feature and they suggest that other ridges may be important in transmitting energy over much longer distances

imaging tertiary carbonate system the maldives indian ocean insights into carbonate sequence interpretation seismic imaging of carbonate systems is often more complicated than their siliciclastic counterparts carbonates sediments typically get cemented very early and their seismic velocities are normally faster than those of siliciclastic sediments this reduces the resolution of carbonates in comparison to the clastics when using identical acquisition design carbonates are also easily dissoved which may lead to karsting a phenomenon unique to carbonate rocks karst surfaces commonly form highly irregular topography that causes energy dispersion and makes it difficult to image the sediments or rocks below it

evolution of a carbonate delta generated by gateway funnelling of episodic currents cool water carbonate sedimentation has dominated mediterranean shelves since the early pliocene skeletal sand and gravel herein consist of remains of heterozoan organisms which are susceptible to reworking due to weak early cementation in non tropical waters this study documents the lower pleistocene carbonate wedge of favignana island italy which prograded from a 5 km wide passage between two palaeo islands into a perpendicular 10 to 15 km wide strait between the palaeo islands at one side and sicily at the other during the emilian highstand 1 6 ma to 1 1 ma the clinoformed carbonate wedge which is 50 m thick and 6 km long formed by east south east progradation of a platform on the submarine sill by currents that were funnelled between the two palaeo islands platform slope clinoforms evolved from initial aggradation thin and low angle into a progradation phase thick and high angle both clinoform types are characterized by a bimodal facies stacking pattern defined by sedimentary structures created by i subaqueous dunes associated with dilute subcritical currents and ii upper flow regime bedforms associated with sediment laden supercritical turbidity currents focusing of episodic currents on the platform by funnelling between the islands controlled the downstream formation of a sediment body here named carbonate delta the carbonate delta interfingers with subaqueous dune deposits formed in the perpendicular strait this study uses a reconstruction of bedform dynamics to unravel the evolution of this gateway related carbonate accumulation

geographical distribution of diurnal and semidiurnal parametric subharmonic instability in a global ocean circulation model abstractthe evidence for baroclinic energetics of and geographic distribution of parametric subharmonic instability psi arising from both diurnal and semidiurnal tides in a global ocean general

observations of internal waves and parametric subharmonic instability in the philippines archipelago 1 internal waves contain a significant fraction of the kinetic energy in the ocean and are important intermediaries between the forcing by wind and tide and interior diapycnal mixing we report here on measurements from mindoro strait in the philippines connecting the south china sea to the sulu sea of an internal wave field with a number of surprising properties that point to previously unrecognized processes at work in the region continuum spectral levels are very close to typical background values found in the open ocean but internal tide energy in both the diurnal and semidiurnal frequency bands is significantly elevated and higher at the northern mooring mp1 than the southern mp2 two particularly energetic depth ranges stand out at mp1 an upper layer centered near 300 m and one at the bottom of the water column near 1800 m the upper layer contains both internal tides and a near inertial band with upward and downward propagating waves and an apparent spring neap cycle the combination is suggestive of parametric subharmonic instability as the forcing for the near inertial band a conclusion supported by bicoherence estimates mixing estimated from density overturns is weak over much of the water column but enhanced by about an order of magnitude in the deep layer and closely tied to the internal tide both diurnal and semidiurnal near inertial currents in this deep layer are dominantly rectilinear and not well correlated with the mixing bulk mixing rates at the two sites are less than required to produce property changes seen in hydrography suggesting additional enhancement elsewhere in the archipelago

intermediate depth ice coring of high altitude and polar glaciers with a lightweight drilling system a total of 11 ice cores to a maximum depth of 460 m have been obtained over the past 3 years from high altitude glaciers on the saddle of mount bona and mount churchill in alaska designated b c and on quelccaya ice cap and nevado coropuna in peru ice coring was conducted using an intermediate depth drilling system the system includes an electromechanical drill emd and an ethanol thermal electric drill eted the emd permitted an average ice core production rate icpr of 7 0 m h 1 down to 150 m an average icpr of 2 m h 1 to 460 m depth was possible with the eted the quality of the b c ice cores is better than that of cores previously drilled with an emd and eted system a new cutter design drilling with a lubricant cutting fluid and a new anti torque assembly were tested in the laboratory and in glacier boreholes we examine the performance of the drills in cold and temperate ice and in clean and particle laden ice the influence of the ethanol drilling fluid on ice core isotopic ionic and dust composition is discussed

paleogene neogene depositional history of the middle u s atlantic continental rise mixed turbidite and contourite depositional systems abstract the construction of the middle u s atlantic continental rise from late paleogene to late pliocene time included downslope sediment gravity flows and alongslope contourite deposition some previous studies have suggested that contour following bottom currents were the dominant control on depositional patterns depositional relief from two large contourite drifts largely built in late miocene to late pliocene time continue to dominate present day lower rise morphology detailed seismic sequence mapping presented here suggests that significant turbidite fan systems also were active and are preserved in the stratigraphic section a single thick depositional sequence is mapped for oligocene to middle miocene sediments deposited when inferred fan systems off the norfolk washington canyons were reworked by bottom currents along the lower rise to form the proto hatteras outer ridge detailed mapping of three late miocene to late pliocene depositional sequences indicates that fan channel systems were active at the same time large contourite drifts were being constructed sediment delivered to the lower rise by these elongate turbidite systems undoubtedly was an important local source for construction of the hatteras outer ridge and through entrainment and resuspension was an important source for regional contourite deposition controlled by the deep western boundary undercurrent we suggest that submarine fans and current controlled sediment drifts may develop simultaneously as companion systems types of fan drift interaction include current only modification of fans transitional fan to drift and adjacent or overlapping submarine fan and sediment drift deposition

the eclipse drill a field portable intermediate depth ice coring drill a derivative of the danish tipping tower intermediate depth ice coring drill system has been developed for use in remote areas the system as described is capable of drilling to depths of 250 m although the drill has been proven to 350 m the drill system is designed to be transported by hand and consumes 700 w maximum power from a 24 v d c battery bank charged by solar panels or a small generator a field test in the st elias mountains yukon produced good quality core to 161 6 m depth drilling at 6500 m a s l on far east rongbuk glacier mount everest produced good core to 42 m depth

global abyssal mixing inferred from lowered adcp shear and ctd strain profiles abstract internal wave wave interaction theories and observations support a parameterization for the turbulent dissipation rate e and eddy diffusivity k that depends on internal wave shear vz2 and strain z2 variances its latest incarnation is applied to about 3500 lowered adcp ctd profiles from the indian pacific north atlantic and southern oceans inferred diffusivities k are functions of latitude and depth ranging from 0 03 10 4 m2 s 1 within 2 of the equator to 0 4 0 5 10 4 m2 s 1 at 50 70 diffusivities k also increase with depth in tropical and subtropical waters diffusivities below 4500 m depth exhibit a peak of 0 7 10 4 m2 s 1 between 20 and 30 latitudes where semidiurnal parametric subharmonic instability is expected to be active turbulence is highly heterogeneous though the bulk of the vertically integrated dissipation e is contributed from the main pycnocline hotspots in e show some correlation with small scale bottom roughness and near bottom flow at sites where st

three dimensional seismic structure of the dragon flag oceanic core complex at the ultraslow spreading southwest indian ridge 49 39 e the southwest indian ridge swir is an ultraslow spreading end member of mid ocean ridge system we use air gun shooting data recorded by ocean bottom seismometers obs and multibeam bathymetry to obtain a detailed three dimensional 3 d p wave tomographic model centered at 49 degrees 39 e near the active hydrothermal dragon flag vent results are presented in the form of a 3 d seismic traveltime inversion over the center and both ends of a ridge segment we show that the crustal thickness defined as the depth to the 7 km s isovelocity contour decreases systematically from the center similar to 7 0 8 0 km toward the segment ends similar to 3 0 4 0 km this variation is dominantly controlled by thickness changes in the lower crustal layer we interpret this variation as due to focusing of the magmatic activity at the segment center the across axis velocity model documents a strong asymmetrical structure involving oceanic detachment faulting a locally corrugated oceanic core complex dragon flag occ on the southern ridge flank is characterized by high shallow crustal velocities and a strong vertical velocity gradient we infer that this occ may be predominantly made of gabbros we suggest that detachment faulting is a prominent process of slow spreading oceanic crust accretion even in magmatically robust ridge sections hydrothermal activity at the dragon flag vents is located next to the detachment fault termination we infer that the detachment fault system provides a pathway for hydrothermal convection

southwest mallorca island a cool water carbonate margin dominated by drift deposition associated with giant mass wasting abstract by means of new high resolution parasound multi channel reflection seismic and multibeam swath bathymetry data we provide new insight into the sedimentation processes at the margin of a cool water carbonate ramp system off southwestern mallorca island western mediterranean sea here we mapped 7 depositional units which we tied to the messinian unconformity the oldest units 1 3 are deposited from late messinian to late pliocene during phases of extreme sea level fluctuations 2 4 ma ago with the global climate cooling when the estuarine water mass circulation switched into an anti estuarine pattern in the mediterranean sea and sea level highstands reached the today level bottom currents started to shape the distal deposits of the cool water carbonate system off southwestern mallorca represented by depositional units 4 7 although sediment supply from the carbonate shelf to the slope is low due to the insignificant input of terrigenous clastics and slow reef growth slope sediments reach thicknesses 140 m our study displays that the accumulation of sediments at the southwestern insular margin was mainly controlled by long lived bottom currents and giant submarine landslides the bottom currents formed elongated mounded drifts that contemporaneously developed in alongslope and upslope direction they are probably controlled by an isobath parallel bottom current in a present water depth of 350 550 m which we associate with the levantine intermediate water liw and an upslope current acting in 250 600 m associated with the algerian mesoscale gyres that enter the balearic straits from se several scarps at the present sea floor indicate the predominance of slope failures in the study area additionally mass transport deposits occur basinwards in almost all depositional units accordingly giant mass wasting events took place from the pliocene into recent times locally above the alongslope drift a field of sediment waves occurs in a water depth of 170 310 m they are oriented slightly oblique to the contour lines with their steep flank facing upslope showing wavelengths of 400 800 m and heights of 10 15 m we suggest that the waves migrate in upslope direction perpendicular to the main isobath parallel flow furthermore the results of this study indicate that sea level fluctuations have only a minor influence on drift deposition instead submarine landslides caused the creation of new drift units they significantly altered the relief of the sea floor and thereby initiate the realignment of the prevailing current pattern the fault scarps and detachment surfaces left behind were the nucleolus for the development of new drift units

a cretaceous carbonate delta drift in the montagna della maiella italy the upper cretaceous campanian maastrichtian bioclastic wedge of the orfento formation in the montagna della maiella italy is compared to newly discovered contourite drifts in the maldives like the drift deposits in the maldives the orfento formation fills a channel and builds a miocene delta shaped and mounded sedimentary body in the basin that is similar in size to the approximately 350 km2 large coarse grained bioclastic miocene delta drifts in the maldives the composition of the bioclastic wedge of the orfento formation is also exclusively bioclastic debris sourced from the shallow water areas and reworked clasts of the orfento formation itself in the near mud free succession age diagnostic fossils are sparse the deposi tional textures vary from wackestone to float rudstone and breccia conglom erates but rocks with grainstone and rudstone textures are the most common facies in the channel lensoid convex upward breccias cross cut ting channelized beds and thick grainstone lobes with abundant scours indi cate alternating erosion and deposition from a high energy current in the basin the mounded sedimentary body contains lobes with a divergent progradational geometry the lobes are built by decametre thick composite megabeds consisting of sigmoidal clinoforms that typically have a channel ized topset a grainy foreset and a fine grained bottomset with abundant irregular angular clasts up to 30 m thick channels filled with intraforma tional breccias and coarse grainstones pinch out downslope between the megabeds in the distal portion of the wedge stacked grainstone beds with foresets and reworked intraclasts document continuous sediment reworking and migration the bioclastic wedge of the orfento formation has been vari ously interpreted as a succession of sea level controlled slope deposits a shoaling shoreface complex or a carbonate tidal delta current controlled delta drifts in the maldives however offer a new interpretation because of their similarity in architecture and composition these similarities include i a feeder channel opening into the basin ii an excavation moat at the exit of the channel iii an overall mounded geometry with an apex that is in shallower water depth than the source channel iv progradation of stacked lobes v channels that pinch out in a basinward direction and vi smaller channelized intervals that are arranged in a radial pattern as a result the upper cretaceous campanian maastrichtian bioclastic wedge of the orfento formation in the montagna della maiella italy is here interpreted as a carbonate delta drift

near inertial motions over a mid ocean ridge effects of topography and hydrothermal plumes we investigate the spatial structure of near inertial motions in the vicinity of the endeavour segment of juan de fuca ridge approximately 48 n 129 w in the northeast pacific ocean on the basis of time series current and water property data collected from september 1984 to september 1987 near inertial motions are ubiquitous features of the 2200 m water column with root mean square rms current speeds comparable to those of the dominant m2 tidal currents within the lower 1000 m of the water column where most of the observations were obtained near inertial oscillations have rms current speeds of o 1 cm s and vertical isotherm displacements of o 10 m the fluctuations are confined to the frequency band 0 966 1 079 f f is the local coriolis parameter and have characteristic event durations of 1 week although the spectra of subsurface motions are dominated by the blue shifted superinertial band significant spectral peaks are found also in the subinertial and inertial frequency bands marked alteration of the near inertial current amplitudes occurs over two well defined depth zones within the study region within the 200 m zone immediately above the 2100 m ridge crest current amplitudes are amplified by a factor of 1 2 1 7 because of bottom reflection and or scattering of the downward propagating energy evidence that the amplification may be linked to bottom reflection rather than to scattering is provided by flattening and cross slope rotation of the near inertial current ellipses with increased proximity to the top of the ridge reflection would occur at grazing angles of less than 1 and would be associated with surface generated waves originating at distances of over 100 km from the observational site in contrast to the enhanced amplitudes immediately above the top of the ridge near inertial currents within the 1600 to 1800 m depth range undergo pronounced attenuation and frequency alteration amplitude attenuation is especially pronounced for motions in the superinertial band and may arise through critical layer absorption of downward propagating waves as they encounter increased vertical shear in the background flow the increased shear is most likely associated with buoyancy induced flow formed by the extensive hydrothermal plume emanating from vent sites in the axial valley along the ridge crest but it could also be related to bottom trapped oscillations over the steep ridge topography near inertial motions are estimated to have vertical coherence scales of the order of 10 100 m while horizontal coherence scales exceed the 50 km separation between the mooring locations minimum vertical and horizontal coherences are found for the depth zone 1600 1800 m while maximum correlation occurs for near bottom motions immediately above the crest of the ridge weak near inertial motions are observed within the 100 m deep axial valley

the maldives a giant isolated carbonate platform dominated by bottom currents abstract the maldives a 900 km north south trending paired chain of atolls that surround a ca 100 km wide central basin the inner sea represent a giant isolated carbonate platform in the indian ocean new reflection seismic multibeam and acoustic doppler current profiler data show that its development exhibits a distinct change from sea level to mainly bottom current controlled sedimentation at the end of the middle miocene the post middle miocene intra basinal fill consists of 9 mega units that have been formed and shaped by strong bottom currents entering the inner sea between the inter atoll passages at the end of the middle miocene the maldives represent a rimmed platform with the inner sea forming an empty bucket a connection to the indian ocean existed only in the northeastern part of the platform namely the kardiva channel probably concomitant with the onset of the indian monsoon the current pattern around the maldives changed significantly resulting in the formation of channels in the rimmed platform margin the latter process was encouraged by local drowning of large platform parts and additionally by bottom current forced submarine erosion that widened the channels to passages disintegrating the platform into separated atolls of different sizes the reworked material was dumped into the inner sea where it formed large contourite fans along slope bottom currents entering the inner sea from the north further redistributed the material leading to a southward shift of these sedimentary bodies in this way the inner sea was filled from west to east mega units 1 5 starting with mega unit 6 the opening of a southern gateway introduced a prominent northward flow of bottom waters in the inner sea it leads to the deposition of giant elongated drifts at the eastern flank of the basin filling it from east to west because the current swept away most of the material around the atolls the system was not able to prograde and the steady subsidence was compensated by aggradation accordingly a fundamental characteristic of a current controlled carbonate system is that aggradation occurs irrespective of platform sediment export or differently worded the rate of change in accommodation is always positive and greater than the rate of sediment supply adcp measurements in the winter monsoon season show two water masses in the inner sea one at the surface 0 150 m and a second below it that reaches its sea floor by the use of the grain size distribution of odp leg 716 located in the inner sea as proxy for the flow intensity of the bottom water we recognized an inverse proportional relationship to the monsoon strength from late upper miocene to present some of the current controlled deposits form sigmoidal prograding clinoforms preferably at the edges of the atolls adjacent to the passages where the velocity of the current entering the inner sea is high the anatomy of these prograding wedges is similar to sea level controlled clinoforms described for large isolated carbonate platforms in the sequence stratigraphic concept however our results document that they are not influenced by sea level fluctuations because of the high current velocities involved in their deposition they predominately consist of coarse sand sized carbonate fragments exhibiting large pore space and high permeability therefore these drift deposits comprise a possible exploration target

development of sediment drifts approaching an active plate margin under the sw pacific deep western boundary current the abyssal pacific ocean is fed by a 1000 km wide deep western boundary current dwbc that flows northward along the continental margin east of new zealand between the passive margin of chatham rise and the subduction zone of kermadec trench a distance of 1200 km the dwbc has formed a suite of sediment drifts over a depth range of 2200 5700 m airgun and 3 5 khz profiles record a variety of drift types that reflect regional variations in bathymetry sediment supply and the tectonic volcanic framework on chatham rise the dwbc has deposited a sinuous linear body along the south flank 3000 m an extensive apronlike drift on the north flank 2200 4500 m and a ridgelike drift about the rise base 4500 5200 m the flow has also deposited a body of sediment over 400 km long within a moat at the base of the nearby louisville seamount chain further downcurrent the 250 km long rekohu drift 3600 4190 m has developed northward to 39 s south of this latitude drifts comprise mainly reworked pelagic hemipelagic material and sediment transported from distant southerly sources in contrast drifts north of 39 s have received a major injection of terrigenous sediment from hikurangi channel which runs 1400 km from new zealand eastward across the hikurangi plateau to disgorge on to the abyssal floor at the plateau edge en route turbidity current overspill from the channel has moved north under the influence of the shallow dwbc to contribute to a series of small ridge and patch drifts among the numerous seamounts on the plateau at 3500 4200 m off hikurangi channel mouth a large fan has accumulated the dwbc has extended the fan into a drift running over 250 km along the base of hikurangi plateau 5150 5770 m toward kermadec trench here drift sediment becomes increasingly disrupted by mass wasting associated with the active subduction in this area the seismic stratigraphy reveals the drifts to rest mainly on a widespread erosional surface that is interpreted to mark the inception of the dwbc in the region with the late oligocene opening of the australian antarctic seaway drift construction commenced during the miocene but was punctuated in the late miocene by another period of erosion that coincided with increased bottom water production in antarctica deposition resumed in plio pleistocene times when large quantities of sediment from the rapidly rising landmass of new zealand were injected into the boundary current the modern flow continues to affect drift deposition as manifest by an active boundary channel along the foot of hikurangi plateau and widespread scour zones and sediment wave fields

butyl acetate an alternative drilling fluid for deep ice coring projects deep drilling operations in glaciers require a fluid to maintain hydrostatic equilibrium and prevent closure due to plastic flow of the ice many past practices have employed various fluid mixtures using fuel oil as the base the case for butyl acetate is presented here as an adequately dense and environmentally safe drilling fluid results from the 1990 drill season are highly favorable

circulation near axial seamount year long moored current measurements from 1987 to 1988 on the saddle 1860 m between axial volcano and brown bear seamount show an oscillating flow with a broadband frequency centered at about 4 days and a long term average flow of about 4 cm s southward the measurements at sill depth also suggest a possible alternating exchange of water north and south of the cobb eickelberg seamount chain with warmer water 0 05 c coming from the north however measurements near the top of the volcano at about 1400 m show a weaker but northward mean flow and the 4 day oscillation barely exists conductivity temperature and depth ctd observations on the east side of the volcano infer possible intensification of southward flow adjacent to the juan de fuca ridge similar to that found with combined ctd and current meter observations about 100 km to the south southward flow is hypothesized along the entire east side of the ridge on the west side the axial brown bear saddle may be near a convergence zone between southward and northward currents which then flow westward the highly energetic environment on the saddle suggests other gaps in the ridge and seamount chains may be important in determining the fate of the hydrothermal effluents the general oceanic flow at the top of the caldera however is more tenuous and the depth of change between these two regimes cannot be determined

surface sediment composition and distribution of hydrothermal derived elements at the duanqiao 1 hydrothermal field southwest indian ridge abstract the study of the hydrothermal component in oceanic sediments provides key data for locating active and inactive hydrothermal systems in this work we report the geochemistry of surface sediments collected in the newly discovered basaltic hosted duanqiao 1 hydrothermal field dhf on the southwest indian ridge swir the sediments are mainly composed of a pelagic fraction carbonate ooze with minor clay contents basaltic debris fe mn oxyhydroxides and hydrothermal components hydrothermal elements in the dhf exhibit a precipitation sequence of cu co zn pb fe mn ni as proximal sediments to the hydrothermal discharge area are characterized by high cu pb zn and co concentrations and higher cu fe and zn fe ratios decreasing abruptly within the first kilometer after this proximal area an evident increase of mn concentrations and mn fe ratios was observed denoting higher oxidizing conditions of the hydrothermal plume hydrothermal derived pb and co precipitated together with the sulfide phases whereas at the distal sediments pb co ni and as seem to be scavenged from seawater by oxyhydroxides this was corroborated by increased ree fe ratios with distance to the discharge zone the geochemistry of dhf sediments is consistent with an effective separation of metals during gradual cooling and concomitant eh and ph modifications of the hydrothermal plume when in contact with seawater this promotes the precipitation of cu sulfides first closest to the discharge area followed by cu zn and fe sulfides and fe mn oxyhydroxides in more distal areas a comparison of the dhf sediments with those from dragon horn another hydrothermal site at the swir suggests that the hydrothermal fluids responsible for mineral precipitation at dhf are less reduced favoring the formation of sulfides and oxyhydroxides at shorter distances from the discharge zone these findings are particularly valuable to the understanding of the variation in hydrothermal anomalies detected in sediments from proximal and distal areas of discharge zones contributing to the development of tools for the discovery of hydrothermal fields

subtropical catastrophe significant loss of low mode tidal energy at 28 9 1 an idealized numerical study of a northward propagating internal tide reveals a dramatic loss of energy to small scale subharmonic instabilities near 28 9 n inspired by observations of the internal tide radiating northward from the hawaiian ridge a three dimensional numerical model is initialized with a northward baroclinic tidal flux of approximately 1 7 kw m after an initial spinup period energy is quickly transferred from the baroclinic tide to subharmonic motions with half the horizontal tidal wavenumber and small vertical scales through nonlinear advection of horizontal tidal velocity gradients potential oceanic implications are twofold first once a steady state has been reached the instability acts as a partial filter of northward tidal flux between 27 5 and 29 5 n in rough agreement with some altimetric tidal observations second elevated shear of the subharmonic motions suggests the potential for elevated near surface dissipation rates near the critical latitude that may be important for upper ocean mixing

surface sediment geochemistry and hydrothermal activity indicators in the dragon horn area on the southwest indian ridge abstract the fluids found in sulfide producing hydrothermal vents are rich in ore forming elements these elements usually precipitate as solid particles that are dispersed by the plumes and deposited as sediments around the hydrothermal field to assess how the geochemical features of such sediments may be used in seafloor sulfide exploration this study analyzed the surface sediment geochemistry in the dragon horn area on the southwest indian ridge the results indicate that the sediments are mainly composed of pelagic material basalt and ultramafic debris mn fe oxyhydroxides and mn oxides hydrothermal components the debris content in this area is lower than that in sediments found at fast and medium spreading mid ocean ridges with relatively high amounts of ultramafic components indicating that ultramafic rock outcrops are present in the study area precipitated hydrothermal elements in the sediments exhibit zonation in the spatial distributions of zn cu fe and mn the sediments proximal to the hydrothermal field display high cu zn and fe contents and low mn contents hydrothermal cu and zn phases occurred within a limited range of the hydrothermal field and zn was found to precipitate earlier than cu during dispersal from the plume hydrothermal fe distributed similarly with cu spatially but with a larger dispersion distance hydrothermal mn was able to migrate beyond the ridge valley to precipitate in the ridge flank sediments two hydrothermal cu and zn anomalies were identified on the south ridge flanks which suggest the presence of undiscovered hydrothermal field the sediments in the study area are additionally characterized by relatively low mn contents and high fe mn molar ratios the relatively reducing environment of the plume may facilitate the migration of hydrothermal mn over large distances which probably accounts for the lower mn contents in the sediments the cu cu fe molar ratio model was proposed to identify mafic and ultramafic related hydrothermal field the low cu fe molar ratios and ree characteristics of the sediments suggest that ultramafic rocks are likely not the only source of the ore forming element for hydrothermal mineralization low temperature diffuse flow on the ridge flanks of the study area probably results from serpentinization of ultramafic rocks

first active hydrothermal vents on an ultraslow spreading center southwest indian ridge the ultraslow spreading southwest indian ridge is a major tectonic province representing one of the important end member mid ocean ridge types for its very slow and oblique spreading and providing the only known route for migration of chemosynthetic deep sea vent fauna between the atlantic and indian oceans we report the investigation of the first active high temperature hydrothermal field found on any ultraslow mid ocean ridge worldwide located on southwest indian ridge at 37 47 s 49 39 e it consists of three zones extending 1000 m laterally and it is one of four recently discovered active and inactive vent sites within a 250 km long magmatically robust section our results provide the first direct evidence for potentially widespread distribution of hydrothermal activity along ultraslow spreading ridges at least along magmatically robust segments this implies that the segment sections with excess heat from enhanced magmatism and suitable crustal permeability along slow and ultraslow ridges might be the most promising areas for searching for hydrothermal activities it is surprising that the special vent fauna appear to indicate some complex affinity to those on the central indian ridge southern mid atlantic ridge and the southwest pacific ocean

cyclic steps review and aggradation based classification abstract cyclic steps are a type of upper flow regime bedform consisting of trains of upstream and upslope migrating bed undulations the overriding flow is characterised by a series of hydraulic jumps occurring in the troughs of the undulations cyclic steps form in open channel flows such as rivers and comprise a common bedform in subaqueous density flows in oceans lakes and reservoirs cyclic steps are associated with alternating froude subcritical and froude supercritical flow on respectively the stoss side and lee side of individual bedforms the transition between these flow states is embodied by the hydraulic jump in the trough of the bedform leading to the permanent or quasi permanent morphology of cyclic steps over the past decade numerous studies affirmed the dominant role of cyclic steps in generating bed undulations in modern and ancient glacial outwash fluvial delta and turbidite environments as reviewed here cyclic steps were previously discriminated as net depositional climbing transportational and net erosional falling in different parts of sedimentary systems the following cyclic step descriptors with distinct depositional signatures are proposed fully depositional partially depositional transportational partially erosional and fully erosional partially depositional cyclic steps are most common they are associated with backset bedded sets high aggradation rate and nested scours filled with massive to backset bedded deposits low aggradation rate the new classification can be used as a predictive tool in the reconstruction of modern and ancient sedimentary successions using repeat bathymetry seismic reflection or outcrop data it is applied in three turbidite case studies classical deep sea system small scale delta slope and line sourced carbonate slope

magnetic properties of early pliocene sediments from iodp site u1467 maldives platform reveal changes in the monsoon system we report a study of the magnetic stratigraphy and the anisotropy of isothermal remanent magnetization of pliocene sediments from international ocean discovery program iodp site u1467 drilled in the maldives platform indian ocean during exp 359 magnetic stratigraphy gives a precise record of geomagnetic reversals of the early pliocene from approximately 5 3 ma to 3 1 ma providing a detailed age model in an interval where the biostratigraphic record is scarce we use the anisotropy of isothermal remanent magnetization airm to investigate the statistical orientation of fine magnetic particles and provide data on the strength and direction of bottom currents during the early pliocene the strength of bottom currents recorded by the airm shows a prominent increase at the top of chron c3n 1n about 4 2 ma and the current direction ne sw is consistent with that of modern instrumental measurements since bottom currents in the maldives are driven by the monsoon we speculate that the 4 2 ma increase of bottom currents could mark the onset of the present day setting probably related to the coeval uplift phase of the himalayan plateau

accumulation map for the greenland ice sheet 1971 1990 an updated accumulation map for the greenland ice sheet for the period 1971 1990 based on kriging of accurate point estimates developed during the past decade at over 40 points plus over 200 historical points shows an average water accumulation of 30 g cm 2yr 1 average uncertainty standard deviation in estimates of point accumulation for the interior part of the ice sheet where the kriging drift surface is well constrained is approximately 7 g cm 2yr 1 or 24 of mean accumulation because there are multiple cores in most regions the regional uncertainty in accumulation should be considerably lower than the 7 g cm 2yr 1 average uncertainty at a point there are still many areas on the ice sheet including northwest and southern greenland where annual accumulation remains poorly constrained

the leaking bucket of a maldives atoll implications for the understanding of carbonate platform drowning abstract seismic and multibeam data as well as sediment samples were acquired in the south male atoll in the maldives archipelago in 2011 to unravel the stratigraphy and facies of the lagoonal deposits multichannel seismic lines show that the sedimentary succession locally reaches a maximum thickness of 15 20 m above an unconformity interpreted as the emersion surface which developed during the last glacial sea level lowstand such depocenters are located in current protected areas flanking the reef rim of the atoll or in infillings of karst dolinas much of the 50 m deep sea floor in the lagoon interior is current swept and has no or very minor sediment cover erosive current moats line drowned patch reefs whereas other areas are characterized by nondeposition karst sink holes blue holes and karst valleys occur throughout the lagoon from its rim to its center lagoonal sediments are mostly carbonate rubble and coarse grained carbonate sands with frequent large benthic foraminifers halimeda flakes red algal nodules mollusks bioclasts and intraclasts some of them glauconitic as well as very minor ooids finer grained deposits locally are deposited in current protected areas behind elongated faros i e small atolls which are part of the rim of south male atoll the south male atoll is a current flushed atoll where water and sediment export with the open sea is facilitated by the multiple passes dissecting the atoll rim with an elevated reef rim and tower like reefs in the atoll interior it is an example of a leaky bucket atoll which shares characteristics of incipiently drowned carbonate banks or drowning sequences as known from the geological record

on the nature of internal wave spectra near a continental slope 1 amongst the most energetic motions in the deep ocean are internal waves supported by stable vertical stratification in density previously these waves were considered freely propagating as described by a smooth continuum internal wave band iwb frequency spectrum we studied details of the iwb using yearlong current observations from the bay of biscay instead of observing continuum iwb spectra near a continental slope we show that at least the first half decade of the iwb is dominated by motions at localized frequencies determined by strong non linear interactions between waves at the fundamental semidiurnal tidal and atmospherically induced inertial frequencies

long range propagation and associated variability of internal tides in the south china sea the variability of internal tides during their generation and long range propagation in the south china sea scs is investigated by driving a high resolution numerical model the present study clarifies the notably different processes of generation propagation and dissipation between diurnal and semidiurnal internal tides internal tides in the scs originate from multiple source sites among which the luzon strait is dominant and contributes approximately 90 and 74 of the baroclinic energy for m2 and k1 respectively to the west of the luzon strait local generation of k1 internal tides inside the scs is more energetic than the m2 tides diurnal and semidiurnal internal tides from the luzon strait radiate into the scs in a north south asymmetry but with different patterns because of the complex two ridge system the tidal beams can travel across the deep basin and finally arrive at the vietnam coast and nansha island more than 1000 1500 km away during propagation m2 internal tides maintain a southwestward direction whereas k1 exhibit complicated wave fields because of the superposition of waves from local sources and island scattering effects after significant dissipation within the luzon strait the remaining energy travels into the scs and reduces by more than 90 over a distance of 1000 km inside the scs the k1 internal tides with long crests and flat beam angles are more influenced by seafloor topographical features and thus undergo apparent dissipation along the entire path whereas the prominent dissipation of m2 internal tides only occurs after their arrival at zhongsha island this article is protected by copyright all rights reserved

late cretaceous neogene trends in deep ocean temperature and continental ice volume reconciling records of benthic foraminiferal geochemistry δ18o and mg ca with sea level history 1 we reconstruct trends in ice volume and deep ocean temperature for the past 108 myr resolving variations on timescales of 2 myr and longer we use a sea level record as a proxy for ice volume a benthic foraminiferal mg cabf record as a proxy for temperature and a benthic foraminiferal 18obf record as a proxy for both this allows us to construct dual estimates of temperature and ice volume variations for the interval 10 60 ma extracting temperature from 18obf by using sea level as a proxy for ice volume to constrain the 18osw component and extracting seawater 18osw which reflects ice volume from 18obf by using mg cabf to constrain the temperature component each of these approaches requires numerous assumptions but the range of plausible solutions are concordant on timescales 2 myr and within an uncertainty of 2 c temperature and 0 4 18osw the agreement between the two approaches for the last 50 myr provides empirical justification for the use of 18obf mg cabf and sea level records as robust climate proxies our reconstructions indicate differences between deep ocean cooling and continental ice growth in the late cenozoic cooling occurred gradually in the middle late eocene and late miocene pliocene while ice growth occurred rapidly in the earliest oligocene middle miocene and plio pleistocene these differences are consistent with climate models that imply that temperatures set by the long term co2 equilibrium should change only gradually on timescales 2 myr but growth of continental ice sheets may be rapid in response to climate thresholds due to feedbacks that are not yet fully understood

the record of oligocene middle miocene paleoenvironmental changes in a carbonate platform iodp exp 359 maldives indian ocean abstract the carbonate edifice of the maldives has an exceptional record of the global climate and ocean current reconfigurations during the neogene that set the path for the establishment of the present day global climate system the study of seismic profiles sedimentary facies microfacies and ichnofabrics of the oligocene miocene platform slopes allows reconstructing the different stages of basin evolution the focus is on the characterization of paleoenvironmental conditions along the slope at iodp expedition 359 drill sites as well as subtle changes of the slope depositional regime controlled by variations in the frequency and intensity of the platform shedding and the basin oxygenation natural gamma radiation and ichnofabric logs are used as a proxy for sea floor ventilation in the inner sea area which is today surrounded by atolls from the oligocene 25 5 ma to the end of the middle miocene climate optimum 15 ma the maldives carbonate platform slopes were poorly oxygenated the opening of the eastern passages through the maldives carbonate platform at 17 5 ma led to an improvement of the sea floor oxygenation sea level fluctuations controlled the sediment input from the platform onto the slope that resulted in changes of the fossil and trace fossil assemblages after the evolution from a ramp to a flat topped platform with well defined bank edges the platform developed hanging shoulder and more sigmoidal clinoforms this correlates to the global decline of coral reefs during the middle miocene climate transition 15 ma and a general sea level lowering the associated slope deposits show abundant channels and cyclic steps are rich in reworked bioclasts and have ichnofabrics pointing towards a good oxygenation of the sea floor the example of the maldives carbonate platform therefore shows how integrating seismic geometries and compositional changes i e microfacies and ichnofabrics allow identifying changes in sea level and in the current pattern as the main controlling factors on carbonate platform evolution

on the interaction between internal tides and wind induced near inertial currents at the shelf edge 1 numerical calculations are performed at the shelf edge to examine the role of nonlinear processes in transferring energy from the m2 tide and wind induced currents close to the inertial frequency f into a wave at the sum of their frequencies termed the fm2 frequency a numerical model of the hebrides shelf edge represented by a cross section initially with idealized topography and subsequently with realistic topography is used in these calculations results show that in the near coastal ocean currents at the fm2 frequency are primarily due to coupling between wind induced inertial oscillations and the m2 internal tide a major source is associated with vertical shear in the inertial oscillations and the vertical velocity due to the internal tide a secondary source is due to the nonlinear momentum advection term in the case in which eddy viscosity is computed from a turbulence energy model shear across the thermocline is larger than when a constant viscosity is used the reduction in shear with a constant viscosity reduces the role of the nonlinear term involving vertical shear and hence the magnitude of the fm2 current in the region of the thermocline increasing the wind stress leads to a deeper thermocline and hence the location of maximum fm2 current in the water column changing the vertical stratification influences the intensity of the inertial oscillations in the surface layer and the distribution of the internal tide and hence changes the pattern of fm2 currents results with realistic topography confirm the major conclusions

little ice age neoglacial paleoenvironmental conditions at siple station antarctica the 550 year records of 18 o and dust concentrations from siple station antarctica suggest warmer and less dusty atmospheric conditions from 1600 to 1830 a d which encompasses much of the northern hemisphere little ice age lia dust and 18 o data from south pole station indicate that the opposite conditions e g cooler and more dusty were prevalent there during the lia meteorological data from 1945 85 show that the lia temperature opposition between amundsen scott and siple inferred from 18 o is consistent with the present spatial distribution of surface temperature there is some observational evidence suggesting that under present conditions stronger zonal westerlies produce a temperature pattern similar to that of the lia these regional differences demonstrate that a suite of spatially distributed high resolution ice core records will be necessary to characterize the lia in antarctica

froude supercritical flow processes and sedimentary structures new insights from experiments with a wide range of grain sizes recognition of froude supercritical flow deposits in environments that range from rivers to the ocean floor has triggered a surge of interest in their flow processes bedforms and sedimentary structures interpreting these supercritical flow deposits is especially important because they often represent the most powerful flows in the geological record insights from experiments are key to reconstruct palaeo flow processes from the sedimentary record so far all experimentally produced supercritical flow deposits are of a narrow grain size range fine to medium sand while deposits in the rock record often consist of a much wider grain size distribution this paper presents results of supercritical flow experiments with a grain size distribution from clay to gravel these experiments show that cyclic step instabilities can produce more complex and a larger variety of sedimentary structures than the previously suggested backsets and scour and fill structures the sedimentary structures are composed of irregular lenses mounds and wedges with backsets and foresets as well as undulating planar to low angle upstream and downstream dipping laminae the experiments also demonstrate that the froude number is not the only control on the sedimentary structures formed by supercritical flow processes additional controls include the size and migration rate of the hydraulic jump and the substrate cohesion this study further demonstrates that froude supercritical flow promotes suspension transport of all grain sizes including gravels surprisingly it was observed that all grain sizes were rapidly deposited just downstream of hydraulic jumps including silt and clay these results expand the range of dynamic mud deposition into supercritical flow conditions where local transient shear stress reduction rather than overall flow waning conditions allow for deposition of fines comparison of the experimental deposits with outcrop datasets composed of conglomerates to mudstones shows significant similarities and highlights the role of hydraulic jumps rather than overall flow condition changes in producing lithologically and geometrically complex stratigraphy

current and sea level signals in periplatform ooze neogene maldives indian ocean abstract periplatform ooze is an admixture of pelagic carbonate and sediment derived from neritic carbonate platforms compositional variations of periplatform ooze allow the reconstruction of past sea level changes periplatform ooze formed during sea level highstands is finer grained and richer in aragonite through the elevated input of material from the flooded platform compared to periplatform ooze formed during the episodes of lowered sea level in many cases however the sea floor around carbonate platforms is subjected to bottom currents which are expected to affect sediment composition i e through winnowing of the fine fraction the interaction of sea level driven highstand shedding and current impact on the formation of periplatform ooze has hitherto not been analyzed to test if a sea level driven input signal in periplatform ooze is influenced or even distorted by changing current activity an integrated study using seismic hydroacoustic and sedimentological data has been performed on periplatform ooze deposited in the inner sea of the maldives the miocene to pleistocene succession of drift deposits is subdivided into nine units limits of seismostratigraphic units correspond to changes or turnarounds in grain size trends in cores recovered at odp site 716 and neoma site 1143 for the pleistocene it can be shown how changes in grain size occur in concert with sea level changes and changes of the monsoonal system which is thought to be a major driver of bottom currents in the maldives a clear highstand shedding pattern only appears in the data at a time of relaxation of monsoonal strength during the last 315 ky results imply 1 that drift sediments provide a potential target for analyzing past changes in oceanic currents and 2 that the ooze composition bears a mixed signal of input and physical winnowing at the sea floor

from the great bahama bank into the straits of florida a margin architecture controlled by sea level fluctuations and ocean currents high resolution multichannel seismic data collected across the great bahama bank margin and the adjacent straits of florida indicate that the deposition of neogene quaternary strata in this transect are controlled by two sedimentation mechanisms 1 west dipping layers of the platform margin which are a product of sea level controlled platform derived downslope sedimentation and 2 east or north dipping drift deposits in the basinal areas which are deposited by ocean currents these two sediment systems are active simultaneously and interfinger at the toe of slope the prograding system consists of sigmoidal clinoforms that advanced the margin some 25 km into the straits of florida the foresets of the clinoforms are approximately 600 m high with variable slope angles that steepen significantly in the pleistocene section the seismic facies of the prograding clinoforms on the slope is characterized by dominant partly chaotic cut and fill geometries caused by submarine canyons that are oriented downslope in the basin axis seismic geometries and facies document deposition from and by currents most impressive is an 800 m thick drift deposit at the confluence of the santaren channel and the straits of florida this santaren drift is slightly asymmetric thinning to the north the drift displays a highly coherent seismic facies characterized by a continuous succession of reflections indicating very regular sedimentation leg 166 of the ocean drilling program odp drilled a transect of five deep holes between 2 and 30 km from the modern platform margin and retrieved the sediments from both the slope and basin systems the neogene slope sediments consist of peri platform oozes intercalated with turbidites whereas the basinal drift deposits consist of more homogeneous fine grained carbonates that were deposited without major hiatuses by the florida current starting at approximately 12 4 ma sea level fluctuations which controlled the carbonate production on great bahama bank by repeated exposure of the platform top controlled lithologic alternations and hiatuses in sedimentation across the transect both sedimentary systems are contained in 17 seismic sequences that were identified in the neogene quaternary section seismic sequence boundaries were identified based on geometric unconformities beneath the great bahama bank all the sequence boundaries could be traced across the entire transect into the straits of florida biostratigraphic age determinations of seismic reflections indicate that the seismic reflections of sequence boundaries have chronostratigraphic significance across both depositional environments

latitudinal dependence of diapycnal diffusivity in the thermocline estimated using a finescale parameterization 1 the spatial distribution of diapycnal diffusivity in the thermocline over a large area in the interior of the north pacific is obtained by incorporating the finescale vertical shear of horizontal velocity measured by expendable current profilers into the empirical formula proposed by gregg 1989 it is clearly shown that diapycnal diffusivity in the north pacific strongly depends on the latitude for example the estimated diapycnal diffusivities near the hawaiian ridge and the izu ogasawara ridge exceed 10 4 m2 s 1 whereas those near the aleutian ridge and the emperor seamounts are of the order of 0 1 10 4 m2 s 1 even though the available semidiurnal internal tidal energy is similar among all of these prominent topographic features the observed results are consistent with the numerical prediction that the cascade of internal tidal energy down to dissipation scales is dominated by parametric subharmonic instability which is expected to occur only below 30 n