relationships between extratropical sea level pressure variations and the central pacific and eastern pacific types of enso abstract this study examines the linkages between leading patterns of interannual sea level pressure slp variability over the extratropical pacific 20 60 n and the eastern pacific ep and central pacific cp types of el nino southern oscillation enso the first empirical orthogonal function eof mode of the extratropical slp anomalies represents variations of the aleutian low and the second eof mode represents the north pacific oscillation npo and is characterized by a meridional slp anomaly dipole with a nodal point near 50 n it is shown that a fraction of the first slp mode can be excited by both the ep and cp types of enso the slp response to the ep type is stronger and more immediate the tropical extratropical teleconnection appears to act more slowly for the cp enso during the decay phase of ep events the associated extratropical slp anomalies shift from the first slp mode to the second slp mode as the second slp mode grows subtropical sst anomalies are induced beneath via surface

a linear decomposition of the southern ocean thermohaline structure the thermohaline structure of the southern ocean is deeply influenced by the presence of the antarctic circumpolar current acc where water masses of the world ocean are advected transformed and redistributed to the other basins it remains a challenge to describe and visualize the complex 3d pattern of this circulation and its associated tracer distribution here a simple framework is presented to analyze the southern ocean thermohaline structure a functional principal component analysis pca is applied to temperature and salinity s profiles to determine the main spatial patterns of their variations using the southern ocean state estimate sose this study determines the vertical modes describing the southern ocean thermohaline structure between 5 and 2000 m the first two modes explain 92 of the combined s variance thus providing a surprisingly good approximation of the thermohaline properties in the southern ocean the first mode 72 of total variance accurately describes the north south property gradients the second mode 20 mostly describes salinity at 500 m in the region of antarctic intermediate water formation these two modes present circumpolar patterns that can be closely related with standard frontal definitions by projecting any given hydrographic profile onto the sose based modes it is possible to determine its position relative to the fronts the projection is successfully applied on the hydrographic profiles of the woce sr3 section the southern ocean thermohaline decomposition provides an objective way to define water mass boundaries and their spatial variability and has useful application for comparing model output with observations

global adjustment of the thermocline in response to deepwater formation the global adjustment of the thermocline in response to deepwater formation is studied in a single mode model on a beta plane the signal is carried from ocean to ocean by kelvin waves which travel equatorward along western boundaries eastward across the equator poleward at the eastern boundaries and then eastward around the southern tip of continents into the next ocean basin the interior is filled by rossby waves emanating from eastern boundaries stronger weaker deepwater formation induces an upward downward motion of the main thermocline in the world oceans the adjustment is completed on centennial time scales

dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the paleocene isotopic records across the latest paleocene thermal maximum lptm indicate that bottom water temperature increased by more than 4 c during a brief time interval 104 years of the latest paleocene 55 6 ma there also was a coeval 2 to 3 excursion in the 13c of the ocean atmosphere inorganic carbon reservoir given the large mass of this reservoir a rapid 13c shift of this magnitude is difficult to explain within the context of conventional hypotheses for changing the mean carbon isotope composition of the ocean and atmosphere however a direct consequence of warming bottom water temperature from 11 to 15 c over 104 years would be a significant change in sediment thermal gradients and dissociation of oceanic ch4 hydrate at locations with intermediate water depths in terms of the present day oceanic ch4 hydrate reservoir thermal dissociation of oceanic ch4 hydrate during the lptm could have released greater than 1 1 to 2 1 1018 g of carbon with a 13c of approximately 60 the release and subsequent oxidation of this amount of carbon is sufficient to explain a 2 to 3 excursion in 13c across the lptm fate of ch4 in oceanic hydrates must be considered in developing models of the climatic and paleoceanographic regimes that operated during the lptm

physical controls on biogeochemical zonation in the southern ocean the primary control on the n s zonation of the southern ocean is the wind induced transport of the antarctic circumpolar current acc the acc divides the southern ocean into three major zones the subantarctic zone saz north of the acc the acc transport zone and the zone south of the acc saccz the zone of acc transport is most often subdivided into two zones the polar frontal zone pfz and the antarctic zone aaz but it may be appropriate to define more subzones or indeed only one at some longitudes to maintain geostrophic balance isopycnals must slope upwards to the south across the acc thus raising nutrient rich deep water closer to the surface as one goes polewards in addition silicate concentrations increase polewards along isopycnals because of diapycnic mixing with silicate rich bottom water surface silicate concentrations therefore decrease northwards from high levels in the saccz to low levels in the saz within the saz and pfz and even in the northern part of the aaz silicate levels may drop to limiting levels for siliceous phytoplankton production during summer nitrate concentrations also decrease northwards but only become limiting in the subtropical zone north of the saz the second circumpolar control is the changing balance of stratification with temperature dominating near surface stratification in the saz and salinity dominating further south because of fresh water input to the surface from melting ice this results in circumpolar features such as the subsurface 2 c temperature minimum and the subduction of the salinity minimum of antarctic intermediate water which are often but not always associated with frontal jets and large transports the transport of the acc is dynamically constrained into narrow bands the number and latitudinal location of which are controlled by the bathymetry and so vary with longitude thus it is not the fronts that are circumpolar but the total acc transport and scalar properties of the salinity and temperature fields evidence of summer silicate and nitrate uptake in all zones saz pfz and aaz shows that there is productivity despite their high nutrient low chlorophyll status blooms covering large areas say 400 km across in the pfz and aaz are found in the vicinity of submarine plateaux which suggest benthic iron fertilizatio

gas migration below gas hydrates controlled by mass transport complexes offshore mauritania abstract interpretation of 3d seismic data from offshore mauritania reveals many localized high amplitude anomalies haas above mass transport complexes mtcs these haas are located at 400 600 m burial depth and have circular or elongate shapes in map view the reflections at the tops of these haas are of negative polarity we divide these haas into two populations aligned and non aligned the aligned haas comprise three linear groups above the structural lineaments of underlying mtcs suggesting a genetic link the aligned haas in the western group are often located in depressions and are interpreted to be caused by both gas migration and coarser grained sediments the other two groups of aligned haas are interpreted to be caused by residual gas after gas migration fed along the margins of underlying mtcs in contrast the non aligned haas are generally smaller located above the mtc bodies they are interpreted to be caused by gas which migrated from underlying mtc bodies rather than along the margins without any evidence for lithology change we propose that the locations of the haas both aligned and non aligned indicate gas migration pathways up mtc margins or local faults associated with the underlying mtcs one possible gas source could be from hydrate dissociation due to the upward resetting of the base of gas hydrates since buried mtcs and gas hydrates can coexist in marine environments the model of buried mtcs channelling fluids upwards may occur widely which can supply gas to the hydrate systems

vertical evolution of fluid venting structures in relation to gas flux in the neogene quaternary of the lower congo basin offshore angola abstract the processes preceding the genesis of pockmarks and their subsequent evolution has been rarely studied since pockmarks were discovered in 1970 a high resolution 3 d seismic survey from the continental slope of the lower congo basin allows us to study in detail the structures that developed before the formation of pockmarks as well as their subsequent geometric evolution this paper aims at reporting new type of fluid related structures that relate to pockmarks the objective of this research is to understand 1 the evolution of these structures and their genetic relationship with pockmarks and 2 how their morphologies may be influenced by changes in venting intensity over time pockmark and chimney structures in this survey are hosted within late miocene to quaternary hemipelagites above clastic reservoirs in a petroliferous basin they are all associated with positive high amplitude anomalies phaas on seismic data phaas are interpreted from their dramatically higher acoustic impedance relative to the background sedimentary values of the host units and by their association with seismic pull up effects from their planform and cross sectional geometry phaas are primarily interpreted as being due to the development of methane related carbonates but they may be associated with gas hydrates in one or two examples methanogenic carbonates form either by authigenic accumulation or by anaerobic methane oxidation amo once upwardly migrating methane enters the sulfate methane transition zone smtz typically within the first few tens of meters below the seabed therefore vertical accumulations of phaas are interpreted to reflect the temporal evolution of methane flux migrating upward to reach the paleo seabed or smtz timing of local scale fluid leakage can therefore be diagnosed by analyzing phaas at seep locations that occur in association with pockmark chimney structures a conceptual model for the evolution of different fluid venting structures is proposed here and is based on the analysis of venting structures that we expressed by 3 d seismic reflections in this model the main features are linked into a process based sequence culminating in the infill and termination of the pockmarks as fluid venting sites our model summarizes different rhythms of venting which are expressed by different types of phaas along with venting structures linear phaas and conduits inferred slow seeps they can be succeeded by sub circular phaas and shallow depressions which inferred slow to moderate seeps then the seep can become either a fast vent and terminated into a pockmark crater or it can become less active and be sealed afterwards subsurface sediment fluidizations and remobilizations are likely to happen in the last case and are probably caused by hydrate dissociations

gas trapped below hydrate as a primer for submarine slope failures abstract interpretation of a three dimensional 3 d seismic dataset from offshore of mauritania reveals a shear zone at the base of a partially developed slope failure the shear zone is at a depth of 220 m below the seabed immediately above a hydrate bottom simulating reflector bsr we propose that a paleo gas accumulation trapped below hydrate was the primer for what would have been a substantial submarine slope failure of 220 m thickness covering 50 km 2 this is based on the following observations a the shear surface is at the level of seismic resolution coincident with some present gas accumulations located immediately below sediment that hosts hydrate b there are remnants of a more extensive paleo gas accumulation that would have generated sufficient buoyancy pressure for the shear surface to be critically stressed and therefore primed the failure c seismic pipes are a common seismic feature within the studied succession but absent in the area of the shear zone which supports the hypothesis that a high gas column could have existed this is a rare example of a shear zone that did not lead to the complete development of a slope failure it provides the first seismic evidence that the buoyancy effect of gas below the hydrate rather than the hydrate dissociation is also a viable mechanism for large scale slope failures

adjustment of the ocean under buoyancy forces part i the role of kelvin waves abstract the early stages of the adjustment of an ocean toward equilibrium is examined using an ocean general circulation model the initial state is one with uniform meridional potential temperature gradients yielding density gradients representative of those observed in the north atlantic the main adjustment in the time examined here the first few months is by coastal kelvin waves the first baroclinic mode dominates so the behavior proves to be very similar to that in a shallow water model also the dynamics is locally close to that for an f plane but account must be taken of the way f varies with latitude the essential result of the coastal adjustment is to reduce the temperature gradients around the perimeter of the basin especially at the level where the first mode has the maximum effect this study also examines the way this adjustment process is distorted in models due to the constraints imposed by computer limitations namely in a model with too coarse a horizontal resolution and an artif

spreading of red sea overflow waters in the indian ocean as a result of its remarkably high salinity and despite its small volume input remnants of red sea water rsw have been identified in the agulhas current over 6000 km distant of their source this provided the motivation to investigate the long term mean spreading of rsw throughout the indian ocean using a comprehensive set of observations taken from the national oceanographic data center archives and from the world ocean circulation experiment hydrographic program for the indian ocean after emerging from the gulf of aden into the arabian sea rsw spreads predominantly southwestward along the african continental slope as indicated by strongly inclined isohalines across the arabian sea there is some monsoon variability so that during the winter monsoon there is more rsw present in the gulf of aden and an intensification of southward spreading along the western boundary between 12 and 5 n elsewhere the intermediate depth salinity field of the indian ocean appears relatively stationary between 5 and 10 s in the region of the south equatorial current isohalines of the rsw layer become quasi zonal across the width of the indian ocean only dipping southward toward the western boundary west of 50 e south of here there is a strong tongue of rsw spreading southward through the mozambique channel and into the agulhas current these conclusions concur with previous localized investigations of intermediate water properties using a simple mixing model the percentage of rsw throughout the indian ocean was quantified it was found that the flux of salt into the gulf of aden from the red sea is similar to that estimated to cross 32 s in the agulhas current this result implies that all the rsw which is mixed into the interior of the indian ocean may eventually be exported at the western boundary furthermore it implies that rsw is the dominant component of the salt budget for the intermediate layer and that input from the indonesian seas and via diapycnic processes are small

methane related carbonate cements in pockmarks of the north sea abstract some pockmarks on the norwegian continental shelf contain patches of cemented sediment that can provide hardgrounds attractive to a variety of benthonic organisms detailed examination of a sandstone sample from a pockmark in norwegian block 25 7 in the north sea has revealed the presence of mg calcite and aragonite cements some of the latter forming botryoids all cement types are characterized by extremely light carbon isotopic compositions with a mean 13c value of 56 1 pdb which shows that the cements contain carbonate produced by oxidation of biogenic methane oxidation occurred in both the oxic and anoxic diagenetic zones trace amounts of interstitial methane mean 13c 40 8 and higher hydrocarbon gases up to c5 with a c1 cn ratio of 0 855 0 874 in the pockmark sediments indicate that some thermogenic methane may be mixed with the biogenic gas

seasonal meandering of the polar front upstream of the kerguelen plateau the location of the antarctic polar front pf is mapped in the southern indian ocean by decomposing the shape of temperature and salinity profiles into vertical modes using a functional principal component analysis we define the pf as the northernmost minimum of temperature at the subsurface andrepresent it as a linear combination of the first three modes this method is applied on an ocean reanalysis data set and on in situ observations revealing a seasonal variability of the pf latitudinal position that is most pronounced between the conrad rise and the kerguelen plateau this shift coincides with variations in the transport across the northern kerguelen plateau we suggest that seasonal changes of the upper stratification may drive the observed variability of the pf with potentially large implications for thepathways and residence time of water masses over the plateau and the phytoplankton bloom extending southeast of the kerguelen islands

the buoyancy and wind driven ventilated thermocline abstract an analytical study of the combined wind driven and buoyancy driven thermocline problem is presented the analysis is an extension of the ventilated thermocline model of luyten et al an exact solution for the ventilated region of the subtropical gyre is found under the condition that the ratio of the buoyancy flux to ekman pumping is a function of latitude alone it demonstrates the general westward shift of streamlines when the subtropical gyre is heated and the enlargement of the eastern shadow zone the flow in the shadow zone is described in most instances for the case of relatively small buoyancy flux it is shown that the shadow zone now in motion due to buoyancy forcing splits into two zones there is a broad eastward zone in which the deep flow is northeastward and a narrow zone between this branch and the ventilated fluid in which the shadow zone flow is returned in a relatively swift current nearly preserving potential vorticity for buoyancy ekman flux ratios which are nearly inde

gravity driven faults migration pathways for recycling gas after the dissociation of marine gas hydrates abstract three dimensional seismic data from offshore of mauritania west africa reveal tiers of normal and tear faults that are approximately parallel to the strike and dip of the slope respectively they formed as a result of multiple episodes of down slope creep and have propagated through 60 140 m of sediment some intersect the seabed while others are buried and formed during earlier down slope translation gas hydrates the base of which is evidenced by aligned amplitude terminations or a bottom simulating reflection form within the faulted succession there are spatial relationships among the fault locations breached gas accumulations and the location of shallower free gas sealed by hydrates these are consistent with the upward resetting of the base of the hydrates the migration of dissociated gas up the faults to either become trapped below its reset base in a new free gas zone or potentially recycled back into a hydrate phase there is little evidence for venting of dissociated gas at the seabed gravity driven faulting is common on continental and island slopes where gas hydrates are expected to occur therefore this model for the utilization of gravitationally driven faults for the migration and recycling of gas should be generally applicable

geophysical and geochemical evidence for subsurface hydrocarbon leakage in the bering sea alaska abstract geophysical and geochemical surveys were conducted on the bering sea shelf to investigate whether hydrocarbons found in the surficial sediment particularly in association with certain geophysical events indicate the presence and nature of more deeply reservoired oil and gas deposits surface sediment samples were obtained at 1000 sites where high resolution seismic data indicated the presence of gas seepage and or surface faulting in addition sites for background measurements were occupied away from areas of gas seepage results of the geophysical survey show seismic wipeout zones to be present throughout the bering sea shelf sediment samples obtained within the shallow wipeout zones confirm that these features are caused by gas in addition compositional and isotopic analyses indicate that the gas is of biogenic and thermogenic origins the thermogenic leakage appears to be related to localized faulting and shallow structuring the high resolution seismic data therefore provide an excellent tool for locating leakage points thereby enhancing the opportunity of finding migrated thermogenic hydrocarbons in surficial sediments an examination of vertical variability shows the hydrocarbon concentrations to be strongly related to various processes occurring in the surficial sediments core samples must be taken from below the zone of maximum disturbance to obtain unaltered hydrocarbon samples

the thinness of the ocean in s θ p space and the implications for mean diapycnal advection abstract it is shown that the ocean s hydrography occupies little volume in the three dimensional space defined by salinity temperature pressure s p and the implications of this observation for the mean vertical transport across density surfaces are discussed although ocean data have frequently been analyzed in the two dimensional temperature salinity s diagram where casts of hydrographic data are often locally tight in s space the relatively empty nature of the world ocean in the three dimensional s p space seems not to have received attention the world ocean s data lie close to a single surface in this three dimensional space and it is shown that this explains the known smallness of the ambiguity in defining neutral surfaces the ill defined nature of neutral surfaces means that lateral motion along neutral trajectories leads to mean vertical advection through density surfaces even in the absence of small scale mixing processes the situation in which the ocean s hydrography occupies a

natural gas formation in the western nile delta eastern mediterranean thermogenic versus microbial abstract the offshore part of the nile delta is an emerging gas and condensate province and one of the most promising areas for future petroleum exploration and supply in northern africa while the nile delta s hydrocarbon potential and its geological and stratigraphic pattern has been the focus of numerous studies the origin of its gas has been poorly studied in order to characterize the natural gas and to determine whether microbial or thermogenic processes are the dominant generation pathways of methane a regional geochemical study was performed on 38 test gases collected from 9 neogene western nile delta discoveries main constituents of the gases are methane 89 4 99 4 up to pentane with traces of carbon dioxide and nitrogen methane carbon isotopic composition ranges from 66 0 to 39 6 displaying a mixed microbial and thermogenic character for most of the gases microbial gas contributions range from 15 to 95 the mixed and the more thermogenic gases are frequently accompanied by light oils and condensates 41 59 api calculated maturities of the thermogenic gas proportions suggest source rock maturities between 0 8 and 1 4 ro which are actually present in the pre miocene section evidence for microbial alteration of petroleum accumulations in the nile delta is largely manifested in anomalously heavy isotopic compositions of both wet gas components propane and n butane and preferential removal of higher n alkanes from condensates the hydrocarbon accumulations affected by biodegradation are trapped within shallow pliocene reservoirs

oceanic teleconnections remote response to decadal wind forcing abstract the transhemispheric and interbasin response to time dependent wind forcing localized in the northern hemisphere of a single basin is examined using the reduced gravity shallow water equations in domains of simple geometry on decadal timescales the pressure on the eastern boundary fluctuates synchronously in both hemispheres and thus communicates a signal to latitudes distant from the forcing the signal then penetrates into the interior through westward radiation of rossby waves associated with the eastern boundary pressure fluctuation is a time dependent mass flux across the equator that in a single basin is balanced by a storage of mass in the unforced hemisphere two oceanic basins connected by a reentrant channel at the high latitude edge of the southern hemisphere are then considered again the forcing is confined to the northern hemisphere of one basin only in this geometry the time dependent mass flux across the equator of the forced basin is not entirely balanced within the same ba

coherent heat patterns revealed by unsupervised classification of argo temperature profiles in the north atlantic ocean abstract a quantitative understanding of the integrated ocean heat content depends on our ability to determine how heat is distributed in the ocean and identify the associated coherent patterns this study demonstrates how this can be achieved using unsupervised classification of argo temperature profiles the classification method used is a gaussian mixture model gmm that decomposes the probability density function of a dataset into a weighted sum of gaussian modes it is determined that the north atlantic argo dataset of temperature profiles contains 8 groups of vertically coherent heat patterns or classes each of the temperature profile classes reveals unique and physically coherent heat distributions along the vertical axis a key result of this study is that when mapped in space each of the 8 classes is found to define an oceanic region even if no spatial information was used in the model determination the classification result is independent of the location and time of the argo profiles two classes show cold anomalies throughout the water column with amplitude decreasing with depth they are found to be localized in the subpolar gyre and along the poleward flank of the gulf stream and north atlantic current nac one class has nearly zero anomalies and a large spread throughout the water column it is found mostly along the nac one class has warm anomalies near the surface 50 m and cold ones below 200 m it is found in the tropical equatorial region the remaining four classes have warm anomalies throughout the water column one without depth dependance in the southeastern part of the subtropical gyre the other three with clear maximums at different depths 100 m 400 m and 1000 m these are found along the southern flank of the north equatorial current the western part of the subtropical gyre and over the west european basin these results are robust to both the seasonal variability and to method parameters such as the size of the analyzed domain

observations of warm water volume changes in the equatorial pacific and their relationship to el nino and la nina abstract this paper describes observed changes in surface winds sea surface temperature sst and the volume of water warmer than 20 c wwv in the equatorial pacific ocean for the period 1980 99 the purpose is to test recent hypotheses about the relationship between variations in wwv and the el nino southern oscillation enso cycle the results confirm inferences based on theory models and previous empirical analyses using proxy data namely sea level that enso involves a recharge and discharge of wwv along the equator and that the cyclic nature of enso results from a disequilibrium between zonal winds and zonal mean thermocline depth the authors also find that the magnitude of enso sst anomalies is directly related to the magnitude of zonal mean wwv anomalies furthermore for a given change in equatorial wwv the corresponding warm el nino sst anomalies are larger than the corresponding cold la nina anomalies this asymmetry between the warm and cold phases of the enso cycle implies differences

contrasting eastern pacific and central pacific types of enso abstract surface observations and subsurface ocean assimilation datasets are examined to contrast two distinct types of el nino southern oscillation enso in the tropical pacific an eastern pacific ep type and a central pacific cp type an analysis method combining empirical orthogonal function eof analysis and linear regression is used to separate these two types correlation and composite analyses based on the principal components of the eof were performed to examine the structure evolution and teleconnection of these two enso types the ep type of enso is found to have its sst anomaly center located in the eastern equatorial pacific attached to the coast of south america this type of enso is associated with basinwide thermocline and surface wind variations and shows a strong teleconnection with the tropical indian ocean in contrast the cp type of enso has most of its surface wind sst and subsurface anomalies confined in the central pacific and tends to onset develop and decay in situ

atlantic pacific asymmetry in deep water formation while the atlantic ocean is ventilated by high latitude deep water formation and exhibits a pole to pole overturning circulation the pacific ocean does not this asymmetric global overturning pattern has persisted for the past 2 3 million years with evidence for different ventilation modes in the deeper past in the current climate the atlantic pacific asymmetry occurs because the atlantic is more saline enabling deep convection to what extent the salinity contrast between the two basins is dominated by atmospheric processes larger net evaporation over the atlantic or oceanic processes salinity transport into the atlantic remains an outstanding question numerical simulations have provided support for both mechanisms observations of the present climate support a strong role for atmospheric processes as well as some modulation by oceanic processes a major avenue for future work is the quantification of the various processes at play to identify which mechanisms are primary in different climate states

tracking the relict bases of marine methane hydrates using their intersections with stratigraphic reflections we describe a three dimensional seismic interpretation approach for identifying the past bases of the methane hydrate stability zone in marine settings the bases commonly crosscut stratigraphic reflections and these lines of intersection can be revealed by maps of seismic amplitude maps for four reflections that are crosscut by the base of the present day offshore mauritania methane hydrate reveal extraordinary curvilinear changes of amplitude that are parallel to one another and extend for as much as 20 km they are interpreted as marking the relict intersections of the bases of the hydrate stability zone and formed by its upward resetting we hypothesize that this was caused by pulsed sedimentation during repeated glacial interglacial cycles over the past 1 25 m y localized deflections in the lines of intersection are indicative of local changes in sediment temperature this technique could provide a wealth of information on local and regional changes in ambient conditions and better estimates of the volumes of methane being released through time it therefore should help test for interdependencies between hydrate dissociation and climate change

gas hydrate fluid flow and free gas formation of the bottom simulating reflector abstract gas hydrate in continental margins is commonly indicated by a prominent bottom simulating seismic reflector bsr that occurs a few hundred metres below the seabed the bsr marks the boundary between sediments containing gas hydrate above and free gas below most of the reflection amplitude is caused by the underlying free gas gas hydrate can occur without a bsr however and the controls on its formation are not well understood here we describe two complementary mechanisms for free gas accumulation beneath the gas hydrate stability zone ghsz the first is the well recognised hydrate recycling mechanism that generates gas from dissociating hydrate when the base of the ghsz moves upward relative to hydrate bearing sediment the second is a recently identified mechanism in which the relationship between the advection and diffusion of dissolved gas with the local solubility curve allows the liquid phase to become saturated in a thick layer beneath the ghsz when hydrate is present near its base this mechanism for gas production called the solubility curvature mechanism is possible in systems where the influence of diffusion becomes important relative to the influence of advection and where the gas water solubility decreases to a minimum several hundred metres below the ghsz we investigate a number of areas in which gas hydrate occurs to determine where gas formation is dominated by the solubility curvature mechanism and where it is dominated by hydrate recycling we show that the former is dominant in areas with low rates of upward fluid flow such as old rifted continental margins low rates of seafloor uplift and high geothermal gradient and or pressure conversely free gas formation is dominated by hydrate recycling where there are rapid rates of upward fluid flow and seabed uplift such as in subduction zone accretionary wedges using these two mechanisms to investigate the formation of free gas beneath gas hydrate in continental margins we are able to resolve a number of problems these include why sub bsr free gas zones are generally thin in convergent margins and thick in passive margins and why areas such as the gulf of mexico and mackenzie delta do not have regional bsrs finally we use our understanding of how bsrs are formed to show that regional gas hydrate is unlikely to exist in substantial quantities in passive continental margins outside areas with an observable bsr

middle miocene mass transport deposits in the southern part of the roer valley graben abstract from the late early miocene onwards a north west propagating delta was built out in the differentially subsiding roer valley graben the lower part of this delta sequence consists of the lignite bearing fine sands and silts of the heksenberg member a maximum flooding surface at the top of a transgressive system tract in the lower part of the delta acted as a shear surface along which the overlying middle miocene deltaic deposits slid downslope forming mass transport deposits seismic events sea level fluctuations and gas charging are considered potential triggers for slope failure the past presence of gas might be indicated by high amplitude seismic anomalies in the rotated blocks in the head domain of the mass transport deposits gas could have had a biogenic origin for example from the lignite in the heksenberg member or migrated from deeper sources into the delta

the transient response of the southern ocean pycnocline to changing atmospheric winds 1 the vertical density structure of the southern ocean is dynamically linked to wind stress at the surface but the nature of this coupling is not fully understood observations from the last several decades show a significant increase in the strength of westerly winds over the southern ocean but an appreciable change in the tilt of constant density surfaces isopycnals has not yet been detected there using a combination of theory and idealized numerical simulations we show that the response of the density structure occurs on centennial timescales making it difficult to detect significant changes with a few decades of hydrographic observations dynamic coupling between the circumpolar current and northern basins regulates the slow adjustment of the density structure our results provide a new interpretation for recent observations and highlight the importance of the interaction between regional southern ocean dynamics and global ocean circulation citation jones d c t ito andn s lovenduski 2011 thetransientresponseofthe southern ocean pycnocline to changing atmospheric winds geophys res lett 38 l15604 doi 10 1029 2011gl048145

opposing decadal changes for the north atlantic meridional overturning circulation hydrographic properties of the north atlantic ocean have changed significantly over the past decades a combination of changes in seawater density calculated from observed properties of sea water and a numerical ocean model reveals that the strength of the meridional overturning circulation has changed in different directions in the subpolar and subtropical basins respectively

a three layer model for wind driven circulation in a subtropical subpolar basin part i model formulation and the subcritical state abstract a three layer model is formulated and run numerically in order to understand the highly nonlinear structure of the wind driven circulation in a basin including both a subtropical and a subpolar gyre the third layer is assumed to be infinitely deep and motionless we emphasize the strong nonlinearity associated with large layer thickness change a broad range of parameters has been tested to explore possible flow patterns for cases of weak wind forcing and large amounts of water in the upper layer the upper layer covers the whole basin the second layer is stagnant except near the western boundary for stronger forcing closed potential vorticity isolines appear within the second layer and there is a strong nonlinear coupling between the first and second layers the numerical results show some flow patterns highly asymmetric with respect to the zero wind curl line the potential vorticity distribution in the second layer shows a strong competition between potential vorticity advection interfa

fluid flow due to the advance of basin scale silica reaction zones the conversion of biogenic silica opal a to opal ct cristobalite and tridymite in biosiliceous sediment causes increased rates of water expulsion because of the reduction in sediment porosity and dehydration of the amorphous opal a phase this release of water occurs over large tracts of sedimentary basins during sediment burial within discrete diagenetic reaction zones analysis of two dimensional and three dimensional seismic data sets from basins in the northern hemisphere provides geophysical evidence for a variety of fluid conduits and roughly circular erosional depressions at the contemporaneous seabed we interpret these features as indicative of water expulsion and focused fluid flow emanating from opal a to opal ct reaction zones at burial depths within the range 200 800 m the rate at which water is expelled depends upon the degree of porosity reduction and the weight fraction of bound water at the reaction zone as well as the rate of advance of the reaction zone where the reaction is actively taking place within homogeneous biosiliceous sediment the rate of water expulsion is independent of the reaction rate this is because water is released across the entire reaction zone therefore slow reaction rates are compensated for by expulsion of water across wider reaction zones we calculate the rate and volume of water expulsion for the faeroe shetland basin where the sediment immediately below the reaction zone contains on average 30 opal ct by weight the estimated volumetric rate of water expulsion per unit surface area at the present day is 6 m3 my 1 per square meter which is greater than the vertical flux of water at the same depth from compaction of the deeper basin fill the average volumetric rate of water expulsion is 120 km3 my 1 across the whole basin biogenic silica is particularly rich in neogene successions in high latitude and equatorial regions and where silica reaction zones are identified they should be factored into sediment compaction and fluid flow histories

localization of abrupt change in the north atlantic thermohaline circulation 1 recent climate model experiments as well as paleoclimate records suggest that the meridional overturning circulation or thermohaline circulation in the atlantic ocean could change abruptly as a result of global warming and that this could have a significant impact on european climate we use a reduced gravity model to investigate the response of the atlantic overturning circulation to changes in forcing we find that variability at decadal and higher frequencies is confined to a single hemisphere this implies that a overturning variability resulting from high frequency changes in buoyancy forcing in the labrador and greenland seas will be limited to the north atlantic and b any observed decadal and higher frequency fluctuations in north atlantic overturning can only result from changes in the surface fluxes within the north atlantic basin itself these results suggest that southern ocean wind forcing is not important for north atlantic overturning on decadal and shorter timescales

mimoc a global monthly isopycnal upper ocean climatology with mixed layers a monthly isopycnal mixed layer ocean climatology mimoc global from 0 to 1950 dbar is compared with other monthly ocean climatologies all available quality controlled profiles of temperature t and salinity s versus pressure p collected by conductivity temperature depth ctd instruments from the argo program ice tethered profilers and archived in the world ocean database are used mimoc provides maps of mixed layer properties conservative temperature absolute salinity sa and maximum p as well as maps of interior ocean properties sa and p to 1950 dbar on isopycnal surfaces a third product merges the two onto a pressure grid spanning the upper 1950 dbar adding more familiar potential temperature and practical salinity s maps all maps are at monthly 0 5 0 5 resolution spanning from 80 s to 90 n objective mapping routines used and described here incorporate an isobath following component using a fast marching algorithm as well as front sharpening components in both the mixed layer and on interior isopycnals recent data are emphasized in the mapping the goal is to compute a climatology that looks as much as possible like synoptic surveys sampled circa 2007 2011 during all phases of the seasonal cycle minimizing transient eddy and wave signatures mimoc preserves a surface mixed layer minimizes both diapycnal and isopycnal smoothing of s as well as preserves density structure in the vertical pycnoclines and pycnostads and the horizontal fronts and their associated currents it is statically stable and resolves water mass features fronts and currents with a high level of detail and fidelity

repeated fluid expulsion through sub seabed chimneys offshore norway in response to glacial cycles abstract focused fluid flow through sub seabed sediments is a common phenomenon on continental margins worldwide however the governing controls and timing of this fluid release have been difficult to understand in particular for fluid flow features buried beneath sub surface sediments a link between fluid flow activity ensuing pockmark formation and the last glacial maximum has been hypothesized on the formerly glaciated norwegian margin new high resolution p cable 3d seismic data from the nyegga area on the mid norwegian margin reveal at least two more periods of fluid expulsion from sub seabed sediments the 3d seismic data show depositional patterns within chimney features expressed as truncations of seismic horizons against the flanks of the chimneys the truncations are interpreted by analogy with present day observations as evidence for buried carbonate mounds and or sediment wash out during formation of pockmarks in the past the truncations are hence an indicator and a chronological marker for fluid expulsion in the past the classification of chimneys results in three major groups 1 chimneys that have been formed and consecutively reactivated one or two times during the last 200 kyr and that have a fluid flow expression at the present seafloor 2 chimneys that are approx 125 160 ka bp old without any associated fluid flow expression at the present day seafloor 3 chimneys with no stratigraphical evidence for reactivation formed after the last glacial maximum 18 25 ka bp the observations suggest that each activity period was likely related to the last stages of maximum glaciations in the region the emplacement of thick sequences of glacigenic debris flow deposits during these maximum stages most likely caused rapid increase in overpressure and subsequently the formation of focused fluid flow features piercing through sediments of the naust formation at nyegga

nonlinear acoustic scattering from a gassy poroelastic seabed a model for difference frequency backscatter from trapped bubbles in sandy sediments was developed a nonlinear volume scattering coefficient was computed via a technique similar to that of ostrovsky and sutin nonlinear sound scattering from subsurface bubble layers in natural physical sources of underwater sound edited by b r kerman kluwer dordrecht 1993 pp 363 373 which treats the case of bubbles surrounded by water biot s poroelastic theory is incorporated to model the acoustics of the sediment biot fast and slow waves are included by modeling the pore fluid as a superposition of two acoustic fluids with effective densities that differ from the pore fluid s actual density and account for its confinement within sediment pores the principle of acoustic reciprocity is employed to develop an expression for the backscattering strength model behavior is consistent with expectations based on the known behavior of bubbles in simpler fluid media

dilution of the northern north atlantic ocean in recent decades declining salinities signify that large amounts of fresh water have been added to the northern north atlantic ocean since the mid 1960s we estimate that the nordic seas and subpolar basins were diluted by an extra 19 000 5000 cubic kilometers of freshwater input between 1965 and 1995 fully half of that additional fresh water about 10 000 cubic kilometers infiltrated the system in the late 1960s at an approximate rate of 2000 cubic kilometers per year patterns of freshwater accumulation observed in the nordic seas suggest a century time scale to reach freshening thresholds critical to that portion of the atlantic meridional overturning circulation

equilibrium structure and dynamics of the california current system this paper addresses the structure and dynamical mechanisms of regional and mesoscale physical variability in the subtropical northeast pacific ocean using the regional oceanic modeling system roms the model is configured with a u s west coast domain that spans the california current system ccs with a mesoscale horizontal resolution up to as fine as 3 5 km its mean seasonal forcing is by momentum heat and water fluxes at the surface and adaptive nudging to gyre scale fields at the open water boundaries its equilibrium solutions show realistic mean and seasonal states and vigorous mesoscale eddies fronts and filaments the level of eddy kinetic energy eke in the model is comparable to drifter and altimeter estimates in the solutions with sufficiently fine resolution because the model lacks nonseasonal transient forcing the authors conclude that the dominant mesoscale variability in the ccs is intrinsic rather than transiently forced the primary eddy generation mechanism is the baroclinic instability of upwelling alongshore currents there is progressive movement of meanseasonal currents and eddy energy offshore and downward into the oceanic interior in an annually recurrent cycle the associated offshore eddy heat fluxes provide the principal balance against nearshore cooling by mean ekman transport and upwelling the currents are highly nonuniform along the coast with important influences by capes and ridges in both maintaining mean standing eddies and launching transient filaments and fronts

the potential volume of oceanic methane hydrates with variable external conditions gas hydrate abundance in marine sediment depends on gas concentration and the available pore space within certain stability limits potentially gas hydrates can occur between the seafloor and a locus of subbottom depths where geothermal gradients intersect gas gas hydrate pore water equilibrium curves perpendicular to a given continental margin the lens shaped area between these two bounding surfaces a sl varies according to seven basic parameters gas composition water activity a w bottom water temperature t b geothermal gradient g slope depth z slb slope gradient z and sea level relative to the shelf break z 0 assuming pure ch 4 gas 35 km 2 of sediment can host gas hydrate across an average continental margin at a pleistocene lowstand a w 0 981 t b 0 c g 0 c z slb 4000 m z 0 04 z 0 0 however this potential area would decrease with smaller a w higher t b greater g shallower z slb steeper z and lower z 0 and increase with opposite external conditions of the basic parameters temperature t b and g and bathymetry z slb and z can particularly influence the distribution of gas hydrate on continental slopes a hydrothermal gradient with e g surface temperatures t b will also decrease a sl although minimally especially if t b is warm the sum of parallel cross sectional areas along a margin combined with porosity o gives the potential volume of gas hydrate v assuming 200 000 km of continental margin with a o of 50 3 5 10 6 km 3 of pore space can contain gas hydrates at present day a volume that compares favorably with previous estimates 1 2 to 6 4 10 6 km 3 although underlying approaches differ fundamentally since the triassic v glob probably has increased significantly because t b has cooled while total margin length has grown this trend was likely punctuated by at least one major decrease nominally 1 5 to 0 7 10 6 km 3 when t b suddenly rose by 5 c during the latest paleocene thermal maximum lptm a prominent global negative 13 c excursion across the lptm may signify massive release of ch 4 associated with this theoretical drop in v glob

effect of gas hydrates melting on seafloor slope instability we present a theoretical study of the thermodynamic chemical equilibrium of gas hydrate in soil by taking into account the influence of temperature pressure pore water chemistry and the mean pore size distribution the model uses a new formulation based on the enthalpy form of the law of conservation of energy the developed model shows that due to a temperature and pressure increase hydrates may dissociate at the top of the hydrate occurrence zone to ensure a chemical equilibrium with the surrounding bulk water this original result confirms what has been already shown through experiments the second part of the paper presents an application of the model through a back analysis of the giant storegga slide on the norwegian margin two of the most important changes during and since the last deglaciation hydrostatic pressure due to the change of the sea level and the increase of the sea water temperature were considered in the calculation simulation results show that melting of gas hydrate due to the change of the gas solubility can be at the origin of a retrogressive failure initiated at the lower part of the storegga slope once again the developed model leads to predictions which are supported by laboratory experiment results but contradictory to previous interpretations and beliefs considering that hydrate dissociation occurs only at the bottom of the gas hydrate stability zone

time dependent adjustment in a simple model of the mid depth meridional overturning cell abstracta recently proposed reduced gravity model of the warm water branch of the middepth meridional overturning circulation in a rectangular basin with a circumpolar connection is extended to include time dependence the model describes the balance between gain of warm water through northward ekman advection across the circumpolar current loss of warm water through eddy fluxes southward across the current net gain or loss of warm water through diabatic processes north of the current and changes in the thickness of the warm water layer the steady solutions are the same as those found previously when the previous parameterization of diabatic fluxes is used time dependent solutions are considered for the approach of the solution to a new equilibrium when the forcing or parameters are abruptly changed and then held fixed an initial adjustment occurs through a combination of boundary and equatorial adjustment followed by planetary wave propagation the longer term adjustment to equilibrium consists p

adjustment of the ocean under buoyancy forces part ii the role of planetary waves abstract a numerical ocean general circulation model is used to investigate the early stages in the adjustment to equilibrium of an ocean initially at rest with imposed uniform meridional potential temperature gradients which yield density gradients representative of those observed in the north atlantic the main feature of the adjustment during the early stages the first year is the formation and decay of a subtropical warm core and a subpolar cold core density gyre the gyres are formed by the irreversible winding up of the initially zonal isotherms by first baroclinic mode coastally trapped dissipative kelvin waves this phase and the north south asymmetry arising from the variation in viscous diffusive kelvin wave properties with latitude were discussed in part i of this series within a month effects become significant especially in the evolution of the southern gyre which develops a distinct east west asymmetry through western intensification and long planetary wave propagation of b

unsupervised clustering of southern ocean argo float temperature profiles the southern ocean has complex spatial variability characterized by sharp fronts steeply tilted isopycnals and deep seasonal mixed layers methods of defining southern ocean spatial structures traditionally rely on somewhat ad hoc combinations of physical chemical and dynamic properties as a step towards an alternative approach for describing spatial variability in temperature here we apply an unsupervised classification technique that is gaussian mixture modelling or gmm to southern ocean argo float temperature profiles gmm without using any latitude or longitude information automatically identifies several spatially coherent circumpolar classes influenced by the antarctic circumpolar current in addition gmm identifies classes that bear the imprint of mode intermediate water formation and export large scale gyre circulation and the agulhas current among others because gmm is robust standardized and automated it can potentially be used to identify structures such as fronts in both observational and model datasets possibly making it a useful complement to existing classification techniques

geological controls on focused fluid flow associated with seafloor seeps in the lower congo basin a synthesis of backscatter imagery coupled with a large 3d seismic dataset in the lower congo basin lcb reveals a patchy distribution of features interpreted to be associated with fluid seepage from 300 m to 2500 m water depth with the exception of one region of anomalous backscatter positive relief mounds all inferred seep sites occur in negative relief pockmarks the extensive 3d seismic dataset in the lcb offers a unique opportunity to study the plumbing system that is feeding surface cold seep systems and in general to reconstruct the relationship between tectonics and fluid flow in continental margins the fluid seeps in the lcb are associated with morphologically stratigraphically or tectonically controlled focused fluid flow the integration of the geophysical datasets backscatter imagery coupled to 3d seismic clearly indicates that fluid seeps are not randomly distributed but their seabed organization reflects 1 the location of the underlying structure reservoir or trap where the fluids are coming from 2 the geometry and morphology of the reservoir trap and 3 the discontinuities in the sedimentary column along which fluids have migrated in the lcb seafloor pockmarks are always associated with underlying tectonic structures fault zones salt diapirs polygonal faults or buried sedimentary bodies turbiditic channels erosional surfaces whereas they never occur above sub horizontal parallel stratified fine grained sediments even if triggering processes can not be clearly defined here we propose a model of seafloor fluid seep organization which represents a new tool for identifying the geometry of flow pathways and the underlying buried bodies where the fluids are originating from this qualitative 3d model provides insight into the geohydrologic processes of continental margins

a simple model of the response of the atlantic to the north atlantic oscillation the response of an idealized atlantic ocean to wind and thermohaline forcing associated with the north atlantic oscillation nao is investigated both analytically and numerically in the framework of a reducedgravity model the nao related wind forcing is found to drive a time dependent leaky gyre circulation that integrates basinwide stochastic wind ekman pumping and initiates low frequency variability along the western boundary thisissubsequentlycommunicated togetherwith the stochasticvariability induced bythermohaline forcing at high latitudes to the remainder of the atlantic via boundary and rossby waves at low frequencies the basinwide ocean heat content changes owing to nao wind forcing and thermohaline forcing are found to oppose each other the model further suggests that the recently reported opposing changes of the meridional overturning circulation in the atlantic subtropical and subpolar gyres between 1950 70 and 1980 2000 may be a generic feature caused by interplay between the nao wind and thermohaline forcing

the dynamical subtropical front 1 the southern ocean subtropical front stf is thought to play a key role in the global climate system theory suggests that the latitude of the stf regulates the volume of saline agulhas leakage into the atlantic ocean from the indian here we use satellite sea surface temperature sst and sea surface height ssh data to study the physical characteristics of the stf water mass boundary we find that the strong currents in this region do not align with the surface water mass boundary therefore we provide a new climatology for these currents which we define as the dynamical stf dstf the dstf is the eastward extension of the western boundary current in each basin and is characterized by strong sst and ssh gradients and no seasonal cycle at the center of each basin it merges with the sub antarctic front on the eastern side of basins the stf surface water mass boundary coincides with a separate region of multiple sst fronts we call this the subtropical frontal zone stfz the fronts in the stfz have a large seasonal cycle and no ssh signature despite lying close to the same water mass boundary the dstf and stfz are completely unrelated we therefore suggest the term stf only be used when referring to the surface water mass boundary when studying the strong currents on the western side of basins the term dstf is more relevant and similarly the term stfz better describes the region of enhanced sst gradients towards the east

numerical simulation of the biot slow wave in water saturated nivelsteiner sandstone the development of rock acoustic models is important for interpreting seismic data these models should relate seismic properties such as wave velocity and attenuation to the production and lithological properties of reservoir rocks such as porosity permeability fluid type and fluid saturation

wind stress curl and enso discharge recharge in the equatorial pacific discharge and recharge of the warm water volume wwv above the 20 c isotherm in an equatorial pacific ocean box extending across the pacific from 156 e to the eastern ocean boundary between latitudes 5 s and 5 n are key variables in enso dynamics a formula linking wwv anomalies zonally integrated wind stress curl anomalies along the northern and southern edges of the box and flow into the western end of the box is derived and tested using monthly data since 1993 consistent with previous work a wwv balance can only be achieved if the 20 c isotherm surface is not a material surface that is warm water can pass through it for example during el nino part of the wwv anomaly entering the box is cooled so that it is less than 20 c and therefore passes out of the bottom of the box the 20 c isotherm surface the observations suggest that the anomalous volume passing through the 20 c isotherm is approximately the same as t the anomalous wwv entering the western end of the box therefore the observed wwv anomaly can be regarded as being driven by the anomalous wind stress curl along the northern and southern edges of the box the curl anomaly changes the wwv both by divergent meridional flow at the edges of the box and vortex stretching that is the sverdrup balance does not hold in the upper ocean a typical amplitude for the rate of change of wwv for the 5 s 5 n box is 9 6 sv sv 10 6 m 3 s 1 the wind stress curl anomaly and the transport anomaly into the western end of the box are highly correlated with the el nino index nino 3 4 the average sea surface temperature anomaly ssta over the region 5 s 5 n 170 120 w and nino 3 4 leads minus the wwv anomaly by one quarter of a cycle based on the preceding results a simple discharge recharge coupled enso model is derived only water warmer than about 27 5 28 c can give rise to deep atmospheric convection so unlike past discharge recharge oscillator models the west central rather than eastern equatorial sstas are emphasized the model consists of two variables t the ssta averaged over the region of strong enso air sea interaction in the west central pacific equatorial strip 5 s 5 n 156 e 140 w and d the 20 c isotherm depth anomaly averaged over the

deepwater north west borneo hydrocarbon accumulation in an active fold and thrust belt in the deepwater acreage of north west borneo an active offshore fold and thrust belt hosts a number of proven hydrocarbon accumulations and promises to deliver considerable additional hydrocarbon volumes as a result of ongoing exploration campaigns typical trapping geometries observed in this neogene large scale linked fold and thrust belt are hanging wall anticlines foreland folds and ridges and sub thrust footwall cut offs commercial drilling targets lie within the deformed miocene and pliocene sediment pile and these have been charged with oil and gas from a petroleum system that is still active at the present day a major challenge here is to avoid drilling traps that have expelled their hydrocarbons during periods of active deformation and uplift in recent geological time studies of tectonic deformation phases are beginning to illuminate some key controls on trap formation and hydrocarbon retention history this paper focuses on providing an overview of the north west borneo deepwater fold and thrust belt and introduces some key issues and challenges for future hydrocarbon exploration in the region

oil and gas wells and their integrity implications for shale and unconventional resource exploitation data from around the world australia austria bahrain brazil canada the netherlands poland the uk and the usa show that more than four million onshore hydrocarbon wells have been drilled globally here we assess all the reliable datasets 25 on well barrier and integrity failure in the published literature and online these datasets include production injection idle and abandoned wells both onshore and offshore exploiting both conventional and unconventional reservoirs the datasets vary considerably in terms of the number of wells examined their age and their designs therefore the percentage of wells that have had some form of well barrier or integrity failure is highly variable 1 9 75 of the 8030 wells targeting the marcellus shale inspected in pennsylvania between 2005 and 2013 6 3 of these have been reported to the authorities for infringements related to well barrier or integrity failure in a separate study of 3533 pennsylvanian wells monitored between 2008 and 2011 there were 85 examples of cement or casing failures 4 blowouts and 2 examples of gas venting in the uk 2152 hydrocarbon wells were drilled onshore between 1902 and 2013 mainly targeting conventional reservoirs uk regulations like those of other jurisdictions include reclamation of the well site after well abandonment as such there is no visible evidence of 65 2 of these well sites on the land surface today and monitoring is not carried out the ownership of up to 53 of wells in the uk is unclear we estimate that between 50 and 100 are orphaned of 143 active uk wells that were producing at the end of 2000 one has evidence of a well integrity failure

experimental detection of a slow acoustic wave in sediment at shallow grazing angles following recent experimental results at sea n p chotiros proc oceans 89 that suggest the existence of a previously undetected type of acoustic wave in sandy sediments an experiment was designed to detect and measure the speed of acoustic waves in an isolated environment the experiment was conducted in a laboratory tank containing 1 m of unwashed river sediment under a 3 m water column observations were made of the travel time and attenuation of a pulse from an acoustic source located above the water sediment interface to a set of probes below the interface it was observed that at normal incidence the pulse traveled at about 1750 m s while at shallow grazing angles the pulse traveled through the sediment at close to 1200 m s an interesting possible explanation exists in the biot model m a biot j acoust soc am 28 168 191 1956 which predicts a slow acoustic wave in porous materials work funded by onr under noarl management

methane hydrate recycling offshore of mauritania probably after the last glacial maximum to what extent methane liberated from marine hydrate will enter the ocean during a warmer world is unknown although methane release due to hydrate dissociation has been modelled it is unclear whether or not methane will reach the seafloor during a warmer world and therefore contribute to oceanic and atmospheric budgets here we show using a new three dimensional 3 d seismic dataset that some hydrate deposits surround the gas chimneys passing through the hsz bottom water warming since the last glacial maximum lgm is interpreted to cause hydrate dissociation but critically some of the released methane was not vented to the ocean the released gas caused seal failure and free gas entered the hydrate stability zone hsz through vertical gas chimneys to where new hydrate accumulations formed this process is a new evidence for methane recycling and could account in part for the lack of methane in ice core records that cover warming events during the late quaternary this research provides new insight into how methane could be recycled rather than vented during a warmer world

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

the thermohaline modes of the global ocean abstractthe first 2000 m of the global thermohaline structure of the ocean are statistically decomposed into vertical thermohaline modes using a multivariate functional principal component analysi

global teleconnections of meridional overturning circulation anomalies there is a wide range of evidence from both models and palaeoclimatic data that indicates the possibility of abrupt changes in the oceanic meridional overturning circulation moc however much of our dynamical understanding of the moc comes from steady state models that rely upon the assumption of thermodynamic equilibrium and are therefore only valid on millennial time scales here a dynamical model for the global teleconnections of moc anomalies on annual to multidecadal time scales is developed it is based on a linear theory for the propagation of zonally integrated meridional transport anomalies in a reduced gravity ocean and allows for multiple ocean basins connected by a circumpolar channel to the south the theory demonstrates that the equator acts as a low pass filter to moc anomalies as a consequence moc anomalies on decadal and shorter time scales are confined to the hemispheric basin in which they are generated and have little impact on the remainder of the global ocean the linear theory is compared with the results of a global nonlinear numerical integration which it reproduces to a good approximation

multidecadal sea level and gyre circulation variability in the northwestern tropical pacific ocean abstractsea level rise with the trend 10 mm yr 1 has been observed in the tropical western pacific ocean over the 1993 2009 period this rate is 3 times faster than the global mean value of the sea level rise analyses of the satellite altimeter data and repeat hydrographic data along 137 e reveal that this regionally enhanced sea level rise is thermosteric in nature and vertically confined to a patch in the upper ocean above the 12 c isotherm dynamically this regional sea level trend is accompanied by southward migration and strengthening of the north equatorial current nec and north equatorial countercurrent necc using a 1 layer reduced gravity model forced by the ecmwf reanalysis wind stress data the authors find that both the observed sea level rise and the nec necc s southward migrating and strengthening trends are largely attributable to the upper ocean water mass redistribution caused by the surface wind stresses of the recently strengthened atmospheric walker circulation based on the lo

a two level model for the wind and buoyancy forced circulation abstract the classic theory of deep circulation by stommel and arons is extended to a two level model instead of specifying the interfacial upwelling a priori it is calculated as part of the solution the model shows strong upwelling along the southern and eastern boundaries while the upwelling in the interior ocean is much weaker with a medium rate of ekman pumping the basin is separated into western and eastern regions by critical characteristics it is shown that across the boundary between the different regions temperature and velocity are continuous but vorticity or some higher order quantity is discontinuous

a mechanism for the formation of methane hydrate and seafloor bottom simulating reflectors by vertical fluid expulsion bottom simulating reflectors bsr are observed commonly at a depth of several hundred meters below the seafloor in continental margin sedimentary sections that have undergone recent tectonic consolidation or rapid accumulation they are believed to correspond to the deepest level at which methane hydrate clathrate is stable we present a model in which bsr hydrate layers are formed through the removal of methane from upward moving pore fluids as they pass into the hydrate stability field in this model most of the methane is generated below the level of hydrate stability but not at depths sufficient for significant thermogenic production the methane is primarily biogenic in origin the model requires either a mechanism to remove dissolved methane from the pore fluids or disseminated free gas carried upward with the pore fluid the model accounts for the evidence that the hydrate is concentrated in a layer at the base of the stability field for the source of the large amount of methane contained in the hydrate and for bsrs being common only in special environments strong upward fluid expulsion into the hydrate stability field does not occur in normal sediment depositional regimes so bsrs are uncommon upward fluid expulsion does occur as a result of tectonic thickening and loading in subduction zone accretionary wedges and in areas where rapid deposition results in initial undercconsolidation in these areas hydrate bsrs are common the most poorly quantified aspect of the model is the efficiency with which methane is removed and hydrate is formed as pore fluids pass into the hydrate stability field the critical boundary in the phase diagram between the fluid plus hydrate and fluid only fields is not well constrained however the amount of methane required to form the hydrate and limited data on methane concentrations in pore fluids from deep sea boreholes suggest very efficient removal of methane from rising fluid that may contain less than the amount required for free gas production in most fluid expulsion regimes the quantity of fluid moved upward to the seafloor is great enough to continually remove the excess chloride and the residue of isotope fractionation resulting from hydrate formation thus as observed in borehole data there are no large chloride or isotope anomalies remaining in the local pore fluids the differences in the concentration of methane and probably of co2 in the pore fluid above and below the base of the stability field may have a significant influence on early sediment diagenetic reactions

circumpolar response of southern ocean eddy activity to a change in the southern annular mode 1 analysis of satellite altimeter data reveals anomalously high eddy kinetic energy eke in the antarctic circumpolar current acc during the period 2000 2002 around 2 3 years earlier 1998 the circumpolar eastward wind stress as quantified by the southern annular mode sam showed a significant positive peak and we have shown previously that the acc peaked around 1998 in response an eddy resolving ocean model is used to investigate the delay between wind forcing and the eddy response and demonstrates that the lag is due to the time taken to influence the deep circulation of the acc winds over the southern ocean have shown a strong climatic increase over the past few decades if this increase in winds is also reflected as an increase in eddy activity as our analysis suggests it might then the increased poleward heat flux may have played a significant role in the observed warming of the southern ocean citation meredith m p and a m hogg 2006 circumpolar response of southern ocean eddy activity to a change in the southern annular mode geophys res lett 33 l16608 doi 10 1029 2006gl026499

on the north atlantic ocean heat content change between 1955 70 and 1980 95 theupper oceanheatcontentofthenorthatlantichasundergonesignificantchangesoverthelast50years but the underlying physical mechanisms are not yet well understood in the present study the authors examine the north atlantic ocean heat content change in the upper 700 m between the 1955 70 and 1980 95 periods consistent with previous studies the large scale pattern consists of warming of the tropics and subtropics and cooling of the subpolar ocean however this study finds that the most significant heat content change in the north atlanticduring these two time periods is the warmingofthe gulfstream region numerical experiments strongly suggest that this warming in the gulfstream region is largely driven by changes of the large scale wind forcing furthermore the increased ocean heat content inthe gulfstream regionappears tofeedbackon tothe atmosphere resulting in warmer surface air temperature and enhanced precipitation there

an exchange window for the injection of antarctic intermediate water into the south pacific antarctic intermediate water aaiw occupies the intermediate horizon of most of the world oceans formed in the southern ocean it is characterized by a relative salinity minimum with a new denser in situ national oceanographic data center dataset the authors have reanalyzed the export characteristics of aaiw from the southern ocean into the south pacific ocean these new data show that part of the aaiw is exported from the subpolar frontal region by the large scale circulation through an exchange window of 10 width situated east of 90 w in the southeast corner of the pacific basin this suggests the origin of this water to be in the antarctic circumpolar current a set of numerical modeling experiments has been used to reproduce these observed features and to demonstrate that the dynamics of the exchange window is controlled by the basin scale meridional pressure gradient the exchange of aaiw between the southern and pacific oceans must therefore be understood in the context of the large basin scale dynamical balance rather than simply local effects

generation accumulation and resource potential of biogenic gas biogenic gas is generated at low temperatures by decomposition of organic matter by anaerobic microorganisms more than 20 of the world s discovered gas reserves are of biogenic origin a higher percentage of gases of predominantly biogenic origin will be discovered in the future biogenic gas is an important target for exploration because it occurs in geologically predictable circumstances and in areally widespread large quantities at shallow depths in rapidly accumulating marine sediments a succession of microbial ecosystems leads to the generation of biogenic gas after oxygen is consumed by aerobic respiration sulfate reduction becomes the dominant form of respiration methane generation and accumulation become dominant only after sulfate in sediment pore water is depleted the most important mechanism of methane generation in marine sediments is the reduction of co2 by hydrogen electrons produced by the anaerobic oxidation of organic matter co2 is the product of either metabolic decarboxylation or chemical decarboxylation at slightly higher temperatures the factors that control the level of methane production after sediment burial are anoxic environment sulfate deficient environment low temperatu e availability of organic matter and sufficient space the timing of these factors is such that most biogenic gas is generated prior to burial depths of 1 000 m in marine sediments most of the biogenic gas formed can be retained in solution in the interstitial pore waters because of higher methane solubility at the higher hydrostatic pressures due to the weight of the overlying water column under certain conditions of high pressures and or low temperatures biogenic methane combines with water to form gas hydrates biogenic gas usually can be distinguished from thermogenic gas by chemical and isotopic analyses the hydrocarbon fraction of biogenic gas consists predominantly of methane the presence of as much as 2 of heavier hydrocarbons can be attributed to admixture of minor thermogenic gas due to low temperature degradation of organic matter the amounts of hydrocarbon components other than methane generally are proportional to temperature age and organic matter content of the sediments biogenic methane is enriched in the light isotope 12c dgr13c1 lighter than 55 ppt owing to kinetic isotope fractionation by methanogens the variations in isotopic composition of biogenic methane are controlled primarily by dgr13c of the original co2 substrate which reflects the net isotopic effect of both addition and removal of co2 the methane isotopic composition also can be affected by mixing of isotopically heavier thermogenic gas the possible complicating factors require that geologic chemical and isotopic evidence be considered in attempts to interpret the origin of gas accumulations accumulations of biogenic gas have been discovered in canada germany italy japan trinidad the united states and ussr in cretaceous and younger rocks at less than 3 350 m of burial and in marine and nonmarine rocks other gas accumulations of biogenic origin have undoubtedly been discovered however data that permit their recognition are not available

long rossby wave basin crossing time and the resonance of low frequency basin modes the ability of long wave low frequency basin modes to be resonantly excited depends on the efficiency with which energy fluxed onto the western boundary can be transmitted back to the eastern boundary this efficiency is greatly reduced for basins in which the long rossby wave basin crossing time is latitude dependent in the singular case where the basin crossing time is independent of latitude the amplitude of resonantly excited long wave basin modes grows without bound except for the effects of friction the speed of long rossby waves is independent of latitude for quasigeostrophic dynamics and the rectangular basin geometry often used for theoretical studies of the wind driven ocean circulation is such a singular case for quasigeostrophic dynamics for more realistic basin geometries where only a fraction of the energy incident on the western boundary can be transmitted back to the eastern boundary the modes have a finite decay rate that in the limit of weak friction is independent of the choice of frictional parameters explicit eigenmode computations for a basin geometry similar to the north pacific but closed along the equator yield basin modes sufficiently weakly damped that they could be resonantly excited

methane recycling between hydrate and critically pressured stratigraphic traps offshore mauritania three dimensional seismic reflection data from offshore mauritania reveal a base hydrate reflection beneath which are 56 stratigraphically trapped gas accumulations 0 08 gt methane only 2 are sealed by the base of the hydrate 0 004 gt methane and therefore constitute the free gas zone some of the stratigraphically trapped gas accumulations are critically pressured there are also 360 gas chimneys beneath the gas hydrate but not outside the region covered by it they are evidence for repetitious leaking from the accumulations only 12 of the chimneys terminate at a contemporaneous seabed showing that methane venting was not significant instead upward resetting of the base hydrate triggered gas dissociation and caused the traps to become charged and critically pressured vertical leaking to intermediate shallower stratigraphic traps or to the gas hydrate itself where it was reincorporated occurred once the seal capacity was exceeded most 95 of the free gas is stored stratigraphically and re migrates in this way and this trapping mechanism may be typical of fine grained passive margin continental slopes only 5 of the gas is within the free gas zone the role of recycling of methane between the hydrate and stratigraphic traps has not previously been recognized and represents a store for this greenhouse gas that is not susceptible to changes in ambient conditions

seismic modeling of the decollement zone at the base of the barbados ridge accretionary complex wide aperture seismic reflection profiling of the northern barbados ridge accretionary complex at 16 12 n has imaged the decollement horizon across more than 100 km of the accretionary complex revealing prominent changes in the amplitude of the decollement reflection synthetic seismograms from models of the variation of velocity and density with depth were compared with selected seismic traces between 2 5 km seaward of the toe of the accretionary complex and 20 km landward of the toe to determine the changes in velocity density properties along the decollement thrust zone modeling requires a thin low velocity layer to develop landward along the decollement beneath the complex while no such layer exists at the corresponding stratigraphic boundary in the undeformed sediments seaward of the complex two dimensional modeling that incorporated lateral variations in topography velocity and density was performed on a 7 km section of the decollement 20 km landward of the deformation front the best fitting model includes a 20 m thick low velocity layer at the decollement horizon with a relative velocity decrease in the layer of 0 2 km s changes in the reflection amplitude along this 7 km section could be produced by either a change in the velocity anomaly within the layer or a thinning of the decollement layer to less than 4 m the shape of the reflected wavelet from the decollement indicates that the principal cause of the amplitude variation is a change in the velocity contrast rather than a change in thickness the low velocity layer can be explained by a decollement shear zone that has high porosity maintained by elevated pore fluid pressure high fracture permeability within the decollement permits dewatering of sediment attached to the oceanic crust and thrust beneath the accretionary complex

gyres driven by combined wind and buoyancy flux abstract the combined effects of buoyancy forcing and wind in a gyre scale steady ocean circulation are modeled using discrete layers with an interfacial flux not necessarily vertical the equations for vorticity conservation of the geostrophic flow in this system are fully nonlinear involving a jacobian for the layer thicknesses these equations are written in a form which can be solved by the method of characteristics the form of these equations invites the interpretation that the geostrophic baroclinic flow is driven by buoyancy and steered by the wind two examples are solved and discussed a subtropical gyre with heating and a subpolar gyre with cooling in each case there are distinct regimes of flow depending upon whether the characteristics originate at the eastern or western boundaries of the model a simple geometrical argument illustrates that the difference between these two regimes the direct and indirect cells depends upon the sign of the true vertical velocity relative to the interf

a theory for the surface atlantic response to thermohaline variability abstract the response of the upper warm limb of the thermohaline circulation in the north atlantic to a rapid change in deep water formation at high latitudes is investigated using a reduced gravity ocean model changes in deep water formation rate initiate kelvin waves that propagate along the western boundary to the equator on a timescale of months the response in the north atlantic is therefore rapid the southern hemisphere response is much slower limited by a mechanism here termed the ldquo uatorial buffer since to leading order the flow is in geostrophic balance the pressure anomaly decreases in magnitude as the kelvin wave moves equatorward where the coriolis parameter is lower together with the lack of sustained pressure gradients along the eastern boundary this limits the size of the pressure field response in the southern hemisphere interior adjustment is by the westward propagation of rossby waves but only a small fraction of the change in thermohaline circulation strength is commun

heaving modes in the world oceans part of climate changes on decadal time scales can be interpreted as the result of adiabatic motions associated with the adjustment of wind driven circulation i e the heaving of the isopycnal surfaces heat content changes in the ocean including hiatus of global surface temperature and other phenomena can be interpreted in terms of heaving associated with adjustment of wind driven circulation induced by decadal variability of wind a simple reduced gravity model is used to examine the consequence of adiabatic adjustment of the wind driven circulation decadal changes in wind stress forcing can induce three dimensional redistribution of warm water in the upper ocean in particular wind stress change can generate baroclinic modes of heat content anomaly in the vertical direction in fact changes in stratification observed in the ocean may be induced by wind stress change at local or in the remote parts of the world oceans intensification of the equatorial easterly can induce cooling in the upper layer and warming in the subsurface layer the combination of this kind of heat content anomaly with the general trend of warming of the whole water column under the increasing greenhouse effect may offer an explanation for the hiatus of global surface temperature and the accelerating subsurface warming over the past 10 15 years furthermore the meridional transport of warm water in the upper ocean can lead to sizeable transient meridional overturning circulation poleward heat flux and vertical heat flux thus heaving plays a key role in the oceanic circulation and climate

an eddy permitting southern ocean state estimate abstract an eddy permitting general circulation model of the southern ocean is fit by constrained least squares to a large observational dataset during 2005 06 data used include argo float profiles ctd synoptic sections southern elephant seals as oceanographic samplers seaos instrument mounted seal profiles xbts altimetric observations envisat geosat jason 1 and ocean topography experiment topex poseidon and infrared and microwave radiometer observed sea surface temperature an adjoint model is used to determine descent directions in minimizing a misfit function each of whose elements has been weighted by an estimate of the observational plus model error the model is brought into near agreement with the data by adjusting its control vector here consisting of initial and meteorological boundary conditions although total consistency has not yet been achieved the existing solution is in good agreement with the great majority of the 2005 and 2006 southern ocean observations and better repr