resistivity structure of the upper crust in iceland the results of numerous resistivity soundings from all parts of iceland are analyzed the resistivity and the p wave velocity of the uppermost kilometer in the crust show a strong correlation with the geological structure of iceland by estimating porosity from p wave velocity archie s law is found not to be valid in general the temperature dependence of nine water saturated basaltic rock samples was investigated in a laboratory showing stronger temperature dependence than expected from pore fluid conduction these two facts suggest that interface conduction is the most important conduction mechanism a relation between bulk resistivity fracture porosity pore fluid resistivity and temperature is established comparison of regional bulk resistivity with different zeolite zones shows that the resistivity is mainly a function of the maximum depth of burial of the lava pile emphasizing the importance of clay minerals and zeolites for the measured bulk resistivity

shifts of the plate boundary in iceland some aspects of tertiary volcanism this paper points out five anomalies of the icelandic upper crust that are not compatible with currently accepted construction models these anomalies are 1 assumption of a single time averaged plate boundary 2 irregular depth distribution to seismic layer 3 3 the assumption of a uniform vertical compositional zonation 4 the excess width of iceland and 5 abnormal variations in the duration and stratigraphic thickness of chron 5 time lavas in western northern and eastern iceland a model involving frequent shifts of the volcanic zones in iceland is outlined that suggests a way to account for the above structural anomalies the model also suggests that detailed studies of basalt lava facies types may be used effectively to trace small scale shifts of the tertiary plate boundary

the iceland research drilling project this issue of the journal of geophysical research contains the collected results of the initial studies of a 3 km vertical section of icelandic crust the material described in these papers was largely presented at a meeting of the investigators working in this project held in reykjavik iceland from may 13 to may 15 1980 iceland presents a very well exposed example of crust formed at an accretional plate margin the crest of the mid atlantic ridge the desirability of a detailed study of a long vertical section of icelandic crust by deep drilling has been expressed in proposals and recommendations of scientists and international scientific committees since the early 1960 s a formal proposal with descriptions of several alternative drill sites was presented in 1975 working group on deep crustal drilling in iceland 1975 but difficulties in financing a drill hole that would penetrate into crustal layer 3 vp 6 5 km s delayed implementation of the recommendations

crustal structure and magmatism of north atlantic continental margins enormous variations exist along rifted margins of the north atlantic in the magmatic activity that accompanied continental breakup the north atlantic margins exhibit a full range of magmatic behaviour one end of the spectrum commonly termed volcanic margins is found in the northern north atlantic off greenland norway and northwest britain where huge volumes of igneous rock were added to the continental crust as it rifted and the adjacent oceanic crust is considerably thicker than normal the other end of the spectrum so called non volcanic margins occurs further south off france and spain where only minor volcanism accompanied rifting and the oceanic crust immediately adjacent to the continental margin is thinner than normal oceanic crust results from seismic profiles are used to determine the volume and distribution of igneous crust accumulated as the continents broke up and seafloor spreading commenced the volume of melt produced during rifting is controlled mainly by the temperature of the underlying asthenospheric mantle and by the amount and rate of decompression as it rises beneath stretching and thinning lithosphere large quantities of igneous rock were produced as the continents broke up to form the north atlantic where the newly initiated iceland plume brought abnormally hot mantle beneath the rift much smaller quantities of melt were produced further south where mantle temperatures were normal during rifting the distribution of igneous rocks across the continental margin and the adjacent oceanic crust is controlled by the rate at which melt is produced and by the ease with which it can intrude laterally in the crust where the rate of stretching is very slow and the rifting is spread over many millions or even tens of millions of years as is common during the initial stages of rifting then the rising asthenospheric mantle has time to cool by conduction heat is lost from the asthenospheric mantle both vertically into the overlying water layer and laterally into the colder wedges of continental lithosphere on either side of the rifted region as a consequence of the reduced mantle temperatures considerably less partial melting occurs in the slowly rising asthenospheric mantle than in mantle which rises rapidly beneath rifts this explains the abnormally thin oceanic crust found adjacent to the non volcanic rifted continental margins in the north atlantic as the new ocean widens the spreading centre becomes isolated from the continental margins on either side by newly formed warm oceanic lithosphere and the rate of rifting generally increases the oceanic crust then attains its equilibrium thickness as conductive heat loss from the rising asthenospheric mantle reduces and all the melt formed by decompression of the mantle solidifies close to the spreading axis basaltic melts can intrude continental crust far more easily than oceanic crust basaltic dykes may extend many hundreds of kilometres away from the magma source beneath continental rifts this redistributes melt laterally towards the continent in both volcanic and to a lesser extent in non volcanic margins and may be a contributory factor in leaving unusually thin crust in the first formed ocean immediately adjacent to some rifted margins it also allows melt formed under volcanic rifts to migrate large distances laterally as is seen in the rockall area northwest of britain

seismic attenuation of atlantic margin basalts observations and modeling paleogene basalts are present over much of the northeastern atlantic european margin in regions containing significant thicknesses of layered basalt flows conducting seismic imaging within and beneath the volcanic section has proven difficult largely because the basalts severely attenuate and scatter seismic energy we use data from a vertical seismic profile vsp from well 164 07 1 that penetrated 1 2 km of basalt in the northern rockall trough west of britain to measure the seismic attenuation caused by the in situ basalts the effective quality factor qeff of the basalt layer is found from the vsp to be 15 35 which is considerably lower more attenuative than the intrinsic attenuation measured on basalt samples in the laboratory we then run synthetic seismogram models to investigate the likely cause of the attenuation full waveform 1d modeling of stacked sequences of lava flows based on rock properties from the same well indicates that much of the seismic attenuation observed from the vsp can b

low frequency modulation of intraseasonal equatorial kelvin wave activity in the pacific from soda 1958 2001 intraseasonal equatorial kelvin wave activity iekw at a low frequency in the pacific is investigated using the simple ocean data assimilation soda oceanic reanalyses a vertical and horizontal mode decomposition of soda variability allows estimation of the kelvin wave amplitude according to the most energetic baroclinic modes a wavenumber frequency analysis is then performed on the time series to derive indices of modulation of the iekw at various frequency bands the results indicate that the iekw activity undergoes a significant modulation that projects onto baroclinic modes and is not related in a straightforward manner to the low frequency climate variability in the pacific linear model experiments corroborate that part of the modulation of the iekw is tightly linked to change in oceanic mean state rather than to the low frequency change of atmospheric equatorial variability

volcaniclastic rocks of the reydarfjordur drill hole eastern iceland 1 primary features a 1918 m deep bore hole in a late miocene shield basalt central volcano mixed sequence contains 58 stratigraphically distinct volcaniclastic interbeds constituting 8 8 of the extrusive part of the section textural mineral chemical and bulk chemical criteria are applied to identify the original nature composition and origin of these rocks which have been metamorphosed by hydrothermal alteration between 100 c top to ca 300 c base of the section units range from a few centimeters to 18 m excluding a 30 m thick ignimbrite with 75 of the units being less than 1 m thick formerly vitric to tachylitic shards and lapilli lithic fragments and crystals are the main components most thin volcaniclastic units are basaltic vitric to tachylitic tuffs thin felsic air fall tuffs a few millimeters to a few centimeters thick occur as separate layers and interbedded with basaltic tuffs nearly all thicker units are made up dominantly of felsic vitric to lithic clasts and crystals phenocrysts common only in felsic air fall tuffs are clinopyroxene ranging from augite to ferrohedenbergite and plagioclase ranging from an80 20 both of which span the entire range of compositions previously known from iceland bulk chemical analyses of major and 11 trace elements of 65 samples allow texturally indistinct rocks to be compositionally identified even though chemical changes due to mixing and alteration are pronounced in many rocks the rhyolitic ignimbrite cooling unit is composed of four flow units the upper most being slightly more mafic in composition because of admixture of a basaltic component the two central flow units show extensive high temperature devitrification with lensoid tridymite rimmed cavities now laumontite filled representing collapsed pumice lapilli the composition of the clastic units corresponds in a general way to that of the interbedded lava flows these data and evidence for weathering and reworking in some units are used to reconstruct the volcanic history of the area phase a 1918 1306 m dominantly mafic shield basalts contains only few and mostly basaltic tuffs except for sparse rhyolitic air fall tuff from distant central volcanoes phase b 1306 653 m composed dominantly of evolved basalts and icelandites culminating with the rhyolitic ignimbrite 920 950 m probably represents the eastern flank of a nearby central volcanic complex whose center may be situated less than 10 km westward downdip the upper extrusive 500 m of the section phase c are basalt lavas of average tholeiitic composition containing abundant chiefly felsic and reworked clastic units in the interval between 286 and 410m these are tentatively interpreted to reflect a phase of structural basin forming not compensated by extrusive volcanic activity allanite phenocrysts were found for the first time in icelandic rocks and the host pumice tuffs may constitute an important stratigraphic marker horizon that may help to tie the drilled section to the subaerial volcanic series in this part of eastern iceland the alteration of the clastic layers is discussed in a companion paper

deep crustal reflectors under reydarfjordur eastern iceland crustal accretion above the iceland mantle plume summary deep seismic reflection profiling on the faroe iceland ridge experiment fire imaged a sequence of dipping events in the upper to crust beneath the tertiary lava pile in reydarfjordur eastern iceland these events dip towards the spreading centre at which the crust was formed and continue down to 6 s two way time corresponding to depths down to 15 18 km similar crustal reflections have been observed previously within the crust in other areas of iceland although the reflective events located by previous surveys have not been interpreted rigorously in this study we calculate synthetic seismograms from modelled sequences of lava flows to examine the reflective character of a lava le the synthesized lava pile sections are based on field measurements made in the reydarfjordur area the 1 km vertical exposures on the sides of the fjord are composed of lava flows within stratigraphic units which have a range of compositions and some intercalated tuffaceous clastic horizons we compare the synthetic seismograms with our seismic reflection data to test the interpretation that the deep dipping reflections are caused by a gently dipping sequence of subaerially erupted lavas we conclude that the upper and mid crustal reflections that we observe can be interpreted as lava flow sequences similar to those seen at the surface with the main reflections resulting from packets of flows of similar thicknesses the reflectors that we have imaged beneath iceland are similar to those defining the seaward dipping reflectors on the volcanic continental margins and we therefore suggest that the seaward dipping reflections commonly result from packets of lavas with similar distributions of thicknesses the occurrence of lavas down to 15 18 km beneath reydarfjordur can be explained partly by multiple episodes of accretion of the upper crust although the volcanic processes forming the crust in iceland generally lead to advection of extrusive material to mid crustal levels the production of an average extrusive layer totalling 15 km thickness is within reasonable estimates of the current volcanic productivity the advection of frozen lava flows to mid crustal levels contributes to the maintenance of the subsolidus crust that is observed on iceland and the zeolite minerals within the extrusive sequence may be responsible for the crustal low resistivity zone that has been observed using magnetotelluric techniques

interface scattering versus body scattering in subbasalt imaging and application of prestack wave equation datuming the presence of high velocity highly heterogeneous layers such as basalt have a detrimental effect on imaging below those structures internal characteristics of the basalt itself can also degrade the subbasalt image quality in many cases the boundary surfaces of a basalt sequence are not smooth but contain some roughness at a scale similar to the seismic wavelength interface scattering resulting from interface roughness can be a serious problem in this paper body and interface scattering problems are addressed through acoustic finite difference modeling interface scattering seems to have the most detrimental effect on imaging at depth a wave equation prestack datuming procedure is applied to synthetic acoustic data to remove the effects of rough interfaces this computational process transforms seismic data to a new datum plane in the subsurface eliminating propagation effects between the surface and the new datum plane the technique leads to a much improved image of below the basalt with the g

poisson s ratio of a seaward dipping reflector series hatton bank summary seismic reflection profiles across the hatton bank continental margin in the eastern north atlantic have imaged thick sequences of seaward dipping reflectors converted shear waves propagating through one of these sequences were recorded on a two ship expanding spread profile just landward of the ocean continent transition synthetic seismogram modelling of these shear waves indicates that poisson s ratio in the sequence is 0 27 0 01 similar to that of normal oceanic layer 2

equatorial wave sequence associated with warm pool displacements during the 1986 1989 el nino la nina in the pacific equatorial band the zonal displacement of the eastern edge of the warm pool subject to insignificant seasonal variations is dominated by strong interannual variations almost in phase with the southern oscillation index over the 1971 1973 el nino la nina period gill 1983 suggested that such displacement was caused solely by horizontal advection by zonal current anomaly basin wide geosat derived zonal surface current anomalies are available during the 1986 1989 period and they agree quite well with observed equatorial currents in the western and central pacific within the 4on 4 s equatorial band the cumulative contribution of first baroclinic kelvin and first symmetric m 1 rossby modes to surface zonal equatorial current anomalies account for most of the geosat derived current variability from these geosat derived currents it is demonstrated that the eastward westward displacement of the eastern edge of the warm pool was primarily due to horizontal advection by zonal current anomalies during the 1986 1987 el nino 1988 1989 la nina as a corollary the el nino la nina warm cold sea surface temperature sst anomaly in the central western pacific was the result of anomalous zonal advection the 1986 1989 displacement of the warm pool appeared as a low frequency movement resulting from high frequency forcing i e a succession of local wind forcing and its remote kelvin and m 1 rossby wave responses in the equatorial waveguide in the central western pacific a in 1 downwelling rossby wave issued from the eastern pacific shifted the displacement from eastward to westward and therefore seems to have been the cause for el nino to turn to la nina in boreal fall 1987 in the eastern pacific the simultaneous occurrence of free propagating downwelling upwelling kelvin waves with the seasonal sst warming cooling appeared necessary for the development of local el nino la nina sst anomaly over most of the equatorial pacific slow migration of sst anomalies clearly showed up distinct during el nino and la nina these zonal migrations appeared to result from the delayed association of two phenomena the zonal advection in the central western pacific and the arrival of downwelling upwelling kelvin waves into the eastern pacific synchronous with the seasonal sst warming cooling albeit different in terms of equatorial wave action as compared to the delayed action oscillator theory our results suggest that zonal advection and wave dynamics were both important for the generation and migration of el nino southern oscillation sst anomalies in 1986 1989 finally our observational results about warm pool displacements equatorial wave sequence the shift from el nino to la nina and the slow sst migrations are presented in a schematic form involving sequential ocean atmosphere coupling which appears continuously over the november 1986 to a

average lithospheric structure of southern africa summary we present a new 1 d earth model for the crust and upper mantle beneath the archaean core of southern africa the crustal structure is constrained by published seismic refraction reflection data and by the modelling of teleseismic receiver function data the mantle structure is constrained by traveltime measurements for p and s waves by waveform inversion of multimode surface waves and by forward modelling of p waveforms for regional earthquakes the 42 km thick crust of southern africa consists of four layers a 2 km thick 5 05 km s surface layer a 5 km thick 6 08 km s upper crust a 20 km thick 6 30 km s l mid crust and a 15 km thick 6 73 km s lower crust below the crust there is an 80 km thick upper mantle lid v and v are 8 09 and 4 62 km s l respectively at the top of the lid and the compressional and shearvelocity gradients through the lid are 0 0008 s l and 0 0013 s respectively below the lid there is a substantial shear wave lvz with the shear wave velocity dropping from 4 72 km s l in the lower part of the lid to 4 32 km s at 250 km depth the shear wave lvz is required to fit both the v measurements and the regional surface waveforms forward modelling of p suggests that no upper mantle p wave lvz exists there is an increase in the p wave velocity gradient at 125 km depth to 0 0015 s and a second increase at 250 km depth to 0 0035 s velocities and densities in the seismic model of the lid agree closely with velocity and density estimates from the geochemical analysis of garnet periodotite nodules from kimberlites of the kaapvaal craton implying that the estimates from kirnberlites are representative of a wider region of the archaean core of southern africa not just of the restricted region beneath which the kimberlites are found a comparison of the southern african seismic model with seismic models for other shield regions where nodule data are not available shows that the nodule results may also be representative of the upper mantle in those regions

seismic response and construction of seaward dipping wedges of flood basalts voring volcanic margin geological and geophysical data from the 900 m of volcanic basement drilled at ocean drilling program site 642 provide the framework for studying seismic properties of huge extrusive constructions on volcanic margins the main part of the drilled section corresponding to a prominent seaward dipping reflector sequence consists of subaerially emplaced tholeiitic basalt flows and thin interbedded sediments the basalts exhibit a characteristic velocity and density lava flow distribution reflecting changing porosity pore aspect ratio distribution and alteration stacks of laterally continuous basalt flows appear to have thin layer transverse isotropic properties for typical wavelengths in multichannel seismic data vertical seismic profiling and average sonic log velocities are similar 3 77 km s and 3 88 km s respectively while comparable refraction velocities are 10 20 higher synthetic seismogram modeling based on downhole logs shows that basement reflectors originate from interference and tuning effects of numerous basalt flow and interbedded sediment interfaces though the most continuous reflectors are related to thick flows seismic models based on the characteristic velocity and density basalt flow distributions and site 642 stratigraphy show that reflector truncation and onlap may be caused by seismic interference phenomena in a sequence of landward thinning flows the base of the dipping reflector sequence reflector k correlates with flows in the lower part of upper series basalts while the transition from basaltic to underlying dacitic andesitic lavas correlates with a locally defined reflector a model for the breakup related volcanism includes 1 prebreakup dacitic andesitic volcanism 2 early breakup basaltic volcanism infilling the prebreakup relief 3 main breakup stage with intense focused volcanism large subsidence and lava pounding and 4 late breakup volcanism during a period of decreased subsidence and local off axis activity

sensitivity of an intermediate ocean atmosphere coupled model of the tropical pacific to its oceanic vertical structure an intermediate ocean atmosphere coupled model icm of the tropical pacific is used to investigate the sensitivity of the coupled system to the oceanic vertical structure the model consists of a three baroclinicmode tropical pacific ocean and a gill tropical atmosphere the mixed layer is similar to the zebiak and cane zc coupled atmosphere ocean model but uses a different parameterization for subsurface temperature derived from xbt data a 100 yr simulation is made and the long term variability of the icm is briefly discussed and compared with the the observations and the zc model the icm reproduces regular enso events with a spectrum peak centered at the period 3 5 yr the presence of higher order vertical modes in the oceanic component of the model allows for reduced off equatorial variability of zonal current and thermocline anomalies and for finer meridional scale for sst anomaly variability in comparison with a simulation with one baroclinic mode only the nature of the processes sustaining the oscillations in the icm is then investigated based on the coupled instabilities and delayed oscillator theories it

in situ evidence for the nature of the seismic layer 2 3 boundary in oceanic crust the igneous oceanic crust is typically thought of as comprising two layers an upper crust seismic layer 2 characterized by a rapid increase in seismic velocity with depth and a thicker lower crust seismic layer 3 which is distinguished from layer 2 by both a higher p wave velocity 6 69 0 26 km s l and a much smaller vertical velocity gradient 1 8 km continuous hole drilled into the oceanic crust11 13 and find that at this location the seismic layer 2 3 boundary lies within the sheeted dyke complex where it is associated with gradual downhole changes in crustal porosity and alteration not a litho logical transition from sheeted dykes to gabbro

the mode of emplacement of neogene flood basalts in eastern iceland facies architecture and structure of the holmar and grjota olivine basalt groups abstract holmar and grjota are two stratigraphically distinct transitional alkaline olivine basalt lava groups within the westward dipping neogene flood basalts of eastern iceland the holmar olivine basalt group separated from the overlying grjota olivine basalt group by only a few tholeiite flows can be traced over 80 km north south with thicknesses varying from 250 m where thickest to 30 m where thinnest the grjota group can be traced over 50 km also north south reaching thicknesses of 250 m and thinning down dip to 10 m in contrast to other groups in eastern iceland that thicken down dip the studied olivine basalt groups thicken up dip the groups filled topographic confinements and formed aprons around central volcanoes we have estimated the minimum volumes to be 119 km 3 for holmar and 86 km 3 for grjota scoria cones are found in the holmar group and two thick olivine dolerite sills cross cut the holmar group and probably belong to the plumbing system that fed the grjota group the architecture of the lava groups are near identical the architecture is compound with lobes stacked horizontally and vertically varying from 1 15 m thick and 2 200 m long but do also encompass a number of thicker 15 20 m and more extensive 1 km long lava lobe in the stacks filled lava tubes are commonly observed within the lava flows the constituent lobes of the flows are often directly emplaced or welded together suggesting rapid buildup but are also found interbedded with redbeds and thicker tuff deposits and occasionally preserve tree molds the internal structure follows the characteristics for lava lobe morphology in general with an upper vesicular crust forming half to one third of the total thickness a massive core with abundant vesicle cylinders and a thin basal vesicular crust flow tops are of the pahoehoe type seldom found with scoria or clinker inflation structures such as tumuli and inflation clefts were identified in the flows the eruptions were dominantly fissure fed with a few vents evolving to short lived point source volcanism the pre existing topography exerted control on the advancement of the flows and influenced the final architecture of the groups at each location it is evident that the large and dense spatial distribution of these groups including numerous thick lava units suggests large volcanic episodes together the groups imply a period with higher production of olivine basalts in iceland and form well defined stratigraphic markers the transitional alkaline olivine basalt groups are interpreted to delineate a fossil southward propagating rift flank volcanic zone located east of a now extinct and buried mature rift zone

the internal structure of oceanic plateaus inferences from obducted cretaceous terranes in western colombia and the caribbean although the structure of mantle plume derived oceanic plateaus has recently been assessed using remote geophysical techniques combined with petrological modelling it is nevertheless desirable to test whether these theoretical rock types dunites gabbros and basalts actually exist and to establish their geochemical nature oceanic plateaus may have initially formed above or near sea level during a short vigorous pulse and thereafter commonly subside to abyssal depths as the lithosphere cools thus making sampling of their deeper levels extremely difficult however the cretaceous age colombian caribbean oceanic plateau was partially accreted against the south american continent so making the imbricated segments available for study during the process of plateau accretion and imbrication it is predominantly the basaltic layers which are obducted but parts of the sequence down to layered and banded gabbros with associated pyroxenites and dunites sometimes foliated can be exposed where the imbricate thrusting brings up deeper levels most of the upper crustal sequence in western colombia is composed of basaltic pillowed and massive flows and sills that are chemically uniform and undepleted relative to normal mid ocean ridge basalts komatiites and more abundant picrites are found at intervals and appear to occur near the base of the sequence in these zones both depleted and moderately enriched basalt and komatiite compositions occur and may result from dynamic partial melting and mixing processes associated with the high temperature part of the plume it is possible to integrate these compositional characteristics into a general model for oceanic plateau structure where the rate of magma supply is in excess of that which can be accommodated by normal spreading processes thus leading to extrusion of flows and the emplacement of sills and high level magma chambers but relatively few dykes the dense and chemically heterogeneous ultramafic magmas intrude the base of the pile and undergo fractionation to form ultramafic cumulates whereas the well mixed basaltic magmas are erupted to form the homogeneous plateau basalts based on these observations a possible structure of the caribbean colombian plateau is proposed which is compatible with geophysical models for other less well exposed oceanic plateaus

vertically propagating annual and interannual variability in an ogcm simulation of the tropical pacific ocean in 1985 94 abstract the annual and interannual variability in the subthermocline equatorial pacific is studied in a simulation of the tropical pacific for the 1985 94 decade using a primitive equation high resolution model the study focuses on temperature variability and vertical energy fluxes similar to the observations made by w s kessler and j p mccreary the annual harmonic of vertical isotherm displacements presents phase lines sloping downward from east to west with off equatorial maxima estimates of zonal and vertical phase speeds the location of the maxima of isotherm displacements and an analysis of ernergy fluxes suggest the presence of l 1 rossby waves the simulated field is somewhat more trapped toward the equator than the observations a linear simulation is carried out for the vertical standing modes with characteristics derived from the ogcm simulation in the linear simulation high order meridional mode rossby waves are more prominent than in the ogcm simulation however the l 1 ros

evaluation of topex and basin wide tropical ocean and global atmosphere tropical atmosphere ocean sea surface topographies and derived geostrophic currents fields of topex derived sea level anomalies are validated against and compared with fields of tropical ocean and global atmosphere tropical atmosphere ocean toga tao dynamic height anomalies for the first 470 days of the topex poseidon mission at periods longer than 35 days topex sea level anomalies compare extremely well with toga tao dynamic height anomalies throughout the whole basin with a mean correlation of 0 79 and mean rms differences of 2 6 cm zonal geostrophic current anomalies are then derived from both data sets and validated against in situ current meter measurements at the equator topex derived geostrophic current anomalies at the equator are in good agreement with in situ currents at 0 165 e 0 170 w 0 140 w and 0 110 w toga tao derived current anomalies compare well with in situ currents at 0 110 w 0 140 w and 0 170 w but poor comparisons at 0 165 e are thought to be linked to the absence of instantaneous salinity measurements in the dynamic height calculation despite localized discrepancies basin wide toga tao geostrophic currents compare reasonably well with topex derived geostrophic currents the discrepancies in the comparison between topex and tao are shown to be mostly associated with gaps in toga tao space time sampling and subsequent gridding procedures

seismic attenuation in the crust at the mid atlantic plate boundary in south west iceland summary very little seismic attenuation occurs in the mid to lower crust of south west iceland the lowest path averaged quality factor for a wave turning in the mid to lower crust 12 20 km is qp 110 for p waves and q5 250 for s waves with most of the data having higher values typically qp 200 300 and qr 400 600 attenuation estimates based on a 1 d layered inversion give correspondingly high values qp 800 and q 800 2000 these q values are inconsistent with thermal models that predict a broad 100 km wide region of above solidus temperatures centred on the volcanic zones the observed attenuation implies an upper limit for mid to lower crustal temperature in the 700 775 c range assuming a gabbroic lithology much higher attenuation qp 60 qr 100 occurs in the uppermost 4 km of crust this is most likely apparent attenuation caused by strong near surface seismic heterogeneity resulting from fissures faults and extreme changes in porosity up to 20 30 per cent the quality factor of the near surface layer varies regionally and is lower in volcanic zones than in either the reykjanes peninsula or the south iceland lowland

evidence of boundary reflection of kelvin and first mode rossby waves from topex poseidon sea level data the topex poseidon sea level data lead to new opportunities to investigate some theoretical mechanisms suggested to be involved in the el nino southern oscillation phenomenon in the tropical pacific ocean in particular we are interested in studying the western boundary reflection a process crucial for the delayed action oscillator theory by using the topex poseidon data from november 1992 to may 1995 we first projected the sea level data onto kelvin and first mode rossby waves then we estimated the contribution of wind forcing to these waves by using a single baroclinic mode simple wave model forced by the ers 1 wind data wave propagation was clearly observed with amplitudes well explained by the wind forcing in the ocean interior evidence of wave reflection was detected at both the western and eastern boundaries of the tropical pacific ocean at the eastern boundary kelvin waves were seen to reflect as first mode rossby waves during the entire period the reflection efficiency in terms of wave amplitude of the south american coasts was estimated to be 80 of that of an infinite meridional wall at the western boundary reflection was observed in april august 1993 in january june 1994 and later in december 1994 to february 1995 although the general roles of these reflection events in the variability observed in the equatorial pacific ocean are not clear the data suggest that the reflections in january june 1994 have played a role in the onset of the warm conditions observed in late 1994 to early 1995 indeed during the january june 1994 period as strong downwelling first mode rossby waves reflected into downwelling kelvin waves easterly wind and cold sea surface temperature anomalies located near the date line weakened and eventually reversed in june july 1994 the presence of the warm anomalies near the date line then favored convection and westerly wind anomalies that triggered strong downwelling kelvin waves propagating throughout the basin simultaneously with the beginning of the 1994 1995 warm conditions

asphalt volcanoes as a potential source of methane to late pleistocene coastal waters natural petroleum seepage emits large volumes of oil and methane to the oceans every year accompanied by the formation of asphalt volcanoes on the sea floor the discovery of seven asphalt volcanoes off the coast of southern california may help to explain high methane emissions recorded during the late pleistocene

the accretion of oceanic crust by episodic sill intrusion seismic refraction data from mature oceanic regions show remarkable consistency in crustal thickness between sites yet the characteristics of different spreading centers show great variation this leads to the paradox that the similarity in mature oceanic crust suggests that the igneous processes in crustal accretion are the same across a range of spreading rates but that the differences in the spreading centers themselves imply the opposite we have developed a simple two dimensional model for crustal accretion at oceanic spreading centers by the episodic addition of sills at a high level within the crust as implied by recent geophysical studies of spreading centers using our model we have calculated velocity and temperature fields in the crust including the effects of hydrothermal cooling we show that oscillations in the temperature field due to episodicities of 200 20000 years have an effect localized to the region within 50 500 m of the intrusion by assuming that all latent heat and excess specific heat introduced by the injection of sills is convected away by hydrothermal circulation we estimate that the maximum heat transfer associated with hydrothermal cooling in the upper crust is of the order of 10 times greater than the conductive flux we have calculated seismic velocities associated with the model temperature field in the crust and we show that these are consistent with results from both seismic refraction and reflection experiments the model may be applied to both fast and slow spreading ridges and it accounts for the apparent increases in both the mean crustal temperature and the frequency of presence of melt with increasing spreading rate hence resolving the paradox we also show that the implications of this model are consistent with geological observations of layering in ophiolites

anomalous warming in the indian ocean coincident with el nino the topex poseidon altimeter has provided further evidence that interannual warming occurs in the indian ocean with a frequency similar to that of el nino in the pacific and has yielded important clues to the dynamics driving the warming the signal is especially strong during the 1997 el nino the altimeter observes long waves which move westward from the southeastern indian ocean at about the same time as westwardly wind anomalies appear in the east central portion of the basin the sea level peaks in the southwestern indian ocean and causes a sea level variation signal that is a near mirror image of el nino in the eastern pacific sea surface temperature data also show a similar correlation an analysis of the altimeter data indicates significant variability in the indian ocean during the 1994 and 1997 el nino events at the first and second baroclinic rossby wave modes sea surface temperature and wind data suggest that the indian ocean warming has occurred during several previous el nino events particularly during the large events of 1982 and 1987 based on these observations it is suggested that the warming begins with wind forced rossby waves in the southeastern indian ocean associated with the southern oscillation similar to the forcing of kelvin waves which precede el nino in the pacific

imaging of intrabasalt and subbasalt structure with full wavefield seismic tomography 1 the imaging of subsurface structure using conventional seismic reflection methods is problematic across areas where high velocity basaltic rocks are intercalated with sedimentary layers of low velocity in this study we use an approach that combines wavefield inversion of both normal incidence and wide aperture seismic data to constrain velocity structure within and beneath the basalt our field data are a two ship long offset profile acquired in the faeroe basin a conventional traveltime inversion of the wide angle data is performed to obtain a starting model for the wavefield inversion synthetic tests are used to explore resolution and to determine a strategy for inversion of the field data our final velocity model shows a high velocity basaltic layer underlain by an intermediate velocity layer that is presumably sedimentary the basalts appear highly heterogeneous in velocity and the degree of heterogeneity increases significantly with depth within the layer a prestack depth migration using the final velocity model reveals a number of new features despite the velocity heterogeneity within the basalts strong reflections are seen within and immediately beneath the basaltic section and these reflectors are more clearly imaged on the final depth migrated section than hitherto the transition from sediments to basalt does not appear to be sharp uniquely defined or locally subhorizontal the data suggest that velocity heterogeneity and the complexity of the transition from basalt to sediment may explain the difficulties of imaging beneath basalts west of britain wavefield tomography can resolve this complexity and hence improve both imaging and confidence of interpretation within and beneath basalts

tropical pacific baroclinic mode contribution and associated long waves for the 1994 1999 period from an assimilation experiment with altimetric data an ocean general circulation model ogcm of the tropical pacific in which combined topex poseidon and ers sea level anomalies are assimilated over january 1994 through july 1999 is used to investigate equatorial wave characteristics during the intense 1997 1999 el nino la nina event near the equator the linear vertical modes are estimated at each grid point of the ogcm simulation with and without assimilation consistently with an increase of the vertical gradient within the thermocline and a rise of the thermocline depth in the eastern basin the assimilation results in an increased contribution of the higher order baroclinic modes in the eastern basin and a decreased contribution of the first baroclinic mode in the western pacific for the zonal current variability for pressure the first baroclinic mode contribution is reduced whereas the higher order modes contribution is weakly impacted kelvin and first meridional rossby waves are then derived for the first two more energetic baroclinic modes in the simulation with assimilation kelvin waves of both modes constructively contribute to the strong warming observed in 1997 with the first second baroclinic mode being more energetic than the second first baroclinic mode in the early mature stage of the warming kelvin waves of both modes reflect as first meridional rossby waves at the eastern boundary reflection efficiency of 95 and contribute to push back the warm pool westward the reversal of the warming is apparently initiated by the second baroclinic mode contribution which controls the position of the 28 c isotherm in the surface layer in the far eastern pacific from january 1998 at the western boundary reflections of rossby waves take place for both modes with an estimated total efficiency of 50 at 165 e this suggests that in our model the delayed oscillator theory is not applicable for explaining the reversal from warm to cold conditions in 1998 while the zonal advective feedback was at work more generally the study suggests that it is necessary to take into account the vertical structure of the ocean when interpreting altimetric data which can be done through an assimilation experiment

megaplume bubble process visualization by 3d multibeam sonar mapping abstract multibeam echosounder data were collected during a surface ship survey of the 22 4b well site in the north sea in september 2011 using a teledyne reson 7125 modern multibeam echosounders are instrumental in providing detection and accurate localization of weak to strong bubble plumes two survey profiles effectively insonified the bubble plumes rising from the main crater at the well site providing snapshot data of bubble plume processes additionally three profiles insonified bubble plumes rising from in and to the south of a secondary crater 1 2 km southeast of the main crater data processing included a simple algorithm that muted mislocated echoes from incomplete sidelobe suppression the data processing produced a cartesian volume of echo intensity from the water column and seabed plume geometry was analyzed to investigate a number of important large scale plume processes including plume bubble detrainment due to currents and stratification downwelling flows sea surface interaction plume heterogeneity and other fluid transport processes the data showed strong upwelling flows with bubble vertical motions generally much faster than currents one important finding was that megaplumes create intrusions above the general thermocline in part because their extensive upwelling flow lifts the thermocline locally as a result the intrusion layer deposits dissolved gases in the upper wave mixed layer of the water column where it is not isolated from the atmosphere unlike dissolved gases in the lower water column the analysis shows that high fidelity multibeam echosounder data can provide a wealth of remote sensing information on bubble plume characteristics and processes with important applications including blowout monitoring and response better understanding of megaplumes such as used in lake destratification and improved characterization of natural seep emission processes

estimates of methane output from mud extrusions at the erosive convergent margin off costa rica four mud extrusions were investigated along the erosive subduction zone off costa rica active fluid seepage from these structures is indicated by chemosynthetic communities authigenic carbonates and methane plumes in the water column we estimate the methane output from the individual mud extrusions using two independent approaches the first is based on the amount of ch4 that becomes anaerobically oxidized in the sediment beneath areas covered by chemosynthetic communities which ranges from 104 to 105 mol yr 1 the remaining portion of ch4 which is released into the ocean has been estimated to be 102 104 mol yr 1 per mud extrusion the second approach estimates the amount of ch4 discharging into the water column based on measurements of the near bottom methane distribution and current velocities this approach yields estimates between 104 105 mol yr 1 the discrepancy of the amount of ch4 emitted into the bottom water derived from the two approaches hints to methane seepage that cannot be accounted for by faunal growth e g focused fluid emission through channels in sediments and fractures in carbonates extrapolated over the 48 mud extrusions discovered off costa rica we estimate a ch4 output of 20 106 mol yr 1 from mud extrusions along this 350 km long section of the continental margin these estimates of methane emissions at an erosional continental margin are considerably lower than those reported from mud extrusion at accretionary and passive margins almost half of the continental margins are described as non accretionary assuming that the moderate emission of methane at the mud extrusions off costa rica are typical for this kind of setting then global estimates of methane emissions from submarine mud extrusions which are based on data of mud extrusions located at accretionary and passive continental margins appear to be significantly too high

bubble momentum plume as a possible mechanism for an early breakdown of the seasonal stratification in the northern north sea the presence of a seasonal thermocline likely plays a key role in restraining methane released from a seabed source in the deeper water column thereby inhibiting exchange to the atmosphere the bubble plume itself however generates an upward motion of fluid e g upwelling and may thereby be partially responsible for an early breakdown of the seasonal thermocline measurements at site 22 4b located at 57 550n 1 380e in the uk central north sea 200 km east of the scottish mainland where gas is still being released since a blow out in 1990 have been used to identify the generation of the seasonal thermocline and thus the depth of the upper mixed layer and its breakdown in autumn data derived from two landers containing an acoustic doppler current profiler and a conductivity temperature depth recorder were used to determine the mixed layer depth and the breakdown of the thermocline mixing of upper layer fluid into the lower layer has been inferred from large amplitude variations in the nearbottom temperature the adcps estimate velocity profiles in four beam directions using doppler shifted frequency from acoustic pings sent out and received by four different transducers in a specific configuration besides that the intensity of the backscattered sound per transducer is also recorded bubbles from the nearby plume contaminate the signal during part of the tidal cycle but in bubble free periods the mixed layer depth can be estimated using the acoustic backscatter signal as local maxima results show that the thermocline broke down between mid october and early november several weeks earlier than the breakdown of the thermocline in nearby comparable areas likely caused by bubble induced downwelling at the site the early breakdown of the thermocline was accompanied by multiple occurrence of a strong jet like structure associated with the seasonal tidal mixing front

use of marine controlled source electromagnetic sounding for sub basalt exploration the problem of imaging sedimentary structure obscured by high velocity layers such as carbonate basalt or salt using conventional seismic techniques is well known when this problem is encountered in offshore areas marine electromagnetic data can provide valuable alternative and complementary constraints on the structure we concentrate on the use of marine controlled source electromagnetic csem sounding in the frequency domain the feasibility of mapping sub basalt sedimentary structure using this technique is explored by means of modelling studies which mimic a type of survey which could readily be employed in practice as a starting point the problem is addressed in terms of 1d resistivity structure we show that sub basalt sediments can be detected and their depth of burial quantified to within 200 m in the examples shown the presence of small scale inhomogeneities in the basalt which cause much of the scattering in seismic data is shown to have no appreciable effect on the ability of the csem data to detect the sediments the modelling is then extended to 2 5 dimensions again the presence of sub basalt sediments can be readily detected and their properties and geometry quantified especially if the electromagnetic data are combined with constraints from complementary geophysical techniques

ongoing methane discharge at well site 22 4b north sea and discovery of a spiral vortex bubble plume motion highlights mega ebullition of biogenic methane from an abandoned offshore gas well north sea evidence for midwater bubble plume intrusion fallback and short circuiting of the plume effective trapping of seabed released methane underneath the thermocline first observation of a spiral vortex methane plume and marginal turbulences megaplumes appear less efficient in terms of vertical methane transport than previously thought abstract first direct evidence for ongoing gas seepage activity on the abandoned well site 22 4b northern north sea 57 55 n 01 38 e and discovery of neighboring seepage activity is provided from observations since 2005 a manned submersible dive in 2006 discovered several extraordinary intense seepage sites within a 60 m wide and 20 m deep crater cut into the flat 96 m deep seafloor capture and isotope chemical analyses of the gas bubbles near the seafloor revealed in situ concentrations of methane between 88 and 90 vol with 13c ch4 values around 74 vpdb indicating a biogenic origin bulk methane concentrations throughout the water column were assessed by 120 niskin water samples showing up to 400 000 nm ch4 in the crater at depth in contrast concentrations above the thermocline were orders of magnitude lower with a median value of 20 nm a dye tracer injection into the gas seeps revealed upwelling bubble and water motion with gas plume rise velocities up to 1 ms 1 determined near the seabed however the dissolved dye did not pass the thermocline but returned down to the seabed measurements of direct bubble mediated atmospheric flux revealed low values of 0 7 0 3 kty 1 much less than current state of the art bubble dissolution models would predict for such a strong and upwelling in situ gas bubble flux at shallow water depths i e 100 m acoustic multibeam water column imaging data indicate a pronounced 200 m lateral intrusion at the thermocline together with high methane concentration at this layer a partly downward orientated bubble plume motion is also visible in the acoustic data with potential short circuiting in accordance to the dye experiment this observation could partly explain the observed trapping of most of the released gas below the well established thermocline in the north sea moreover 3d analyses of the multibeam water column data reveal that the upwelling plume transforms into a spiral expanding vortex while rising through the water column such a spiral vortex motion has never been reported before for marine gas seepage and might represent an important process with strong implication on plume dynamics dissolution behavior gas escape to the atmosphere and is considered very important for respective modeling approaches

the crustal magma chamber of the katla volcano in south iceland revealed by 2 d seismic undershooting summary results of a 2 d seismic undershooting experiment on the katla central volcano in south iceland are reported large localized traveltime anomalies 0 4 s are observed on an array within the katla caldera the traveltimes are forward modelled using a wavefront tracker developed in appendix a thus non linear effects encountered in traveltime tomography are avoided as well as common problems with ray tracing in the presence of strong lateral heterogeneity an extreme variation in compressional velocity is required to extend over a significant volume in order to model the data the resulting model is not unique but constraints on the allowable range of velocities 2 5 6 0 km s render the basic features well constrained a clear s wave shadow is closely associated with delays in traveltime due to a shallow slow anomaly low amplitude p waves go hand in hand with early arrivals due to thin structural features flanking the slow anomaly the model is interpreted in terms of a magma chamber containing extensively molten rock the magma chamber is shallow with a bottom at a depth of about 1 5 km below sea level 3 0 km below surface and measures about 5 km across the depth of the chamber is roughly at the level of buoyant equilibrium for basaltic melt in the crust owing to poor vertical resolution at shallow depths in the undershooting geometry the top of this shallow magma chamber is not well resolved on the other hand the bottom of the chamber is well resolved the chamber is underlain by rocks of average or high velocity for that depth the magma chamber is a persistent feature big enough 10 km3 to supply magma for large eruptions and to supply heat to permit remelting of hydrated basaltic crust to produce silicic magmas at shallow levels the chamber is fed by magma fracturing from below the model agrees with phenomenological models of magma chambers in iceland based on geological observations and provides a quantification of those models in terms of depth and size on the other hand it is fundamentally different from recent models of magma chambers at mid ocean ridges which may be more akin to the pervasive region of partial melt at depth beneath iceland this underlines the important effect of the icelandic hotspot on tectonics and volcanism in iceland and implies a substantially different crustal and thermal structure in iceland from that of normal mid ocean ridges

determination of ocean tides from the first year of topex poseidon altimeter measurements an improved geocentric global ocean tide model has been determined using 1 year of topex poseidon altimeter measurements to provide corrections to the cartwright and ray 1991 model cr91 the corrections were determined on a 3 3 grid using both the harmonic analysis method and the response method the two approaches produce similar solutions the effect on the tide solution of simultaneously adjusting radial orbit correction parameters using altimeter measurements was examined four semidiurnal n2 m2 s2 and k2 four diurnal q1 o1 p1 and k1 and three long period ssa mm and m constituents along with the variation at the annual frequency were included in the harmonic analysis solution the observed annual variation represents the first global measurement describing accurate seasonal changes of the ocean during an el nino year the corrections to the m2 constituent have an rms of 3 6 cm and display a clear banding pattern with regional highs and lows reaching 8 cm the improved tide model reduces the weighted altimeter crossover residual from 9 8 cm rms when the cr91 tide model is used to 8 2 cm rms comparison of the improved model to pelagic tidal constants determined from 80 tide gauges gives rms differences of 2 7 cm for m2 and 1 7 cm for k1 comparable values when the cr91 model is used are 3 9 cm and 2 0 cm respectively examination of topex poseidon sea level anomaly variations using the new tide model further confirms that the tide model has been improved

lithology and structure of the volcanic sequence in eastern iceland the reydarfjordur drill hole penetrated 1919 m of section with a recovery of 99 7 subaerial lava flows comprise 54 of the core dikes 41 and clastic rocks about 5 in addition a 1 3 km thick section of lava flows and clastic rocks exposed above the drill site was measured and sampled the lava flows are chiefly low magnesia basalts and ferrobasalts with lesser amounts of icelandite basaltic andesite and olivine tholeiite in the drill core complete flows range from 1 2 to 19 7 m in vertical thickness and average 7 1 m in the exposed section flow thicknesses range from 1 to 30 m and average 5 2 m most of the flows are characterized by an upper scoriaceous zone 1 3 meters thick grading downward through a zone of fractured basalt into a massive flow interior some flows also have a thin basal breccia olivine tholeiites and some low magnesia basalts are commonly mottled whereas ferrobasalts and icelandites have well developed flow banding most of the flows are fine to medium grained aphyric to sparsely phyric and moderately vesicular a few are moderately plagioclase phyric others have scattered phenocrysts of clinopyroxene and olivine over 100 intrusive units are recognized in the core and they span approximately the same compositional range as the flows they are characterized by having steeply inclined chilled contacts a uniform massive aspect and an absence of vesicles or brecciation most are less altered than the intruded lava flows and contain secondary minerals only along sparse high angle fractures many of the dikes are also aphyric but some contain notable plagioclase or olivine phenocrysts groundmass textures range from fine grained intersertal to coarse grained ophitic clastic units occur typically as thin varicolored layers between lava flows rarely exceeding 0 5 m in thickness these are chiefly fine to coarse grained poorly to well bedded lithic tuffs showing varying degrees of reworking most are silicic to intermediate in composition but basaltic units are also present two rhyolitic ash flow tuffs occur in the section one of which is 30 m thick we interpret the lavas to be the flanking portions of broad low angled central volcanoes centered on silicic complexes some of the flows may have erupted from central vents but most probably represent flank eruptions from rift zones represented by the dike swarms most of the dikes were probably intruded laterally into the lava pile thus they do not represent direct vertical feeders from deep magma bodies

seismic structure of basalt flows from surface seismic data borehole measurements and synthetic seismogram modeling we use the wide angle wavefield to constrain estimates of the seismic velocity and thickness of basalt flows overlying sediments wide angle means the seismic wavefield recorded at offsets beyond the emergence of the direct wave this wide angle wavefield contains arrivals that are returned from within and below the basalt flows including the diving wave through the basalts as the first arrival and p wave reflections from the base of the basalts and from subbasalt structures the velocity structure of basalt flows can be determined to first order from traveltime information by ray tracing the basalt turning rays and the wide angle base basalt reflection this can be refined by using the amplitude variation with offset avo of the basalt diving wave synthetic seismogram models with varying flow thicknesses and velocity gradients demonstrate the sensitivity to the velocity structure of the basalt diving wave and of reflections from the base of the basalt layer and below the diving wave amplitudes of the

crustal structure beneath the faroe islands and the faroe iceland ridge abstract we have mapped the transition from the continental faroe block the faroe islands and surrounding shelf to the thickened oceanic crust of the faroe iceland ridge in the north atlantic using the results of a detailed sea to land seismic profile with wide angle to normal incidence recordings of explosive and airgun shots fired at sea along the faroe iceland ridge interpretation of all available seismic and gravity data indicates that this aseismic ridge is composed of 30 3 km thick oceanic crust with a gradual transition to ancient continental crust from 100 to 40 km northwest of the faroe islands close to the shelf edge this confirms that the crust beneath the faroe islands which may be up to 46 km thick comprises continental material in agreement with previous seismic and geochemical results results suggest that the upper 5 2 0 7 km of the faroe crust consists of tertiary basalts generated during continental breakup overlying the continental crust beneath the lower crust where seismic constraint is poor may exhibit high seismic velocities 7 1 7 6 km s 1 which we attribute to underplating or intrusion by mafic melts during continental breakup in the early tertiary

equatorially trapped rossby waves in the presence of meridionally sheared baroclinic flow in the pacific ocean topex poseidon altimeter data are analyzed for the 8 5 year period november 1992 to may 2001 to investigate the sea surface height ssh and geostrophic velocity signatures of quasi annual equatorially trapped rossby waves in the pacific the latitudinal structures of ssh and both components of geostrophic velocity are found to be asymmetric about the equator across the entire pacific with larger amplitude north of the equator the westward phase speeds are estimated by several different methods to be in the range 0 5 0 6 m s 1 these observed characteristics are inconsistent with the classical theory for first vertical first meridional mode equatorially trapped rossby waves which predicts a phase speed of about 0 9 m s 1 with latitudinally symmetric structures of ssh and zonal velocity and antisymmetric structure of meridional velocity the observations are even less consistent with the latitudinal structures of ssh and geostrophic velocity components for other modes of the classical theory the latitudinal asymmetries deduced here have also been consistently observed in past analyses of subsurface thermal data and altimeter data and have been variously attributed to sampling errors in the observational data a superposition of multiple meridional rossby wave modes asymmetric forcing by the wind and forcing by cross equatorial southerly winds in the eastern pacific we propose a different mechanism to account for the observed asymmetric latitudinal structure of low frequency equatorial rossby waves from the free wave solutions of a simple 1 5 layer model it is shown that meridional shears in the mean equatorial current system significantly alter the potential vorticity gradient in the central and eastern tropical pacific the observed asymmetric structures of sea surface height and geostrophic velocity components are found to be a natural consequence of the shear modification of the potential vorticity gradient the mean currents also reduce the predicted westward phase speed of first meridional mode rossby waves improving consistency with the observations

chemical stratigraphy iceland research drilling project reydarfjordur eastern iceland major element data are reported for lavas and dikes from a 3 2 km section through the eastern iceland lava pile the section includes a 1 2 km subaerial exposure and a 1 9 km sequence drilled by the iceland research drilling project irdp alteration increases down section although primary magma chemistry may be interpreted from feo al2o3 tio2 p2o5 and to a lesser extent sio2 in addition to immobile trace elements lava compositions range from mgo rich tholeiite to sio2 rich icelandite while dikes range from tholeiite to tholeiitic andesite three stratigraphic divisions based on volcanologic and trace element criteria e g ce yb and zr y are confirmed by major elements upper and lower groups are predominantly basaltic while the middle group includes a high proportion of silicic lavas variation within groups conforms to simple models combining fractional crystallization and accumulation of phenocryst plagioclase during build up of the pile eruptive phases commenced with silica rich magma and frequently terminated with basaltic or intermediate plagioclase cumulates dike chemistry resembles that of upper group lavas and also the nearby breiddalur volcano middle group lavas probably derive from a buried silicic complex while the lower group may represent the flank of a more deeply buried complex chemical stratigrapy is consistent with iterative burial with spreading of lenticular basaltic silicic complexes magma supply fractionation and mixing are considered in the framework of such kinematic models although it is premature to draw direct analogies to the active neogene zone of iceland

enso simulated by intermediate coupled models and evaluated with observations over 1970 98 part i role of the off equatorial variability enso simulations are investigated in 30 yr integrations of various intermediate coupled models and compared with observed sst wind and thermocline depth anomalies over the tropical pacific the cane and zebiak model simulates warm events with a period close to the observations but with westerlies that are located 30 8 east of them and thermocline anomalies in the western pacific that are much shallower between two warm events the model simulates a series of three weak and short cold sst peaks and hardly ever simulates easterlies the sst in the eastern equatorial pacific is not sensitive to thermocline depth anomalies but to the anomalous downwelling of surface currents induced by ekman shear the model simulates a pair of very strong cyclonic wind stress curl anomalies on both sides of the equator in the eastern off equatorial domain between 78 and 158 lat these are necessary to maintain the oscillatory regime so are the ocean meridional rossby modes higher than 5 the thermocline zonal slopes required to balance the off equatorial curl anomalies are about three times steeper than the ones required to balance the zonal stress along the equator thus the off equator exerts an excess of zonal pressure which by continuity affects the equatorial ocean and plays a crucial role in reversing and triggering the growing events six months after the warm peaks the whole ocean between 158s and 158n is significantly upwelled the equatorial oceanic heat content is recharged from the south prior to a warm event contrary to simulations when the model is driven by observed wind anomalies increasing the friction in the baroclinic ocean does not decrease the off equatorial variability but significantly alters the low frequency oscillations that are no longer enso like introducing the parameterization of subsurface temperature derived from hydrographic profiles in the ocean component does not allow the coupled model to recover cold events as in a forced context introducing the parameterization of convection derived from high cloud temperature measurements is the most effective improvement but results still poorly agree with observations and are in contrast with the simulations driven by observed sst biased toward westerlies in the central pacific upwelled thermocline in the west and warm sst in the east thus modifying the ocean component only or the atmosphere only does not have the same impact on simulations as in a forced context the coupling allows new mechanisms to grow and govern the model behavior one of them is the slow meridional oceanic mass adjusment in quasi sverdrup balance with the winds

roles of equatorial waves and western boundary reflection in the seasonal circulation of the equatorial indian ocean an ocean general circulation model ogcm is used to study the roles of equatorial waves and western boundary reflection in the seasonal circulation of the equatorial indian ocean the western boundary reflection is defined as the total kelvin waves leaving the western boundary which include the reflection of the equatorial rossby waves as well as the effects of alongshore winds off equatorial rossby waves and nonlinear processes near the western boundary the evaluation of the reflection is based on a wave decomposition of the ogcm results and experiments with linear models it is found that the alongshore winds along the east coast of africa and the rossby waves in the off equatorial areas contribute significantly to the annual harmonics of the equatorial kelvin waves at the western boundary the semiannual harmonics of the kelvin waves on the other hand originate primarily from a linear reflection of the equatorial rossby waves the dynamics of a dominant annual oscillation of sea level coexisting with the dominant semiannual oscillations of surface zonal currents in the central equatorial indian ocean are investigated these sea level and zonal current patterns are found to be closely related to the linear reflections of the semiannual harmonics at the meridional boundaries because of the reflections the second baroclinic mode resonates with the semiannual wind forcing that is the semiannual zonal currents carried by the reflected waves enhance the wind forced currents at the central basin because of the different behavior of the zonal current and sea level during the reflections the semiannual sea levels of the directly forced and reflected waves cancel each other significantly at the central basin in the meantime the annual harmonic of the sea level remains large producing a dominant annual oscillation of sea level in the central equatorial indian ocean the linear reflection causes the semiannual harmonics of the incoming and reflected sea levels to enhance each other at the meridional boundaries in addition the weak annual harmonics of sea level in the western basin resulting from a combined effect of the western boundary reflection and the equatorial zonal wind forcing facilitate the dominance by the semiannual harmonics near the western boundary despite the strong local wind forcing at the annual period the rossby waves are found to have a much larger contribution to the observed equatorial semiannual oscillations of surface zonal currents than the kelvin waves the westward progressive reversal of seasonal surface zonal currents along the equator in the observations is primarily due to the rossby wave propagation

constraints on crustal structure in eastern iceland based on extremal inversions of refraction data in the summer of 1978 the iceland research drilling project undertook the drilling of a deep crustal hole near reydarfjorder in eastern iceland as a part of this project the scripps institution of oceanography and the university of washington undertook a small scale seismic refraction experiment near the drill site in an attempt to compare surface geophysical measurements with observations of samples from and logging in the hole using recent advances in the methods of extremal inversion of seismic data we have determined an approximate one dimensional velocity structure for the drill site this structure indicates that the 1 9 km hole failed to penetrate the layer 2 layer 3 transition which was at some 3 0 4 5 km beneath the drill site the transition appears to be rather abrupt unlike that beneath the ocean with velocity increasing from 5 2 5 5 km s in the upper crust to about 6 7 km s in layer 3 we observe a steep eastward dip and a shallow westward dip in the lower crust away from the nearby thingmuli and reydarfjordur volcanic centers respectively in agreement with previous work associating shallow depths to layer 3 with tertiary volcanic centers as a result of increasing metamorphic grade and increased dyke swarm intensity

coefficients for sea surface wind stress heat flux and wind profiles as a function of wind speed and temperature surface layer coefficients for wind profiles wind stress and heat flux in typical open sea conditions are briefly reviewed businger dyer flux gradient relationships and a charnock wind stress formula fit the empirical data and are dimensionally consistent these have been solved by an iterative method and the results are presented in a tabular form suitable for climatological calculations from marine wind and temperature data

coupling of long oceanic waves in the pacific ocean and the rotating elastic earth during the 1986 1987 el nino we investigate interannual coupling between the rotating elastic earth and the ocean during the 1986 1987 el nino we find the e w component of the ground tilt at the esashi earth tides station up toward the east about 50 milliarc seconds mas a rise of sea level about 5 cm at chichijima and oshima in the northwestern pacific ocean and an increase of 1 cm in the very long baseline interferometry vlbi baseline distance between kashima japan and kauai hawaiian islands as well as the residual length of day lod changes of 50 s during the 1986 1987 el nino the residual lod is derived from elimination of changes in atmospheric angular momentum our discussions are based on the coupling between torsional motions of the elastic earth and reflection of the eastward traveling equatorial kelvin waves at the eastern boundary of the pacific ocean these equatorial waves exchange the momentum and the axial and zonal component of vorticity with the elastic earth during the 1986 1987 el nino reflection of the kelvin wave is expected to excite torsional motions and to couple with differential zonal rotation in the elastic earth and to generate anomalies of 1 6 s in lod the differential zonal rotation due to the torsional motion can produce the same order of the magnitude of changes in the vlbi baseline distance between kashima and kauai variations of the e w component of the tilt cannot be explained by torsional motion we infer that the midlatitude rossby waves are generated in the central pacific ocean this conclusion is consistent with sea level changes at chichijima and oshima and changes of the tilt at esashi

methane sources distributions and fluxes from cold vent sites at hydrate ridge cascadia margin to constrain the fluxes of methane ch4 in the water column above the accretionary wedge along the cascadia continental margin we measured methane and its stable carbon isotope signature 13c ch4 the studies focused on hydrate ridge hr where venting occurs in the presence of gas hydrate bearing sediments the vent ch4 has a light 13c ch4 biogenic signature 63 to 66 pdb and forms thin zones of elevated methane concentrations several tens of meters above the ocean floor in the overlying water column these concentrations ranging up to 4400 nmol l 1 vary by 3 orders of magnitude over periods of only a few hours the poleward undercurrent of the california current system rapidly dilutes the vent methane and distributes it widely within the gas hydrate stability zone ghsz above 480 m water depth the methane budget is dominated by isotopically heavier ch4 from the shelf and upper slope where mixtures of various local biogenic and thermogenic methane sources were detected 56 to 28 pdb the distribution of dissolved methane in the working area can be represented by mixtures of methane from the two primary source regions with an isotopically heavy background component 25 to 6 pdb methane oxidation rates of 0 09 to 4 1 per day are small in comparison to the timescales of advection this highly variable physical regime precludes a simple characterization and tracing of downcurrent plumes however methane inventories and current measurements suggest a methane flux of approximately 3 104 mol h 1 for the working area 1230 km2 and this is dominated by the shallower sources we estimate that the combined vent sites on hr produce 0 6 104 mol h 1 and this is primarily released in the gas phase rather than dissolved within fluid seeps there is no evidence that significant amounts of this methane are released to the atmosphere locally

geophysical response of flood basalts from analysis of wire line logs ocean drilling program site 642 voring volcanic margin analysis of wire line logs at ocean drilling program site 642 the deepest scientific hole yet drilled in an offshore large igneous province lip reveals the main features of the extrusive complex constructed during formation of a volcanic margin a characteristic cyclic log response images tholeiitic lavas and thin tuffs corresponding to the seaward dipping reflectors in the seismic record flows are 0 6 18 5 m thick while most sediments are 2000 m focused resistivity kicks the large flow scale variations in physical properties show that representative core measurements must be densely sampled both on flow and sequence scales site 642 log responses appears typical for the north atlantic volcanic province and other lips but differs from logs in deep ocean crustal holes principal differences in log responses reflect mode of crustal emplacement and alteration hence site 642 forms a reference hole for extrusive constructions on oceanic lips

flux and dispersion of gases from the drachenschlund hydrothermal vent at 8 18 s 13 30 w on the mid atlantic ridge hydrothermal emission of mantle helium appears to be directly related to magma production rate but other processes can generate methane and hydrogen on mid ocean ridges in an on going effort to characterize these processes in the south atlantic the flux and distribution of these gases were investigated in the vicinity of a powerful black smoker recently discovered at 8 17 9 s 13 30 4 w the vent lies on the shoulder of an oblique offset in the mid atlantic ridge and discharges high concentrations of methane and hydrogen measurements during expeditions in 2004 and 2006 show that the ratio of ch4 to 3he in the neutrally buoyant plume is quite high 4 108 the ctd stations were accompanied by velocity measurements with lowered acoustic doppler current profilers ladcp and from these data we estimate the methane transport to have been 0 5 mol s 1 in a wsw trending plume that seems to develop during the ebb tidal phase this transport is an order of magnitude greater than the source of ch4 calculated from its concentration in the vent fluid and the rise height of the plume from this range of methane fluxes the source of 3he is estimated to be between 0 14 and 1 2 nmol s 1 in either case the 3he source is significantly lower than expected from the spreading rate of the mid atlantic ridge from the inventory of methane in the rift valley adjacent to the vent it appears that the average specific rate of oxidation is 2 6 to 23 yr 1 corresponding to a turnover time between 140 and 16 days vertical profiles of methane in the surrounding region often exhibited gaussian like distributions and the variances appear to increase with distance from the vent using a gaussian plume model we obtained a range of vertical eddy diffusivities between 0 009 and 0 08 m2m2 s 1 these high values may be due to tidally driven internal waves across the promontory on which the vent is located

seismic properties density and composition of the icelandic crust near reydarfjordur compressional and shear wave velocities densities and porosities have been measured to confining pressures of 6 kbar for 135 samples from a deep borehole in eastern iceland the measurements are from a random sampling of dikes and sills through the 2 km hole velocities within the flow units are relatively low at the tops reach maximum values within central portions and decrease near the bases this systematic variation correlates well with variations in porosity and bulk density the dike units on the other hand have relatively uniform velocities which are approximately equivalent to maximum velocities measured within flow units below 700 m the dike and flow velocities show systematic increases with depth which are related to changes in mineralogy and independent of porosity the velocity gradient dvp dz t within this region is well defined by the laboratory velocities and approximates 0 6 s 1 in good agreement with field refraction measurements high velocities near the base of the hole are related to increasing alteration with depth and in particular an increase in epidote content the abundance of epidote in the rocks recovered from the lower portion of the drillhole supports a metamorphic origin for the layer 2 layer 3 boundary in this region

flare imaging with multibeam systems data processing for bubble detection at seeps multibeam sonar surveys have been conducted since their invention in the 1970s however mainly reflections from the seafloor were considered so far more recently water column imaging with multibeam is becoming of increasing interest for fisheries buoy mooring or gas detection in the water column using elac seabeam 1000 data we propose a technique to detect gas bubbles flares although this system is originally not designed to record water column data the described data processing represents a case study and can be easily adapted to other multibeam systems multibeam data sets from the black sea and the north sea show reflections of gas bubbles that form flares in the water column at least for reasonably intense gas escape the detection of bubbles is feasible the multibeam technique yields exact determination of the source position and information about the dimension of the gas cloud in the water compared to conventional flare imaging by single beam echo sounders the wide swath angle of multibeam systems allows the mapping of large areas in much shorter time

satellite altimetry the marine geoid and the oceanic general circulation for technical reasons the general circulation of the ocean has historically been treated as a steady laminar flow field the recent availability of extremely highaccuracy and high precision satellite altimetry has provided a graphic demonstration that the ocean is actually a rapidly time evolving turbulent flow field to render the observations quantitatively useful for oceanographic purposes has required order of magnitude improvements in a number of fields including orbit dynamics gravity field estimation and atmospheric variability with five years of very high quality data now available the nature of oceanic variability on all space and time scales is emerging including new findings about such diverse and important phenomena as mixing coefficients the frequency wavenumber spectrum and turbulent cascades because the surface elevation is both a cause and consequence of motions deep within the water column oceanographers soon will be able to provide general circulation numerical models tested against and then combined with the altimeter data these will be complete three dimensional timeevolving estimates of the ocean circulation permitting greatly improved estimates of oceanic heat carbon and other property fluxes

an examination of some enso mechanisms using interannual sea level at the eastern and western equatorial boundaries and the zonally averaged equatorial wind abstract theory suggests that the interannual equatorial kelvin wave sea level w in the western pacific should be directly proportional to the interannual western australian sea level and the interannual equatorial kelvin wave sea level e in the eastern pacific should be directly proportional to the equatorial interannual sea level at the south american coast forced equatorial kelvin wave dynamics implies approximately that e t t w t b t t where t b and are respectively a lag corresponding to equatorial kelvin wave propagation zonally averaged equatorial interannual eastward wind stress a forcing coefficient and a number between 0 and 1 a regression analysis of the sea level difference above using western australian and south american sea levels against shows that the correlation is high and is highest 0 83 when deltat 3 months and deltat 1 month the values for deltat and b are reasonable for they are appropriate for a signal dominated by the first

linked sediment and water column methanotrophy at a man made gas blowout in the north sea implications for methane budgeting in seasonally stratified shallow seas large quantities of the greenhouse gas methane ch4 are stored in the seafloor the flux of ch4 from the sediments into the water column and finally to the atmosphere is mitigated by a series of microbial methanotrophic filter systems of unknown efficiency at highly active ch4 release sites in shallow marine settings here we studied ch4 oxidation and the methanotrophic community at a high ch4 flux site in the northern north sea well 22 4b where ch4 is continuously released since a blowout in 1990 vigorous bubble emanation from the seafloor and strongly elevated ch4 concentrations in the water column up to 42 m indicated that a substantial fraction of ch4 bypassed the highly active up to 2920 nmol cm 3 d 1 zone of anaerobic ch4 oxidation in sediments in the water column we measured rates of aerobic ch4 oxidation up to 498 nm d 1 that were among the highest ever measured in a marine environment and under stratified conditions have the potential to remove a significant part of the uprising ch4 prior to evasion to the atmosphere an unusual dominance of the water column methanotrophs by type ii methane oxidizing bacteria mob is partially supported by recruitment of sedimentary mob which are entrained together with sediment particles in the ch4 bubble plume our study thus provides evidence that bubble emission can be an important vector for the transport of sediment borne microbial inocula aiding in the rapid colonization of the water column by methanotrophic communities and promoting their persistence close to highly active ch4 point sources

intrabasement reflections of the ontong java plateau implications for plateau construction 1 the early cretaceous ontong java plateau ojp in the western equatorial pacific earth s most voluminous large igneous province lip is characterized by intrabasement seismic reflections to investigate the nature of such reflections we have analyzed new and all older multichannel seismic mcs reflection data from the ojp using an instantaneous phase velocity analysis technique and synthetic seismograms intrabasement reflections are most prevalent on the main ojp especially on its crest on mcs data the reflections 1 are semicontinuous and subparallel to the top of igneous basement 2 in places have the opposite phase of seafloor reflections and 3 have an average frequency of 20 hz we calculate synthetic seismograms using impedance contrasts between massive lava and pillow lava flows obtained from downhole logs at ojp scientific drill sites and show that these lithologies can produce intrabasement reflections similar to those observed in the mcs data alternatively to evaluate the possibility of sediment sedimentary rock interbeds causing the intrabasement reflections we use published early cretaceous sedimentation rates determined from ojp drill sites and published estimates for the duration of massive ojp volcanism to calculate a range of possible sedimentary interbed thicknesses the range proves to be below the detection limit of the mcs data therefore we conclude that the intrabasement reflections arise from alternating relatively thin lava flows produced at low effusion rates and relatively thick massive flows generated at high effusion rates both originating from vents and fissures on the ojp probably on its crest

crustal accretion model for iceland predicts cold crust we present a self consistent crustal accretion model of iceland that includes heat and mass transfer between an upwelling melting mantle and an accreting cooling crust this models predicts sub solidus crustal temperatures that are consistent with seismic estimates of the maximum depth of earthquake hypocenters and mid crustal shear wave quality factors and with geothermal estimates of surface heatflow

oceanic equatorial waves and the 1991 93 el nino abstract equatorial kelvin and rossby waves associated with the 1991 93 el nino warm event were detected in temperature observations made by the tropical atmosphere ocean tao buoy array intraseasonal kelvin waves were a prominent part of equatorial thermocline depth variability and were well represented by a simple model consisting only of first and second mode baroclinic kelvin waves the second mode was essential to properly represent the observed amplitude thermocline depth variability at 5 n and 5 s was dominated by annual and interannual rossby waves which were found to have been largely wind forced in midbasin with little if any signal associated with eastern boundary reflection an evaluation of the wyrtki buildup hypothesis and the delayed oscillator hypothesis in connection with the 1991 93 events showed that a long lag about two years occurred between the arrival of the downwelling signal in the west and the subsequent initiation of el nino this was considerably longer than suggested

interaction between a bubble plume and the near field in a stratified lake a steady state bubble plume model is evaluated using full scale temperature salinity and dissolved oxygen data collected in a swiss lake the data revealed a plume generated near field environment that differed significantly from the ambient far field water column properties a near field torus of reduced stratification developed around the plume the extent of which is on the same lateral scale as the horizontal dislocations generated by persistent first mode seiching the plume fallback water was found to penetrate much deeper than expected thereby maintaining reduced vertical gradients in the near field torus the plume entrains a portion of the fallback water leading to short circuiting which generates a complex plume lake interaction and reduces far field downwelling relative to the upward plume flow as the integral plume model incorporates the entrainment hypothesis it is highly sensitive to the near field environmental conditions after identifying appropriate near field boundary conditions the plume model predictions agree well with the field observations

evidence for the delayed oscillator mechanism for enso the observed oceanic kelvin mode in the far western pacific abstract observed surface winds from 1961 through september 1992 are used to force a reduced gravity shallow water ocean model results from the hindcast of ocean variability are found to be consistent with the results li and clarke presented in a previous study of sea level and zonal wind variability in the tropical pacific in this note the apparent discrepancies are reconciled between delayed oscillator theory and calculated lag correlations between the observationally based records of western boundary kelvin mode amplitude w and zonal wind forcing evidence for sensitivity in these delayed oscillator lag correlations is presented from a variety of sources including the hindcast data output from the standard version of the zebiak and cane coupled ocean atmosphere model and through three cases with idealized time series of enso variability the authors demonstrate that the low lag correlations for w leading the zonal wind forcing by 1 to 1 years is not inconsistent with the hypothesized rol

dynamics and thermodynamics of a warming event in a coupled tropical atmosphere ocean model abstract a simple coupled ocean atmosphere model similar to that of zebiak and cane is used to examine the dynamic and thermodynamic processes associated with el nino southern oscillation enso the model is run for 300 years the interannual variability which results is regular with a period of either 3 or 4 years quantized by the annual cycle the amplitude 1 5 m s 1 wind and 2 c sst anomalies period and structure of the interannual variability compare well with observations the model warm event is initiated in the spring prior to the event peak and is well described as an instability of the coupled system during instability growth the sea surface temperature sst anomaly is primarily generated by vertical upwelling processes the sst anomaly can be approximately described by the expression t t kth t where t is the sst anomaly t time h the upper layer thickness pycnocline perturbation and an effective damping time which includes heat loss to the atmosphere kt parameterizes

long term acoustic monitoring at north sea well site 22 4b highlights first study using long term passive acoustic monitoring of methane seeps at well blowout site 22 4b seep acoustic temporal variations correlated with ocean tides major acoustic transient event recorded on 8 december 2011 with high temporal resolution abstract marine seeps produce underwater sounds as a result of bubble formation and fragmentation upon emission from the seabed the frequency content and sound levels of these emissions are related to bubble size distribution and emission flux providing important information on methane release from the seafloor long term passive acoustic monitoring was used to continuously record seep sounds over a 7 month period within the blowout crater at the abandoned well site 22 4b in the central north sea also recorded were water column fluid velocities and near seafloor water conductivity temperature and pressure acoustic signatures were primarily from 1 to 10 khz key features were relatively broad spectral peaks at about 1 0 1 5 2 2 3 1 3 6 and 5 1 khz temporal variations in spectral levels were apparently associated with tides the recordings also documented a series of major episodic events including a large and persistent increase 10 db in overall sound levels and spectral broadening on 8 december 2011 the acoustic temporal pattern of this event was consistent with other recorded large transient events in the literature and the major event was correlated with dramatic changes in other measurements including increased water column fluid velocities increased pressure and decreased salinity indicating real changes in emission flux observed seabed morphology changes reported elsewhere in this special issue also likely were related to this event these data demonstrate the dynamic nature of marine seepage systems show the value of monitoring systems and provide direct supporting evidence for a violent formation mechanism of many widespread seep associated seabed features like pockmarks

imaging and regional distribution of basalt flows in the faeroe shetland basin we demonstrate that the use of long offset seismic data allows wide angle reflections and diving waves to be recorded and that these can be used in conjunction with prestack depth migrations to constrain and to image the base of the basalt flows and the underlying structure in the faeroe shetland basin crustal velocity models are built first by inverting the traveltimes of the recorded reflections and diving waves using ray tracing methods finer details of the velocity structure can then be refined by analysis of the amplitudes and waveforms of the arrivals we show that prestack depth migration of selected wide angle arrivals of known origin such as the base basalt reflection using the crustal velocity model allows us to build a composite image of the structure down to the pre rift basement this has the advantage that the wide angle first arriving energy must be primary and not from one of the many multiples or mode converted phases that plague near offset seismic data this allows us to tag these primary arrivals with confidence and then to identify the same arrivals on higher resolution prestack migrations that include data from all offsets examples are drawn from the faeroe shetland basin with a series of regional maps of the entire area showing the basalt depths and the thickness of the basalt flows and underlying sediment down to the top of the pre rift basement the maps show how the basalts thin to the southeast away from their presumed source west of the present faeroe islands and also show the extent to which the structure of the pre rift basement controls the considerable variations in sediment thickness between the basement and the cap formed by the overlying basalt flows

the fate of bubbles in a large intense bubble megaplume for stratified and unstratified water numerical simulations of 22 4b expedition field data extremely intense bubble plumes like the north sea 22 4b blowout megaplume defined as more than 10 6 l day 1 create very strong upwelling flows 1 m s 1 which lead to detrainment of methane enriched water but leave direct bubble mediated transport unaffected dissolved ch4 depth profiles and atmospheric measurements during a fall 2011 survey of the 22 4b site suggest strong constraint of seabed ch4 below the thermocline seabed bubbles were nearly pure ch4 the effect of the upwelling flow on the fate of bubble plume ch4 was investigated with a numerical bubble propagation model the model considered different representative bubble plume size distributions phi and a global total megaplume bubble size distribution phi synthesized from video survey data and phi from the literature simulations showed that none of the literature plumes or variations in the upwelling flow could constrain ch4 sufficiently below the thermocline two new bubble megaplume processes were simulated vortical bubble trapping slow rise and a hypothesized enhanced bubble gas exchange k be an enhancement factor applied to the normal bubble gas exchange rate k b the latter could arise from plume turbulence increasing bubble boundary layer turbulence and thus its gas exchange observations could not be reproduced solely by slow rise however simulations with k be similar to 6 reproduced observational constraints as could weaker k be in conjunction with slow rise field validation of k be is needed given its implications for the fate of megaplume ch4 emissions anthropogenic or natural for stratified and unstratified conditions kbe suggests marine ch4 geologic contributions to the atmosphere from all but shallow waters primarily arises from bubble plumes that are less than megaplume size

lower crustal intrusion on the north atlantic continental margin near the faroe islands in the north atlantic the majority of basalt is intruded into the continent ocean transition rather than extruded onto the surface this melt is intruded into the lower crust as sills which cross cut the continental fabric rather than as an underplate of 100 melt as has often been assumed when continents break apart the rifting is sometimes accompanied by the production of large volumes of molten rock1 2 3 the total melt volume however is uncertain because only part of it has erupted at the surface furthermore the cause of the magmatism is still disputed specifically whether or not it is due to increased mantle temperatures we recorded deep penetration normal incidence and wide angle seismic profiles across the faroe and hatton bank volcanic margins in the northeast atlantic here we show that near the faroe islands for every 1 km along strike 360 400 km3 of basalt is extruded while 540 600 km3 is intruded into the continent ocean transition we find that lower crustal intrusions are focused mainly into a narrow zone 50 km wide on the transition although extruded basalts flow more than 100 km from the rift seismic profiles show that the melt is intruded into the lower crust as sills which cross cut the continental fabric rather than as an underplate of 100 per cent melt as has often been assumed evidence from the measured seismic velocities and from igneous thicknesses are consistent with the dominant control on melt production being increased mantle temperatures with no requirement for either significant active small scale mantle convection under the rift or the presence of fertile mantle at the time of continental break up as has previously been suggested for the north atlantic ocean4 5 6

depth imaging of basalt flows in the faeroe shetland basin summary we use a grid of long offset seismic profiles to map the thickness of subsurface basalt flows that extend southeastward across the faeroe shelf offsets of up to 38 000 m were achieved using two seismic acquisition vessels towing multichannel streamers and shooting alternately wide angle reflections and refractions from the basalt and underlying strata allow us to build a velocity model down to the basement lewisian crust the velocity models give a first order picture of the geology of low velocity sediments of presumed early palaeocene and mesozoic age beneath the basalt flows they also provide a control for pre stack depth migration of the entire seismic data set thus allowing us to produce good depth images of the structure pre stack depth migration of selected wide angle arrivals such as those from the base of the basalt and the underlying basement produce strong reflection images that allow these interfaces to be identified unambiguously on the higher resolution images produced from the entire data set in particular we can distinguish between the primary and multiple reflections we thus use the very long offset arrivals to tag the velocities of the geological interfaces from which they are returned and hence to improve both the imaging and the interpretation of the basalt flows and the underlying structure

an advective reflective conceptual model for the oscillatory nature of the enso recent findings about zonal displacements of the pacific warm pool required a notable modification of the delayed action oscillator theory the current leading theory for the el nino southern oscillation enso simulations with a linearized coupled ocean atmosphere model resulted in 3 to 6 year enso like oscillations with many of the variable model parameters found to be very close to their observed values this simple model suggests that ocean processes that are ignored or underestimated in the delayed action oscillator theory such as zonal current convergence zonal advection of sea surface temperature and equatorial wave reflection from the eastern ocean boundary are fundamental to the development of the enso in particular to its manifestations in the central equatorial pacific

volcanism on the rockall continental margin continental break up in the northern north atlantic was accompanied by massive volcanism this produced sequences of basalt flows erupted close to sea level which are imaged as seaward dipping reflectors on seismic reflection profiles they comprise the upper 4 5 km of transitional crust in both the edoras bank and hatton bank areas of the rockall continental margin detailed surveys show that the reflectors are convex upward on dip lines but are sub horizontal on strike lines they are probably produced by aerially extensive basalt flows extruded from linear fissure vents aligned parallel to the rift axis along the rockall margin two sequences of seaward dipping reflectors occur separated by an 9outer high9 near the foot of the continental margin the landward set of basalt flows are emplaced above stretched and intruded continental crust while the seaward set span the region from highly distended continental crust to fully oceanic crust the outer high is capped by basalt flows but exhibits little coherent internal reflectivity we discuss possible origins of the outer high as a residual block of thinned and intruded continental crust or as a central volcano cut by fissure vents similar to those found in the neovolcanic zones of iceland

continuous inline mapping of a dissolved methane plume at a blowout site in the central north sea uk using a membrane inlet mass spectrometer water column stratification impedes immediate methane release into the atmosphere highlights mims used to quantify the dissolved ch4 inventory around a bubble emission site conservative estimate of well 22 4b seabed ch4 emission was 1 8 ktons yr 1 stratification impedes immediate ch4 release into the atmosphere the dissolved methane ch4 plume rising from the crater of the blowout well 22 4b in the central north sea was mapped during stratified water column conditions geochemical surveys were conducted close to the seafloor at 80 3 m water depth below the thermocline 61 1 m and in the mixed surface layer 13 2 m using membrane inlet mass spectrometry mims in combination with a towed ctd seawater was continuously transferred from the respective depth levels of the ctd to the mims by using an inline submersible pump close to the seafloor a well defined ch4 plume extended from the bubble release site 460 m towards the southwest along this distance ch4 concentrations decreased from a maximum of 7872 nmol l 1 to less than 250 nmol l 1 below the thermocline the well defined ch4 plume shape encountered at the seafloor was distorted and filaments were observed that extended towards the west and southwest in relation to current direction where the core of the bubble plume intersected this depth layer footprints of high ch4 concentrations of up to 17 900 nmol l 1 were observed in the mixed surface layer the ch4 distribution with a maximum of up to 3654 nmol l 1 was confined to a small patch of 60 m in diameter the determination of the water column ch4 inventories revealed that ch4 transfer across the thermocline was strongly impeded as only 3 of the total water column inventory was located in the mixed surface layer best estimate of the ch4 seabed release from the blowout was 1751 tons yr 1 the fate of the trapped ch4 97 that does not immediately reach the atmosphere remains speculative in wintertime when the water column becomes well mixed as well as during storm events newly released ch4 and the trapped ch4 pool can be transported rapidly to the sea surface and emitted into the atmosphere

dynamics of volcanic systems in iceland example of tectonism and volcanism at juxtaposed hot spot and mid ocean ridge systems volcanic systems are swarms of tectonic fractures and basalt volcanoes formed as a result of plate pull as the plates are pulled apart associated with the mid ocean ridges and the magma dynamics of the iceland mantle plume most systems are 40 150 km long 5 20 km wide and develop a central volcano they supply magma to all eruptions in iceland data obtained in the last few years have greatly improved our knowledge of their volcanotectonic environment as a result the geometry of the plume is better constrained and the crust previously considered thin 10 km is now modeled as thick 20 40 km depending on the location of the volcanic systems their activity either decreases or increases faulting in the two main seismic zones from this we can infer that emplacement of the feeder dike to the largest historical eruption in iceland that of laki in 1783 increased shear stress in the south iceland seismic zone and almost certainly triggered the largest m 7 1 in 1784 historical earthquake in ic

stratigraphy and correlation of the region surrounding the irdp drill hole 1978 reydarfjordur eastern iceland the 1 9 km thick sequence intersected by the irdp 1978 drill hole at reydarfjordur eastern iceland is correlated with the westward dipping exposed volcanic pile on the basis of extensive field mapping and paleomagnetic studies the base of the landward extension of sea floor magnetic anomaly 5 epoch 9 can be located at the top of the grjota olivine tholeiite formation intersected at about 1 5 km in the core and exposed at sea level about 9 km east of the drill site other exposed marker formations can also be identified in the core the drill site is located within a relatively dense dyke swarm 10 crustal dilation previously interpreted as the feeder of the overlying volcanic formations field observations including vertically discontinuous dykes and the lack of lava feeder relationships suggest that the dykes were intruded by lateral injection from the south and are considerably younger than and thus could not have fed the exposed lava pile in the vicinity of the drill site it is proposed that about 10 3 m y ago roughly coinciding with the lower boundary of anomaly 5 the reydarfjordur volcanic center ceased to be active and volcanic activity shifted about 20 km to the west where the breiddalur and thingmuli volcanic centers later developed the exposed stratigraphic succession between thingmuli and reydarfjordur volcanic centers is composed of lavas deposited in flank areas relatively removed from a paleovolcanic zone which now probably lies buried to the west of the irdp drill site the lowest 450 m of the core may represent lavas related to activity in the reydarfjordur volcanic center 13 5 km to the east

crustal accretion and dynamic feedback on mantle melting of a ridge centred plume the iceland case abstract a major consequence of the interaction of a plume with an oceanic ridge is the enhanced melt production and associated crust generation in the case of iceland crustal thickness as large as 20 to 40 km has been reported crustal seismic velocities are high and have to be explained by thermal or chemical effects in the first part of the paper we address the question whether extraction of melt out of the plume beneath a slowly spreading ridge and deposition of extracted basalt volumes at the surface produces a dynamic feedback mechanism on mantle melting to study this question we solve the convection equations for a ridge centred plume with non newtonian rheology including melting melt extraction associated with deposition of cold crust at the surface of the model and using a simplified approach for compaction the assumption of cold crust is justified if the thickness of each deposited basaltic layer is less than roughly 1 km depending on the buoyancy flux of the plume crustal thicknesses between 10 and 40 km are modelled showing characteristic dipping structures resembling the rift ward dipping basaltic layers of east and western iceland comparing the resulting crustal thickness and magma generation rate with models in which the dynamic effect of crust deposition has been suppressed indicates that melt generation beneath a slowly spreading ridge is considerably damped by the dynamic feedback mechanism if the plume buoyancy flux exceeds 400 to 600 kg s based on the observed crustal thickness of iceland our models predict a plume buoyancy flux of 1140 kg s in the second part we study the accretion of the icelandic crust by a thermo mechanical model in more detail based on the navier stokes the heat transport and the mass conservation equations including volumetric sources hot 1200 c molten crustal material is injected into the newly forming crust with a constant rate at different crustal source regions a deep widespread emplacement of dykes and sills including crustal underplating b magma chambers at shallow to mid crustal level and c surface extrusions and intrusions in fissure swarms at shallow depth connected to volcanic centres we identify the material from the different source regions by a marker approach varying the relative dominance of these source regions characteristic crustal structures evolve showing shallow dipping upper crustal layers with dip angles between 10 and 15 the thermal structure of the crust varies between cold crust shallow source region dominating and hot crust deep source region dominating we use observations of maximum depth of seismicity to constrain the depth of the 650 c isotherm and seismological inferences on the lower crust to constrain temperatures in that region the best agreement with our models is achieved for crust formation dominated by deep dykes and underplating with a considerable influence of magma chamber accretion

crust generated above the iceland mantle plume from continental rift to oceanic spreading center since the north atlantic continental breakup in the early tertiary the process of rifting above a mantle plume has produced large thicknesses of igneous crust we report results of an integrated offshore onshore seismic study of the crust and upper mantle along a transect of the aseismic faroe iceland ridge between the continental fragment beneath the faroe islands and the present day spreading center in northeast iceland normal incidence seismic data provide an image of the uppermost crust which is complemented by a velocity model from streamer refraction analyses these data together image four sedimentary basins up to 400 m deep along the crest of the ridge the streamer refraction velocity model in turn forms the uppermost section of a full crustal velocity model derived from travel time modeling of air gun and explosive data the compressional to shear wave velocity ratio in the crust of the faroe iceland ridge is 1 77 0 02 similar to that of eastern iceland the moho beneath the faroe iceland ridge lies at a depth of 25 30 km crust of 25 30 km thickness requires an upper mantle potential temperature elevated 200 250 c above normal if formed by passive adiabatic decompression melting of the mantle or a somewhat less elevated temperature if as is likely there is a component of active convection in the underlying mantle plume core

the uk22 4b blowout 20 years on investigations of continuing methane emissions from sub seabed to the atmosphere in a north sea context abstract the 22 4b blowout occurred in the uk sector of the north sea in november 1990 but during a survey in september 2011 strong gas emissions still were occurring this manuscript summarizes the findings of the 2011 survey and subsequent studies considering them in the context of previous investigations and the regional geologic and oceanographic environments the seabed crater formed during the initial event is still there as is a previously undiscovered secondary crater seabed bubble flux estimates indicate methane emission rates of 90 l s 1 at the time however this methane s fate s remains unclear the very strong thermocline that persists for more than half the year acts as an effective barrier to upward migration despite the presence of strong upwelling flows around the bubble plume clearly a large proportion of the methane is advected away from the site to be either oxidized microbially in the water column or released to the atmosphere as a result of normal sea air gas exchange processes nevertheless a significant atmospheric methane anomaly persists in the vicinity of the blowout site this has been constrained to likely less than 0 72 mscfd 5 kton yr 1 and possibly less than 0 36 mscfd 2 5 kton yr 1 during the late fall to early spring months when there is no thermocline direct methane emissions to the atmosphere are expected to increase significantly also long term monitoring has shown that periodic eruptive events occur which likely expel great quantities of methane these demonstrate the dynamic nature of the system and suggest that migration pathways in and between the deep sub seabed and seabed remain active the 22 4b study resulted in the development and adaptation of novel techniques that are applicable to other studies of seabed seepage and the development of a number of critical hypotheses with application to megaplume seepage by natural migration pathways or from the result of anthropogenic intervention

faroe iceland ridge experiment 2 crustal structure of the krafla central volcano the seismic velocity structure of the krafla central volcano is characterized by large variations in compressional velocity a 40 km wide high velocity dome extends from the lower crust 11 14 km depth beneath the volcano narrowing upward a magma chamber sits at its top near 3 km depth it is defined by both 0 2 0 3 s compressional wave delays and shear wave shadowing to be 2 3 km n s 8 10 km e w and 0 7 1 8 km thick the near surface structure uppermost 2 5 km of the krafla caldera is approximately flat lying with only minor lateral heterogeneities the crust beneath the magma chamber has low shear wave attenuation and anomalously high compressional and shear wave velocities shear waves reflected from a 19 km deep moho are clearly visible for some paths through the crustal volume below the magma chamber even though the more shallow diving s waves are severely attenuated the midcrust beneath the shallow magma chamber cannot contain partial melt or even be at near solidus temperatures the krafla central volcano plays a major role in crustal genesis along the plate boundary the high velocity dome in our view represents crust generated in and around the magma chamber which has subsequently been advected to greater depths

ridge plume interaction in the north atlantic and its influence on continental breakup and seafloor spreading development of the rifted continental margins and subsequent seafloor spreading in the north atlantic was dominated by interaction between the iceland mantle plume and the continental and oceanic rifts there is evidence that at breakup time a thin sheet of particularly hot asthenospheric mantle propagated beneath the lithosphere across a 2500 km diameter region this event caused transient uplift massive volcanism and intrusive magmatism and a rapid transition from continental stretching to seafloor spreading subsequently the initial plume instability developed to an axisymmetric shape with the 100 km diameter central core of the iceland plume generating 30 40 km thick crust along the greenland iceland faeroes ridge the surrounding 2000 km diameter region received the lateral outflow from the plume causing regional elevation and the generation of thicker and shallower than normal oceanic crust we document both long term 10 20 ma and short term 3 5 ma fluctuations in the temperature and or flow rate of the mantle plume by their prominent effects on the oceanic crust formed south of iceland lateral ridge jumps in the locus of rifting are frequent above the regions of hottest asthenospheric mantle occurring in both the early history of seafloor spreading when the mantle was particularly hot and throughout the generation of the greenland iceland faeroes ridge the northern north atlantic is a classic example of continental breakup above a thermal anomaly in the mantle created by a mantle plume the most obvious consequence of this was the generation of huge volumes of basaltic magmas by partial melting of the mantle at the time of continental breakup white mckenzie 1989 most of this magma bled up to the crust where the majority perhaps 60 80 was frozen in the lower crust while the rest was extruded as lava flows and pyroclastic deposits often after undergoing fractionation in crustal magma chambers if the extrusion rates are sufficiently high lava flows may form extensive sub horizontal flood basalts extending across the continental hinterland on rifted continental margins the extrusive lavas are typically expressed as seaward dipping reflector sdr sequences that are readily identifiable on seismic reflection profiles all these features have been found along more than 2 500 km of the rifted continental margins in the northern north atlantic on both the european and the greenland sides furthermore the mantle plume that gave rise to the thermal anomaly in the mantle is still going strong lying beneath iceland at the present day the northern north atlantic is therefore an excellent natural laboratory in which to investigate the interplay between rifting and magmatism because the history of rifting is well recorded from the time of continental breakup in the early tertiary to the seafloor spreading at the present day magnetic anomaly field reversals during the tertiary were frequent and south of iceland there are few large fracture zones in the oceanic crust so the seafloor spreading magnetic anomalies contain a clear record of the rifting history throughout the ocean basin there have also been several well constrained wide angle seismic surveys of the continental margins on both sides of the atlantic e g barton white 1997b smallwood et al 1999 korenaga et al 2000 often with coincident drilling penetrating the volcanic rocks so both the structure of the rifted margins and the timing of the igneous activity are well known in this paper we examine the interaction between the mantle plume and rifting first of the continental lithosphere to form the present continental margins and subsequently of the oceanic lithosphere to form the mid ocean ridge and the land mass of iceland smallwood white rift plume interaction 3

a review of oceanographic and meteorological controls on the north sea circulation and hydrodynamics with a view to the fate of north sea methane from well site 22 4b and other seabed sources abstract the north sea hydrodynamics are key to the redistribution of methane released at the 22 4b site located at 57 55 n 1 38 e in the uk central north sea 200 km east of the scottish mainland this review summarizes the current state of knowledge on the north sea circulation stratification and variability therein and briefly discusses the potential consequences for the distribution and fate of methane released from site 22 4b or other seabed sources astronomical tidal waves follow a counter clockwise path and tide topography interaction generates a residual circulation in the same direction wind stress forcing can enhance reduce or even reverse this circulation variations in the strength of the fair isle current fic are important the fic enters the north sea between the orkneys and shetland follows approximately the 100 m isobath passes along site 22 4b and ends up in the norwegian trench the north atlantic oscillation nao also causes variability a positive negative nao index is associated with stronger weaker than normal westerly winds nao situations strengthen the circulation in the north sea whereas it weakens during nao conditions and is directed northeastward high positive correlations exist between the sst at site 22 4b and the nao index climate change can have a long term effect on the hydrodynamics of the north sea seasonal stratification has potentially the most important imprint on methane derived from the well 22 4b site summertime heating stratifies the northern part of the north sea in autumn loss of heat to the atmosphere causes the stratification to breakdown until tides and storms mix the entire water column during the period of stratification the bulk of dissolved methane released from site 22 4b gets trapped below the thermocline the loss of methane to the atmosphere thus becomes a function of the relative time scales of transport and horizontal and vertical mixing processes versus the time scale of microbial degradation oxidation in the water column

physical limitations of dissolved methane fluxes the role of bottom boundary layer processes in situ methane emission measurements from sediments are combined with water column backscatter anomalies recorded with an acoustic doppler current profiler adcp integrated on a benthic observatory during cruise so191 to the hikurangi margin new zealand the fluid flux observatory flufo was deployed at a cold seep site at omakere ridge the sediments incubated in the two benthic chambers of flufo contained seep associated fauna including small and larger tubeworms juvenile bivalves of the genus acharax and some juvenile clams the first 26 h of in situ incubation revealed low to moderate methane fluxes of 0 01 to 0 4 mmol m 2 d 1 into the overlying water of the backup and flux chamber respectively in the following sampling sequence however the methane concentration in the flux chamber reached 3 fold higher concentrations whereas the methane concentration in the backup chamber remained low and unchanged simultaneous to the sudden methane increase a significant backscatter anomaly was recorded and persisted for 30 min and covered the entire depth range 100 m of the upward looking adcp data analyses revealed that a single phase plume no bubbles outburst likely occurred during this time while bubbles appeared to be present during some periods plume simulations revealed that the volume of gas required rate of 8 ton day does not support a bubble plume a second data set was obtained during lander deployments at rock garden where visual observations by rov confirmed the transient pattern of free gas injection into the water column acoustic flares and methane concentration increase in the bottom water hint towards a pressure tidal induced discharge mechanism the presented data demonstrate the temporal and spatial variability of seabed methane emission and very short methane signal lifetime in the water column hours to a few days due to turbulent diffusion both have to be considered when methane budgets are extrapolated from single methane emission rates

origin of seaward dipping reflectors in oceanic crust off the norwegian margin by subaerial sea floor spreading a remarkable layered acoustic stratification is observed in the upper oceanic crust adjacent to the norwegian continental margin it comprises a seaward dipping complex of reflectors in the form of a wedge we suggest that it is a layered igneous sequence that results when crustal accretion occurs at a subaerial spreading axis and that this phenomenon may commonly occur during the earliest phase of ocean basin genesis

engineered and natural marine seep bubble driven buoyancy flows abstract bubble plume upwelling flows were studied in the marine environment through dye releases into engineered plumes and a natural hydrocarbon seep plume for engineered plumes these experiments measured the water column averaged upwelling flow vup zo from release depth zo to the sea surface for a wide range of flows q and zo from vup zo the local upwelling flow vup z where z is depth was calculated and found to vary with q as vup z q0 23 for plumes strong enough to penetrate a shallow thermally stratified layer which was in good agreement with published relationships between vup z and q these data were used to interpret data collected at a natural marine seep for the seep the upwelling flow decelerated toward the sea surface in contrast to the engineered plumes which accelerated toward the sea surface data showed the seep bubble plume upwelling flow lifted significantly colder and more saline water the increased density difference between this upwelling fluid and the surrounding

seismic volcanostratigraphy of large volume basaltic extrusive complexes on rifted margins large volume extrusive basaltic constructions have distinct morphologies and seismic properties depending on the eruption and emplacement environments the presence and amount of water is of main importance while local rift basin configuration erosion and resedimentation determine the overall geometry of the volcanic constructions we have developed the concept of seismic volcanostratigraphy a subset of seismic stratigraphy to analyze volcanic deposits imaged on seismic reflection data the method places special focus on identification and mapping of seismic facies units and the volcanological interpretation of these units interpretation of seismic reflection data along the atlantic and western australia rifted margins reveals six characteristic volcanic seismic facies units named 1 landward flows 2 lava delta 3 inner flows 4 inner seaward dipping reflectors inner sdr 5 outer high and 6 outer sdr these units are interpreted in terms of a five stage tectonomagmatic volcanic margin evolution model comprising 1 explosive volcanism in a wet sediment broad basin setting 2 subaerial effusive volcanism forming gilbert type lava deltas along paleoshorelines 3 subaerial effusive volcanism infilling a fairly narrow rift basin 4 shallow marine explosive volcanism as the injection axis is submerged below sea level and finally 5 deep marine sheet flow or pillow basalt volcanism further erosion and resedimentation processes are particularly important during the shallow marine stages seismic volcanostratigraphy provides important constraints on rifted margin development in particular on the prevolcanic basin configuration relative timing of tectonomagmatic events total amount of volcanic rocks location of paleoshorelines and margin subsidence history these parameters give key boundary conditions for understanding the processes forming volcanic margins and other large volume basaltic provinces

increased fluid flow activity in shallow sediments at the 3 km long hugin fracture in the central north sea the north sea hosts a wide variety of seafloor seeps that may be important for transfer of chemical species such as methane from the earth s interior to its exterior here we provide geochemical and geophysical evidence for fluid flow within shallow sediments at the recently discovered 3 km long hugin fracture in the central north sea although venting of gas bubbles was not observed concentrations of dissolved methane were significantly elevated up to six times background values in the water column at various locations above the fracture and microbial mats that form in the presence of methane were observed at the seafloor seismic amplitude anomalies revealed a bright spot at a fault bend that may be the source of the water column methane sediment porewaters recovered in close proximity to the hugin fracture indicate the presence of fluids from two different shallow 500m sources i a reduced fluid characterized by elevated methane concentrations and or high levels of dissolved sulfide up to 6 mmol l 1 and ii a low chlorinity fluid cl 305 mmol l 1 that has low levels of dissolved methane and or sulfide the area of the seafloor affected by the presence of methane enriched fluids is similar to the footprint of seepage from other morphological features in the north sea