eddy resolving observation of the north pacific subtropical mode water abstracthydrographic data obtained by high resolution shipboard observations and argo profiling floats have been analyzed to study the mesoscale structure and circulation of the north pacific subtropical mode water stmw the float data show that in the late winter of 2008 stmw having a temperature of approximately 18 8 17 7 and 16 6 c formed west of 140 e at 140 150 e and east of 150 e respectively in the recirculation gyre south of the kuroshio extension after spring the newly formed stmw gradually shift southward decreasing in thickness simultaneously the stmws of 16 6 and 17 7 c are gradually stirred and then mixed in terms of properties in late fall they seem to be integrated to form a single group of stmws having a temperature centered at 17 2 c such stmw circulation in 2008 is much more turbulent than that in 2006 which was investigated in a previous study the difference between the two years is attributed to the more variable state of the kuroshio extension in 2008 associate

growth and collapse of the reunion island volcanoes this work presents the first exhaustive study of the entire surface of the reunion island volcanic system the focus is on the submarine part for which a compilation of all multibeam data collected during the last 20 years has been made different types of submarine features have been identified a coastal shelf debris avalanches and sedimentary deposits erosion canyons volcanic constructions near the coast and seamounts offshore criteria have been defined to differentiate the types of surfaces and to establish their relative chronology where possible debris avalanche deposits are by far the most extensive and voluminous formations in the submarine domain they have built four huge submarine bulges to the east north west and south of the island they form fans 20 30 km wide at the coastline and 100 150 km wide at their ends 70 80 km offshore they were built gradually by the superimposition and or juxtaposition of products moved during landslide episodes involving up to several hundred cubic kilometers of material about 50 individual events deposits can be recognized at the surface the landslides have recurrently dismantled piton des neiges les alizes and piton de la fournaise volcanoes since 2 ma about one third are interpreted as secondary landslides affecting previously emplaced debris avalanche deposits on land landslide deposits are observed in the extensively eroded central area of piton des neiges and in its coastal areas analysis of the present day topography and of geology allows us to identify presumed faults and scars of previous large landslides the submarine bulges are dissected and bound by canyons up to 200 m deep and 40 km long filled with coarse grained sediments and generally connected to streams onshore a large zone of sedimentary accumulation exists to the north east of the island it covers a zone 20 km in width extending up to 15 km offshore volcanic constructions are observed near the coast on both piton des neiges and piton de la fournaise volcanoes and are continuations of subaerial structures individual seamounts are present on the submarine flanks and the surrounding ocean floor a few seem to be young volcanoes but the majority are probably old eroded seamounts this study suggests a larger scale and frequency of mass wasting events on reunion island compared to similar islands the virtual absence of downward flexure of the lithosphere beneath the island probably contributes to this feature the increased number of known flank failure events has to be taken into consideration when assessing hazards from future landslides in particular the probability of landslide generated tsunamis

complete gravity study of piton de la fournaise volcano reunion island abstract we present an interpretation of all available gravity data on piton de la fournaise volcano including a new set collected in the summit area using a linear programming inversion method we determine the extension of the grand brule ultramafic body drilled in 1985 and we propose to interpret it as a frozen magma chamber related to a disappeared volcano we also present evidence for superficial mass excess under the enclos fouque and plaine des sables calderas that we link to the present piton de la fournaise volcanism the detailed gravity survey of the summit area allows us to emphasize the great importance of dyke complexes associated with the ne and se rift zones and with the shallow magma chamber a negative anomaly in the summit area is interpreted as a relative minimum between the positive anomaly of grand brule and those of the western part of the enclos fouque and plaine des sables calderas

dynamic processes of co2 storage in the field 1 multiscale and multipath channeling of co2 flow in the hierarchical fluvial reservoir at cranfield mississippi

volcano destabilization by magma injections in a detachment we report the discovery of a detachment fault within reach of structural investigation in a shield volcano piton des neiges la reunion hotspot the detachment exposed by erosion is located between the top of an extinct gabbroic magma chamber and a cover of debris avalanche deposits the fault simultaneously concentrated ductile and brittle deformation toward the sea hydrothermal alteration in the greenschist facies and repeated sill injections of basic magmas our study of this exceptional detachment exposure suggests that two mechanisms participate in volcano flank failure hydrothermal alteration likely sustained by the heat of cooling sills induces slow inter intrusion deformation whereas sill injections in the fault may trigger rapid cointrusive slip this twofold process of destabilization might currently occur on some active volcanoes such as piton de la fournaise

deposits related to degradation processes on piton des neiges volcano reunion island overview and geological hazard abstract piton des neiges pn volcano on reunion island offers a rare opportunity to study deposits related to degradation processes in a deeply eroded oceanic shield volcano both the inner parts and flanks reveal a large amount of resedimented volcaniclastic material including extensive debris avalanche deposits pn litho structural units first studied by upton and wadsworth 1965 philos trans r soc lond a 271 pp 105 130 are re examined this review highlights the importance of long volcanic repose periods and erosion processes during pn history volcaniclastic deposits have been studied in the field in order to evaluate the spatial and temporal distribution of the three main types of pn degradation processes the deposits of these processes have been classified into 1 talus 2 mudflow and debris flow and 3 debris avalanche lithology frequency and estimated volumes of each deposit type imply that the structural evolution of pn can be considered in terms of the competition between the volcanic productivity and the degradation and erosion processes the occurrence of huge catastrophic avalanches produced by flank failure is convincingly linked to the basaltic activity of pn which implies a very low risk at present on the contrary mudflows and debris flows pose an important risk due to the high population density focussed around the basin outlets moreover if smaller debris avalanches can occur in the cirques of pn another major risk must be evaluated

gravity driven deformation of tenerife measured by insar time series analysis 1 we study the state of deformation of tenerife canary islands using differential synthetic aperture radar interferometry dinsar we apply the small baseline subset sbas dinsar algorithm to radar images acquired from 1992 to 2005 by the ers sensors to determine the deformation rate distribution and the time series for the coherent pixels identified in the island our analysis reveals that the summit area of the volcanic edifice is characterized by a rather continuous subsidence extending well beyond las canadas caldera rim and corresponding to the dense core of the island these results coupled with gps ones structural and geological information and deformation modeling suggest an interpretation based on the gravitational sinking of the dense core of the island into a weak lithosphere and that the volcanic edifice is in a state of compression we also detect more localized deformation patterns correlated with water table changes and variations in the deformation time series associated with the seismic crisis in 2004

impact of aleutian low activity on the stmw formation in the kuroshio recirculation gyre region 1 to understand the formation of north pacific subtropical mode water stmw in the kuroshio recirculation gyre region the cause of stmw thickness variation is investigated using temperature profiles in a historically archived data set the thickness variation is predominantly controlled by the main thermocline depth mtd when the main thermocline deepens shoals the wintertime mixed layer depth can develop not develop and consequently thicker thinner stmw is observed in summer the large scale atmospheric forcing controlling the mtd is explored using a wind driven hindcast ocean model the mtd variation stems primarily from a baroclinic response in the ocean to the aleutian low al activity especially the meridional movement of the al exerts a remarkable influence

three dimensional shear velocity anisotropic model of piton de la fournaise volcano la reunion island from ambient seismic noise we cross correlate 4 years of seismic noise from the seismic network of piton de la fournaise volcano la reunion island to measure the group velocity dispersion curves of rayleigh and love waves we average measurements from vertical and radial components to obtain 577 rayleigh wave dispersion curves the transverse components provided 395 love wave dispersion curves we regionalize the group velocities measurements into 2 d velocity maps between 0 4 and 8 s finally we locally inverted these maps for a pseudo 3 d anisotropic shear velocity model down to 3 km below the sea level using a neighborhood algorithm the 3 d isotropic shear wave model shows three distinct high velocity anomalies surrounded by a low velocity ring the anomaly located below the present plaine des sables could be related to an old intrusive body at the location of the former volcanic center before it migrated toward its present location the second high velocity body located below the summit of the volcano likely corresponds to the actual preferential dyke intrusion zone as highlighted by the seismicity the third high velocity anomaly located below the grandes pentes and the grand brule areas and is an imprint of the solidified magma chamber of the dismantled les alizes volcano radial anisotropy shows two main anomalies positive anisotropy above sea level highlighting the recent edifice of piton de la fournaise with an accumulation of horizontal lava flows and the second one below the sea level with a negative anisotropy corresponding to the ancient edifice of piton de la fournaise dominated by intrusions of vertical dykes

three dimensional structure of the submarine flanks of la reunion inferred from geophysical data 1 la reunion indian ocean constitutes a huge volcanic oceanic system of which most of the volume is submerged we present a study of its submarine part based on the interpretation of magnetic and gravity data compiled from old and recent surveys a model of the submarine internal structure is derived from 3 d and 2 d models using constraints from previous geological and geophysical studies two large scale previously unknown buried volcanic construction zones are discovered in continuation of the island s construction to the east the alizes submarine zone is interpreted as the remnants of les alizes volcano eastward flank whose center is marked by a large hypovolcanic intrusion complex to the southwest the etang sale submarine zone is interpreted as an extension of piton des neiges probably fed by a volcanic rift zone over a large extent they were predominantly built during the matuyama period and thus probably belong to early volcanism a correlation exists between their top and seismic horizons recognized in previous studies and interpreted as the base of the volcanic edifice their morphology suggested a lithospheric bulging beneath la reunion not required to explain our data since the seismic interfaces match the top of our volcanic constructions the coastal shelf coincides with a negative bouguer anomaly belt often associated with magnetic anomalies suggesting a shelf built by hyaloclastites a detailed analysis of the offshore continuation of la montagne massif to the north confirms this hypothesis the gravity analysis confirms that the bathymetric bulges forming the northern eastern southern and western submarine flanks are predominantly built by debris avalanche deposits at the surface

subsurface subtropical fronts of the north pacific as inherent boundaries in the ventilated thermocline the two subsurface temperature density fronts and the accompanying eastward currents in the central and southern part of the north pacific subtropical gyre are identified using the repeat hydrographic sections in the western part of the gyre and the sections over the whole gyre from the world ocean circulation experiment woce hydrographic program whp the northern eastward current corresponds to what has been known as the subtropical countercurrent stcc the countercurrent front is located typically near 24 8n and extends from the western boundary approximately to the international date line the previous suggestion that the stcc is located at the southern edge of north pacific subtropical mode water stmw is confirmed the front appears as the southern boundary of the lower potential vorticity pv apparent oxygen utilization aou waters on the isopycnals within the stmw layer the southern eastward current corresponds to what was recognized earlier but has not been documented in detail the southern countercurrent front is located near 188n west of the date line but shifts to the north east of the date line its eastern limit is around 165 8 wa t 258n the southern front corresponds to the southern boundary of the lower pv aou waters on the isopycnal surfaces of the wide su range including those of north pacific central mode water cmw while cmw seems to contribute largely to this low pv aou layer the contribution from other waters is still significant judging from the associated pv aou features in the thermocline both fronts can be regarded as inherent boundaries in the ventilated thermocline dividing the regions ventilated to distinctive degrees correspondence between the observational features of the countercurrents fronts and some of the previously proposed theories concerning the stcc are discussed

magnetic anomalies at the puna ridge a submarine extension of kilauea volcano implications for lava deposition high resolution magnetic data collected along the axis and the south flank of the puna ridge the submarine extension of the east rift zone of kilauea volcano hawaii aid in the interpretation of magmatic processes crustal magnetization varies along the crest of the puna ridge three prominent magnetization highs are located between the depths of 1500 and 2500 m each of the magnetization highs is associated with distinct volcanic features a flow field on top of fissured terrain a lava plateau and a staircase of lava terraces the crustal magnetization highs can be explained by the presence of these volcanic features taking into account water depth and the size of the volcanic features and making simple assumptions about the rock magnetization no measurements of rock magnetization have been made on puna ridge samples we assume two values 50 and 10 a m based on values from mid ocean ridge basalts and the estimated ages of puna ridge rocks clague et al 1995 the two shallowest magnetization highs are adjacent to a landslide scarp lava flows overtop the scarp suggesting that a dike has been emplaced and eruptions have occurred since the landslide formed a possible scenario is that the collapse of the flank led to dike intrusion and eruption at this location because of a reduction in the horizontal stress within the edifice our observations suggest that flows are starting to fill in the gap caused by the landslide to rebuild this section of the north flank

the north pacific climatology of winter mixed layer and mode waters abstract a climatology of the winter mixed layer in the north pacific ocean was constructed using hydrographic data from historical archives and recent observational programs including the world ocean circulation experiment the main aim was to provide better knowledge about source areas of upper water masses the authors have endeavored to preserve water properties near the frontal regions by keeping the smoothing scale as small as possible the resulting climatology shows considerable differences in the mixed layer depth and its water properties from those derived from the world ocean atlas woa maps of the potential vorticity field of the north pacific pycnocline are presented using the isopycnally averaged climatology hydrobase three distinct lateral minima of potential vorticity are identified as subtropical mode water stmw central mode water cmw and eastern subtropical mode water estmw in the western central and eastern parts of the subtropical gyre respectively the hydrobase isopy

deep atmospheric response to the north pacific oceanic subtropical front in spring abstract the north pacific subtropical front stf is a zone of high sea surface temperature sst gradients located around 25 n in the western basin and is most pronounced in spring the stf s atmospheric effects are investigated using satellite observations and an atmospheric reanalysis during april may along the stf surface wind stress curl turns weakly cyclonic in the general background of anticyclonic curls atmospheric column integrated water vapor displays a pronounced meridional maximum along this surface trough suggesting a deep vertical structure cyclonic wind curls occur intermittently at intervals of a few days along the stf in subsynoptic low pressure systems accompanying larger synoptic highs in the main storm track to the north in the subsynoptic surface lows convective rain takes place with deep upward motion moistening the entire troposphere the lows are enhanced by condensational heating leading to the formation of weak cyclonic wind curls the lows display vertical structure ch

subtropical mode water variation in the northwestern part of the north pacific subtropical gyre temporal variations of the north pacific subtropical mode water npstmw are studied with the proper temporal resolution for describing seasonal variation using monthly hydrographic data taken near the ogasawara islands npstmw is formed south of the kuroshio and the kuroshio extension in late winter and advected to the ogasawara site by the southwestward westward return flow of the kuroshio recirculation while npstmw is seen at the ogasawara site in almost all seasons npstmw with a higher core temperature is advected to this site in april and one with a colder core follows in and after june until january or february furthermore the npstmw core temperature shows considerable interannual variation this variation is very similar to that found in other regions away from the ogasawara site suggesting that it reflects the larger scale variation of npstmw and that this site would be suitable for monitoring the npstmw core temperature a significant correlation between the npstmw core temperature and the monsoon index which is defined as the difference in the sea level pressure between nemuro japan and irkutsk russia as an indicator of wintertime cooling indicates that npstmw contains a memory of wintertime cooling in the preceding winter

new 230th 238u and 14c age determinations from piton des neiges volcano reunion a revised chronology for the differentiated series 230th 238u age measurements were carried out on 18 samples of lava from piton des neiges these lavas range from basalts to trachytes the ages extend from 270 000 to 12 000 a b p and cover most of the period in which differentiated series magmas were erupted new 14c ages on charcoal sampled in young pyroclastic deposits are also presented the late stage evolution of the volcano is discussed in the light of these new ages and a chronology of the explosive activity is given the oldest pyroclastic flow ste suzanne is more than 220 000 a other pyroclastic units were emitted between 220 000 and 110 000 a there were at least two episodes of caldera formation the first episode is inferred to have occurred about 190 000 a ago the last occurred between 190 000 and 110 000 a although the most recent lava activity was mainly concentrated on the eastern flank of the volcano and around the summit area it also occurred about 70 000 a ago in the southwestern part of the massif the youngest explosive activity about 50 000 to 20 000 a on the eastern flank of the volcano has now been dated by three different methods 14c and th u ages on block and ash flow tuffs k ar ages on associated domes youthful explosive activity about 35 000 to 12 000 a on the western flank of the volcano is also demonstrated by 14c dating on charcoal and by a th u age of 12 000 a on a trachytic nuee ardente deposit this last age indicates that activity may have persisted later than previously supposed and implies a high rate of erosion for the cirque de mafate

role of olivine cumulates in destabilizing the flanks of hawaiian volcanoes the south flank of kilauea volcano is unstable and has the structure of a huge landslide it is one of at least 17 enormous catastrophic landslides shed from the hawaiian islands mechanisms previously proposed for movement of the south flank invoke slip of the volcanic pile over seafloor sediments slip on a low friction decollement alone cannot explain why the thickest and widest sector of the flank moves more rapidly than the rest or why this section contains a 300 km3 aseismic volume above the seismically defined decollement it is proposed that this aseismic volume adjacent to the caldera in the direction of flank slip consists of olivine cumulates that creep outward pushing the south flank seawards average primary kilauea tholeiitic magma contains about 16 5 wt mgo compared with an average 10 wt mgo for erupted subaerial and submarine basalts this difference requires fractionation of 17 wt 14 vol olivine phenocrysts that accumulate near the base of the magma reservoir where they form cumulates submarine erupted kilauea lavas contain abundant deformed olivine xenocrysts derived from these cumulates deformed dunite formed during the tholeiitic shield stage is also erupted as xenoliths in subsequent alkalic lavas the deformation structures in olivine xenocrysts suggest that the cumulus olivine was densely packed probably with as little as 5 10 vol intercumulus liquid before entrainment of the xenocrysts the olivine cumulates were at magmatic temperatures 1100 c when the xenocrysts were entrained olivine at 1100 c has a rheology similar to ice and the olivine cumulates should flow down and away from the summit of the volcano flow of the olivine cumulates places constant pressure on the unbuttressed seaward flank leading to an extensional region that localizes deep intrusions behind the flank these intrusions add to the seaward push this mechanism ties the source of gravitational instability to the caldera complex and deep rift systems and therefore limits catastrophic sector failure of hawaiian volcanoes to their active growth phase when the core of olivine cumulates is still hot enough to flow

gravity structure of piton de la fournaise volcano and inferred mass transfer during the 2007 crisis abstract the structure and mass transfers at piton de la fournaise volcano are investigated using new on land and offshore measurements the structures are defined using 2 and 3d modelling short wavelength positive anomalies to the w and nw of the central area are interpreted as shallow piles of thick dense lava flows which have filled volcano tectonic depressions several hundreds of meters deep this finding provides a new insight into the evolution of the volcano conversely the currently active central zone is associated with a negative short wavelength anomaly explained by the presence of thin highly vesiculated and fractured lava flows building the central cone low density formations underlying the rivieres des remparts and langevin areas are tentatively interpreted as breccias related to erosion and or to southward directed mass wasting topographic offshore highs in the continuation of the se and ne rift zones of piton de la fournaise are characterized by negative gravity anomalies and reversed magnetic anomalies their interpretation as hyaloclastites and pillow lavas provides a coherent explanation for both types of signals if we admit that they were primarily constructed during the alizes stage two main deeper dense structures are also found the grand brule complex and a complex beneath the plaine des sables and the enclos fouque both are identified as hypovolcanic intrusive complexes the grand brule complex 10 18 km in plan view shows a marked ns elongated geometry unlike the other comparable complexes on the island the plaine des sables enclos fouque complex a more or less circular structure 5 km in diameter is attributed to the ancient shield stage it is centered on the enclos fault suggesting that the paleo center of the ancient shield was located about 1 5 km eastward of the previously proposed location an apparent contradiction was noted between the seismic and gravity images of the internal structure in particular a high velocity cylinder beneath the summit was not detected in the gravity models this contradiction was resolved by considering the relative sensitivity of each method finally the gravity dataset has allowed us to explore the gravity changes that accompanied the major eruptive and volcano tectonic crisis of april 2007 350 m collapse of the summit and emission of 120 106 m3 of lava the change in the mass distribution of the edifice was large enough to be detected in the survey data collected in the central area before and after the crisis however the accuracy was too low for the precise assessment of the mass transfers

volcano structural evolution of piton des neiges reunion island indian ocean four volcano structural stages have accompanied the building of piton des neiges 1 emergent growth stage of the island the major eruptive system is a rift zone trending n 120 associated with dextral strike slip faults trending n 30 and en echelon extensional fissures trending n 70 breccias and lava tubes produced by aerial and phreatomagmatic activity are injected with outward dipping dike swarms along ring fractures suggesting a mechanism analogous to cauldron subsidence 2 shield building stages of growth are related to fissures along the main rift zone and three minor rifts trending n 160 n 45 and n 10 the summit of the basaltic shield volcano is stretched and collapsed in a graben like caldera depression along normal and antithetic faults 3 differentiated lavas are erupted during two stages separated by the opening of a new caldera corresponding to an explosive activity a silicic cone sheet system and a collapse structure 4 younger volcanic activity restricted to the inside caldera has presumably emptied the underlying magma reservoir building a central volcano collapsed along ring internal dip fractures

structure of reunion island indian ocean inferred from the interpretation of gravity anomalies abstract reunion is a volcanic edifice whose origin is related to a hot spot in the indian ocean only 3 of its volume is emergent many geological and geophysical studies were carried out on reunion island during the 1980 s but few of them allow study of the internal structure of the edifice several gravity surveys have been carried out on the island since 1976 and we have compiled the available data set the lack of data on the western side of the island led us to conduct a regional survey in 1993 to obtain a more homogeneous distribution of the stations computation of bouguer anomalies for different correction densities accounts for the variable density of the rocks constituting the edifice and provides a distribution of gravity anomalies interpreted as dense bodies of intrusive rocks inside the edifice two very large intrusive complexes can be unambiguously recognised one beneath piton des neiges and one beneath the grand brule area both have been penetrated by geothermal exploration drill holes and the first is also known from outcrop observations 2 5d simple models were constructed to reveal the geometry and extent of the buried intrusives they are deeply rooted extending several kilometres below sea level and extensive 20 25 km long and 10 13 km wide for the piton des neiges complex 12 15 km long and some kilometres wide for the grand brule complex the development of such complexes implies that the activity of the two volcanic centres was long lasting and remained stable while the volcanoes were growing the grand brule complex has been interpreted as relics of an old volcano named alizes volcano the interpretation of the gravity maps suggests the presence of a ridge of dense rocks to the north of the axis joining the centres of piton des neiges and piton de la fournaise volcanoes by analogy with the other structures 2 5d models show that this structure would culminate between 0 and 1 km below sea level and be 15 km wide this complex induces a maximum anomaly in takamaka valley and we thus propose to name it takamaka volcano no geological evidence of the nature of these dense rocks is available but the ridge coincides with structures revealed by magnetic and seismic data interpretation of the bouguer anomaly maps suggests that the inner gravity structure of piton de la fournaise is not characterised by the presence of a voluminous dense body but probably by more restricted concentrations of dense rocks some structures can be recognised along the present ne and se rift zones and in the previous central part of piton de la fournaise to the west of the present summit the recent eastward migration of the centre of activity of piton de la fournaise accounts for the lack of a large positive anomaly beneath the active craters

deformation of basaltic shield volcanoes under cointrusive stress permutations we performed a microstructural study of piton des neiges la reunion island to understand how intrusions and stresses control each other in basaltic volcanoes our study reveals that three perpendicular intrusions trends coexisted during the 2 myr history of the volcano a n120 140 e rift zone a perpendicular dike trend and swarms of subhorizontal intrusions hereafter called sill zones independently the inversion of fault slip data shows that incompatible paleostress fields recurrently occurred along with the intrusions a dominant nne ssw extension a perpendicular extension and strike slip or compressional regimes the orientations of paleostresses are consistent with the orientations of the three perpendicular intrusion populations we propose that stress accumulation in the edifice under the effect of repeated magma injections resulted in permutations of the principal axes of the stress tensor causing a reorientation of subsequent intrusions stress permutations were cyclical each cycle started with dike injections in an extensional stress field reducing the deviatoric stress and switching the axes of principal stresses and finished with sill intrusions in a compressional stress field sill zones acted as detachment planes restoring the extensional stress field and initiating a new cycle of permutations our model of stress permutations is in agreement with the pattern of eruptions and deformation observed at piton de la fournaise in contrast with the hawaiian model of spreading on a decollement stress permutations in la reunion s volcanoes imply that the basal deformation of the edifices if any is not sufficient to compensate the reduction of deviatoric stress caused by intrusions

ventilated thermocline strongly affected by a deep mixed layer a theory for subtropical countercurrent abstract the mixed layer thickness generally increases in the northward direction but its change seems to occur rather sharply in a narrow transition zone referred to as a mixed layer front here the author investigates the effects of the mixed layer front on the ventilated thermocline structure especially focusing on the generation mechanism of the subtropical countercurrent if the mixed layer front is not parallel to the surface density contour the fluid with minimum isopycnal potential vorticity is formed around the intersection of the mixed layer front and outcrop line if the mixed layer front slants northeastward and surface density is zonally uniform as seen in a numerical experiment carried out by kubokawa and inui in which the subtropical countercurrent was reproduced the minimum potential vorticity on a high density isopycnal occurs in the east while that in a low density isopycnal occurs in the west in the present study analytic solutions for a simple three layer model are presented f

differential formation and circulation of north pacific central mode water abstract a repeat hydrographic section along 165 e was analyzed to verify a westward extension of the formation region of the north pacific ocean central mode water cmw suggested by previous synoptic observations and to investigate the relation between the formation region and thermohaline fronts the cmw formation region extends at least as far west as 155 e much farther than recognized in a previous study based on climatology it is located in two interfrontal regions between the kuroshio extension front and the kuroshio bifurcation front kbf and between the kbf and the subarctic front where two types of cmw namely the lighter variety with potential density of 25 8 26 2 kg m 3 and the denser one of 26 3 26 4 kg m 3 are formed how this differential formation of cmw is reflected in its gyrewide distribution was examined using one time sections of the world ocean circulation experiment woce hydrographic program in the north pacific the main circulation paths of the two types of cmw diverge eas

the april 2007 eruption and the dolomieu crater collapse two major events at piton de la fournaise la reunion island indian ocean abstract after 10 years of high activity and 24 eruptions piton de la fournaise pdf produced on april 2 to may 1 2007 one of its most voluminous and intense eruption since at least one century the eruption focused at 590 m elevation in the grand brule on the south east flank of piton de la fournaise 7 km away from summit it was located close to the southern cliff of the grand brule volcano tectonic collapse the rempart du tremblet and to the nearby village called le tremblet and it highly rattled and incommoded the inhabitants there eleven hours after the beginning of the eruption the lava flows reached the sea and created 0 45 km 2 of new land until the end of eruption in the night of april 5 the rock column beneath dolomieu crater started to collapse with a piston like mechanism into the magma chamber forming within less than 24 h the most recent and well documented caldera we report in this paper the chronology of these two related major events in the recent activity of pdf seen by the scientific networks of the volcanological observatory ovpf ipgp and field observations which allowed to back up the scenario of the caldera formation and to quantify these two exceptional episodes

landslides and spreading of oceanic hot spot and arc shield volcanoes on low strength layers lsls an analogue modeling approach during their growth and evolution oceanic hot spot and arc shield volcanoes are destabilized and often dismantled by landslides the origin of these phenomena can be found in the structure of volcanic islands marked by several low strength horizons such as a marine substratum hyaloclastites hydrothermally altered rocks or volcaniclastic deltas that weaken the edifice we have tested the role of these low strength layers lsls with analogue modeling we conclude that destabilization on these levels can lead to landslides or gravitational spreading in our experiments slides generated are only slump type events but they can evolve in nature into debris avalanches through mechanisms that could not be modeled in the laboratory the role of fan shaped volcaniclastic deltas included in the edifice structure is critical to slump formation this process is more efficient if deltas are associated with another upper lsl such as weak hyaloclastites or hydrothermally altered rocks in this case recurrent events generating collapses with increasing width are observed gravitational spreading of a volcanic island occurs when it is underlain by a ductile marine substratum or includes an intra edifice unconstrained hyaloclastic layer 1 3 as thick as the height of the subaerial edifice spreading is characterized by edifice stretching and subsidence and does not generate major landslides we also show that gravitational spreading on a marine substratum tends to inhibit slide generation especially when the thickness of the sedimentary layer is larger than 1 10 of the edifice total height however if this value is lower spreading occurs later and slumps can develop initially we describe natural examples of oceanic hot spot or arc shield volcanoes affected by both spreading and landslides such as reunion island hawaii island james ross island antarctica and martinique island antilles the main structural features on these volcanoes can be explained solely by deformation on lsls we therefore propose a general model for the destabilization of volcanic islands that is applicable to all shield like edifices

effect of decadal kuroshio extension jet and eddy variability on the modification of north pacific intermediate water abstract salinity modifications in the north pacific intermediate water npiw core layer of 26 7 26 8 in the western north pacific ocean are investigated using temperature salinity data from available profiling float and hydrographic measurements in 2002 09 during 2002 05 when the kuroshio extension ke jet was intense and zonally elongated coherent positive salinity anomalies appeared along the inflow ke jet southeast of japan and in the downstream mixed water region east of 152 e broad scale negative salinity anomalies were detected south of the ke jet and in the upstream mixed water region west of 152 e the signs of these observed salinity anomalies were reversed in 2006 09 when the ke jet transitioned to a weakened and zonally contracted dynamic state by adopting an isopycnal advection diffusion model and conducting model runs with the time dependent advective field inferred from the eddy resolving satellite altimeter sea surface height data it is found that the observed salinity anomalie

the grand brule exploration drilling new data on the deep framework of the piton de la fournaise volcano part 1 lithostratigraphic units and volcanostructural implications abstract the sinking of a deep drill hole 3003 5 m for geothermal exploration in the eastern part of the piton de la fournaise volcano on reunion island has revealed the underlying lithostratigraphy six principal sequences have been distinguished a 0 440 m subaerial basaltic flows b 440 556 m a transitional series characterized by an alternation of subaerial basalts and submarine deposits vitric basalt and hyaloclastic breccia c 556 616 m submarine basalts lavas and hyaloclastites d 616 890 m subaerial basalts e 890 1010 m submarine basaltic deposits with abundant argillized hyaloclastites f 1010 3003 5 m a layered intrusive complex comprising 1010 1129 m sheets of gabbro injected into the lavas and hyaloclastites 1129 1920 m a zoned intrusion of olivine gabbro and dolerite 1920 3003 5 m stratified cumulate gabbro wehrlite dunite the occurrence of the first submarine deposits at 440 m 268 m below present sea level and the intercalation with them of subaerial basalts down to 890 m 728 m below present sea level tend to support a hypothesis of sliding into the sea during subsidence of the eastern flank of the fournaise massif the intrusive complex intersected from 1010 m down is regarded as a former shallow magma chamber a large positive gravimetric anomaly 40 50 mgal centered over grand brule and attributed to the intrusive complex indicates that its roof forms an ellipse 3 20 km elongated n s stratigraphic and morphological structural evidence on land and on the sea floor and the results of the mineralogical study of this very large magma chamber suggest that it belongs to an ancient edifice proto fournaise whose activity preceded the construction of the present fournaise and ended at about 0 6 ma on land the evidence for this earlier edifice is to be found in the zeolitised basalts and differentiated lavas hawaiites forming the basement to the phase 1 lava flows certain subsidence structures on the present fournaise are interpreted as resulting from settling of the proto fournaise volcano the destabilization of the east flank grand brule area of the currently active volcano and the location of the ne and se rift zones could be attributed to the same phenomenon

geothermal model of the salazie cirque reunion island volcanic and structural implications abstract a programme of geothermal exploration on reunion island resulted in the siting of an exploration geothermal borehole in salazie cirque the borehole attained a depth of 2108 m and encountered temperatures of 192 8 c no aquifer was found the geological information obtained made it possible to propose a stratigraphy for the deep underlying rocks beneath the salazie cirque on the piton des neiges and thus to interpret the events that followed the emergence of the island a large gabbroic mass was intersected and found to comprise several successive intrusions the principal one of which is continuously zoned from melagabbro to monzonite modeling of geophysical data on the drillhole results enabled the distribution of the different geological units to be interpreted a geothermal model is proposed taking into account the dynamic evolution of the hydrothermal processes that preceded and accompanied emplacement of the gabbro responsible for the geothermal anomaly underlying the three cirques that sculpt the flanks of the piton des neiges

magma transport and storage at piton de la fournaise la reunion between 1972 and 2007 a review of geophysical and geochemical data abstract since the middle of the 20th century improvement in volcano monitoring techniques has provided large data sets that can be used to evaluate the evolution of magma plumbing systems at piton de la fournaise volcano abundant eruptive activity and a dense monitoring network are especially conducive to such research analysis of the extensive set of geophysical and geochemical data associated with the last 35 years of activity provides new insights into magma transport and storage at piton de la fournaise two periods of frequent eruptions separated by 6 years of inactivity can be distinguished 1972 1992 and 1998 2007 considering these two periods we show evidence of major changes in the shallow plumbing system in 2000 during 1972 1992 and 1998 2000 lava compositions and weak long term eruptive precursors lasting few days to three weeks characterized by no significant summit inflation and about ten volcano tectonic earthquakes per day suggest that eruptions were fed by the progressive drainage of an occasionally recharged shallow magma reservoir geophysical evidence of shallow magma reservoir recharge was only recorded in 1986 and 1998 in contrast from 2000 to 2007 geophysical data highlight the appearance of long term precursors 1 to 5 months of summit inflation and strong seismicity up to one hundred events per day preceding cycles of successive eruptions each cycle of eruptions was characterized by a sequence 3 to 10 months in duration of summit and near summit proximal eruptions and ended with a distal low altitude oceanite olivine abundances 20 vol eruption on the eastern flank of the volcano within each of these eruptive cycles lavas became progressively more magnesian over time the preferential motion of the eastern flank caused by continuous recharging of the shallow reservoir since 2000 would favour the occurrence of distal eruptions towards this flank at the end of a cycle

using principal components analysis and na cl ratios to trace groundwater circulation in a volcanic island the example of reunion abstract the development of the limited groundwater resources of a remote tropical island in particularly rugged volcanic terrain requires a thorough understanding of the hydrogeology the cost of a classical approach by borehole investigations often precludes a reasoned approach resulting in a somewhat haphazard development of resources wuth varying success this paper describes the use of multivariate statistical analysis to trace groundwater circulation in volcanic terrains the method involves the use of principal components analysis pca based on both structural and hydrochemical parameters of 243 springs of reunion western indian ocean as a result we propose the use of na cl analysis in differentiating three types of volcanic aquifers this analysis is consistent with a geological and hydrogeological conceptual model developed from a combined hydrochemical and geological reconnaissance of 27 springs

anatomy of piton de la fournaise volcano la reunion indian ocean the aim of this work is to propose a general model of piton de la fournaise volcano using information from geological and geophysical studies firstly we make a graphical compilation of all available geophysical information along a w e profile secondly we construct a geological section that integrates both the geophysical information and the geological information the lithosphere beneath piton de la fournaise is not significantly flexed and the crust is underlain by an underplating body which might represent the deep magma reservoir for la reunion volcanism piton de la fournaise is a relatively thin volcano lying on a huge volcanic construction attributed mostly to les alizes volcano indeed if the differentiated rocks observed at the bottom of the riviere des remparts are the top of les alizes volcano the interface with piton de la fournaise may be located at about sea level beneath the summit area the endogenous constructions intrusive complexes related to les alizes and piton de la fournaise volcanoes represent a large volume the huge intrusive complex of les alizes volcano probably rests on the top of the oceanic crust and appears to have a buttressing effect for the present eastern volcano tectonic activity of piton de la fournaise the early piton de la fournaise edifice was built around a focus located beneath the plaine des sables area the center subsequently moved 5 6 km eastward to its current location the dense high velocity body beneath the plaines des sables and the western part of the enclos probably corresponds to the hypovolcanic intrusive complex that developed before the volcanic center shifted to its present day position magma reservoirs may have existed and may still exist as illustrated by the march 1998 crisis at the mechanical and density interface between the oceanic crust and the les alizes edifice strong evidence also exists for the presence of a shallower magma reservoir located near sea level beneath the summit the march 1998 pre eruptive seismic pattern location and upward migration seems to be evidence for a transfer of magma between the two reservoirs the dominant structural feature of the central zone is a collapse structure beneath the summit craters above the inferred magma reservoir near sea level the collapsed column constitutes a major mechanical heterogeneity and concentrates most of the seismic intrusive and hydrothermal activity because of its higher permeability and weaker mechanical strength

subduction of north pacific central mode water associated with subsurface mesoscale eddy 1 during a shipboard high density hydrographic survey carried out in the western north pacific in fall 2008 we observed an anticyclonic eddy with a thickness of 150 dbar and a diameter of 40 km near 500 dbar depth at 27 5 n 145 e this subsurface mesoscale eddy contains the north pacific central mode water cmw which has anomalously low potential vorticity and high dissolved oxygen compared to the climatological cmw properties in the same region profiling float measurements detect similar cmw patches near and south of the kuroshio extension as well as southward cmw migration within the cmw formation region north of the kuroshio extension these observed facts suggest that cmw is subducted into the permanent pycnocline not only through large scale eastward advection near the northern edge of the subtropical gyre but also through southward cross frontal advection associated with the formation and migration of subsurface mesoscale eddies

reunion hotspot activitity through tertiary time initial results from the ocean drilling program leg 115 abstract leg 115 of the ocean drilling program recovered basaltic rocks from four sites along the ancient trail of the reunion hotspot the age of volcanism determined from biostratigraphic data at the four sites increases to the north and records the motion of india away from the reunion hotspot through tertiary time hotspot activity began with the eruption of the deccan flood basalt flows at the time of the cretaceous tertiary boundary the reunion hotspot has been stationary with respect to other hotspots in the atlantic and indian ocean basins through tertiary time the geochemical compositions of the drilled basalts differ according to the relative contributions of magmas from hotspot and morb mantle sources

a new model for the evolution of the volcanic island of reunion indian ocean the island of reunion has been studied using data from airborne and shipborne magnetic surveys the subaerial history of reunion spans the last 2 1 million years the brunhes matuyama geomagnetic reversal enables differentiation of volcanic rocks older and younger than 0 78 ma the lower submarine flanks are poorly magnetized and are interpreted as landslide deposits the core of the island is composed of highly magnetized rocks piton des neiges volcano is composed mostly of rocks older than 0 78 ma only its western flank and central area include thick piles of younger rocks piton de la fournaise is a highly and normally magnetized edifice but its northern and eastern flanks are underlain at shallow depth by reversely magnetized formations the latter are regarded as remnants of les alizes volcano associated with the grand brule hypovolcanic complex at the matuyama brunhes transition the island was composed of at least two main volcanoes piton des neiges and les alizes and perhaps also of a third old volcano takamaka at the center north of the island piton de la fournaise grows on the flank of les alizes and is a relatively young focus of volcanism these volcanoes have had successive phases of construction and destruction the analysis of magnetic anomalies over reunion was decisive in defining a new coherent model for the evolution of the island

kar chronology of the ultimate activity of piton des neiges volcano reunion island indian ocean abstract potassium argon measurements were made on 17 samples from the ultimate stage of activity of the piton des neiges volcano which corresponds to a differentiated series with lavas varying from basalt to quartz trachyte samples were selected as a function of their stratigraphic position and their representativity of magmatic evolution the technology used to date young volcanic rocks pleistocene by k ar is described and discussed because of its reliability in the analysis of modern lavas and because of the agreement of the results with the available stratigraphic control the data together with those previously obtained by mcdougall yield ages ranging between 330 000 and 30 000 years with two mainly effusive periods separated by a phase of instability of chiefly explosive volcanism around 190 000 to 150 000 years ago a rough correlation between the chemical evolution of the lavas and time is established the kudo and weil geothermometer when cross checked with buddington and lindsley s geothermometer allows us to establish that the lavas of the two effusive periods correspond to two different depths of the magmatic chamber in the oceanic crust 17 and 10 km respectively the phase of instability may correspond to a sudden variation in these magmatic conditions the differentiated series the ultimate stage of activity of the piton des neiges volcano 330 000 and 30 000 years can be explained as the evolution of a magmatic body isolated when the focus of basaltic eruption migrated southeast from piton des neiges to piton de la fournaise

variability of the kuroshio extension jet recirculation gyre and mesoscale eddies on decadal time scales abstract twelve years of sea surface height ssh data from multiple satellite altimeters are used to investigate the low frequency changes and the interconnections of the kuroshio extension ke jet its southern recirculation gyre and their mesoscale eddy field the dominant signal is characterized by the steady weakening of the ke jet recirculation gyre from 1993 to 1996 followed by a gradual strengthening after 1997 during the weakening period of 1993 96 the ke path migrated southward in general and this path migration reversed in direction during the strengthening period of the ke jet and recirculation gyre after 1997 by hindcasting the ssh signals using linear vorticity dynamics it was found that weakening strengthening in the ke jet and recirculation gyre is consistent with westward propagation of negative positive ssh anomalies generating in the eastern north pacific and strengthening during their westward propagation when the ke jet and recirculation gyre were in a weak mode during 19

petrology of 1977 to 1998 eruptions of piton de la fournaise la reunion island abstract geochemical and petrological variations in 35 historical eruptions from the april 1977 eruption to the march september 1998 eruption at piton de la fournaise have been analyzed for their major elements abundances mineralogical compositions and oxygen isotopes during these 21 years samplings were repeated several times during eruptions as well as temperature measurements since the end of 1977 to 1998 all erupted lavas except 1998 hudson basalt belong to the group of the steady state basalts however evidences for small degrees of differentiation can be observed during a single eruption the earliest lava emitted contains less phenocrysts and has almost always a lower mgo mgo mgo feot than the lavas emitted later suggesting that it is more differentiated from one eruption to the next between 1977 and 1998 bulk compositions are more and more differentiated towards the first lava of 1998 a process of low pressure fractional crystallisation involving small proportions of olivine clinopyroxene and plagioclase allows to explain this evolution a major result of our study is to show that this evolution does not correspond to a unique fractionation trend over the considered period different types of magmas have been distinguished the first magma type erupted from october 1977 to november 1987 was richer in potassium 0 82 wt in average than the second one and apparently derived from the liquid of the 1977 oceanite the second magma type erupted from february 1988 to september 1998 was poorer in k 2 o 0 69 wt in average in march 1998 a third magma type hudson basalt erupted simultaneously with the second one hudson basalt has undergone a wehrlitic fractionation and is supposed to come from a deeper magma reservoir than other erupted magmas for this period from 1977 to 1998 the 18 o of lavas progressively decreases with time together with the increasing chemical differentiation of the magmas by low pressure fractional crystallisation this variation agrees with the hypothesis of magma contamination at shallow level in the altered volcanic edifice vlastelic i deniel c bosq c telouk p boivin p bachelery p famin v staudacher t 2009 this issue pb isotope geochemistry of piton de la fournaise historical lavas journal of volcanology and geothermal research 184 63 78 this issue overall petrological variations are in good agreement with a model of a shallow reservoir discontinuously supplied between 1977 and 1998 the speed of the variations the coexistence of different liquids the chemical evolution of the lavas during the same eruption lead us to favour a model of chemically stratified reservoir constituted by a complex of dykes sills and laccoliths close to the surface and located above larger magma bodies

geothermal study of reunion island audiomagnetotelluric survey abstract in 1979 and 1980 535 magnetotelluric soundings using a frequency range of 1700 hz 8 hz were performed on the island of reunion for geothermal exploration few direct hot water discharges were observed consequently geophysical methods particularly the audiomagnetotelluric method were used extensively favourable geological conditions for this method were encountered and the results which were controlled using classical electrical methods on test areas suggest an unusual distribution of resistivities for lava flows situated above suspected geothermal areas these layers progressively decrease in resistivity down to a very conductive layer in areas where these conductive layers were nearest the surface detailed studies were carried out showing a close correlation between decreasing resistivity and increasing hydrothermal alteration in addition gradient wells reveal high geothermal gradients in such areas the conductive layers revealed by audiomagnetotelluric soundings seem to correlate with thermal effects creating progressive hydrothermal alteration from an inferred hot water reservoir up toward the surface

eddy mean flow interaction in the decadally modulating kuroshio extension system abstract satellite altimeter sea surface height ssh data of the past 16 years are used to investigate the decadal changes of the kuroshio extension ke system that oscillated between a stable and an unstable dynamic state during the stable state of 10 1992 06 1995 and 01 2002 12 2004 the ke jet was intense and had a northerly zonal mean path and a well defined southern recirculation gyre during the unstable state of 07 1995 12 2001 and 01 2005 present the ke jet had a reduced eastward transport and a more southerly flow path transitions between the two dynamic states are caused by the basin scale wind stress curl forcing in the eastern north pacific related to the pacific decadal oscillations pdos or the north pacific gyre oscillations npgos during the positive pdo or negative npgo phase the intensified aleutian low generates negative ssh anomalies in the eastern north pacific through ekman divergence as these wind induced negative ssh anomalies propagate to the west as baroclinic rossby waves they weaken the zonal ke jet and shift its path southward as its path is pushed southward 32 n the deep reaching ke jet has to ride over the shallow shatsky rise generating localized disturbances that lead to their subsequent development along the ke jet west of the shatsky rise the sequence opposite to that listed above occurs when the pdo npgo changes to its negative positive phase after the ke system transitions from a stable to unstable state the enhanced eddy interaction is found to strengthen both the southern recirculation gyre and quasi stationary meanders along the ke s upstream path this nonlinear eddy interaction is important in determining the amplitude of the observed decadal ke variability

a role of eddies in formation and transport of north pacific subtropical mode water 1 hydrographic data acquired during 2001 by argo profiling floats deployed in the kuroshio recirculation region are used to verify the inference that mesoscale eddies prevailing in the recirculation region play an important role in the formation and transport of north pacific subtropical mode water npstmw that is the deeper winter mixed layer is formed preferentially where the thermocline is deeper in association with anticyclonic eddies in the succeeding seasons mesoscale eddies retain npstmw during their southwestward movement from the npstmw formation region so that anticylonic eddies contribute substantially to the npstmw transportation the spatial distributions of the mixed layer depth in winter and the npstmw thickness in the succeeding seasons based on the float data suggest that they are affected by mesoscale eddies the float data also provide the statistical relations of these quantities against the thermocline depth which are consistent with our inference

hydrothermal history of piton des neiges volcano reunion island indian ocean abstract the piton des neiges volcano on reunion island represents a unique example of an oceanic volcano where the extreme development of amphitheatre headed valley erosion has led to the formation of three large cirques they are so large that the island s volcano structural and petrological history can be traced from its emergence to the latest stages of its sub aerial evolution 2 1 m y to 22 000 years ago the various magmatic series of the piton des neiges are moreover abundantly invaded by hydrothermal mineralization it is this post magmatic feature represented by the hydrothermal alteration of the series which is examined here mineralogical studies x ray microprobe scanning electron microscope reveal a large number of hydrothermal species of these zeolites are the most common and five are described here for the first time in reunion gonnardite levynite erionite garronite herschelite six hydrothermal facies characteristic of weak metamorphism are defined chabazitephillipsite natrolite thomsonite analcime thomsonite laumontite thomsonite albiteprehnite and prehnite pumpellyite the paleo temperatures covered by these facies range from 0 to 380 c on the basis of these data and supporting field observations three main hydrothermal phases were determined and fitted into the known chronostratigraphy these three phases have succeeded one another over the last two million years in the piton des neiges massif the extent and mineralogical facies of each phase can be related to the volcanotectonic structures this sequence has been directly linked to the geological evolution of the massif the progressive restriction with time of the hydrothermal manifestations to the present piton des neiges occurred alongside the focusing of volcanism centralized on this same relief an attempt is made to reconstruct the island s hydrothermal history

complex structure of piton de la fournaise and its underlying lithosphere revealed by magnetotelluric 3d inversion abstract la reunion is a large volcanic construction resting on paleocene oceanic crust through the 3d inversion of a large set of magnetotelluric mt soundings our results reveal the general resistivity structure of the western part of piton de la fournaise volcano down to its base and the first 10 km or so of the underlying lithosphere the resistivity pattern shows a general stratification of the resistivity values an upper resistive layer corresponds to unsaturated and water saturated lava flows this layer is thinner a few hundred meters in the enclos and plaine des sables fond de la riviere de l est areas than on the nw flank where it reaches 2000 m below the rest of the edifice is distinctly more conductive and shows highly conductive patches the origin of the globally weak resistivity of the lower part of the construction is not established but can be tentatively attributed to a higher degree of alteration the case of the highly conductive patches is different resistivity values of a few m imply the presence of highly conductive fluids and or minerals that in this context are generally associated with hydrothermal phenomena in the enclos highly conductive patches are unambiguously attributed to the presently active hydrothermal system beneath the plaine des sables fond de la riviere de l est they may be associated with hydrothermal alteration developed by the ancient volcanic center the larger highly conductive patches are found below sea level and above the crust beneath the nw flank at depths significantly larger than the others this area coincides with the path of the n120 rift zone with a deep 10 20 km seismicity and with diffuse co2 degassing a relationship may therefore be inferred between deep magmatic activity eruptive activity and postulated highly conductive hydrothermal zone in the globally weak resistivity of the lower part of the edifice a dome of moderate resistivity 1 k m is centered beneath the plaine des sables it is wider than a dense intrusive complex inferred from gravity models in the same area the present resolution of both mt and gravity models cannot exclude a same nature for both structures beneath 4 km b s l an increase of the resistivity values underlines an interface in good agreement with the inferred location of the transition between the edifice and the oceanic crust at this depth the upper lithosphere shows resistivity values above 250 m this study therefore demonstrates the capability of the method to image major shallow structures such as the hydrothermally altered zones and deeper ones such as the heterogeneity of the crust

three hawaiian calderas an origin through loading by shallow intrusions the calderas of kilauea and mauna loa are highly dynamic structures and in the 200 year historic period have varied in volume by a factor of 2 and gained or lost 1 km3 per century the deeply eroded caldera of the extinct koolau volcano in oahu is wider than active hawaiian calderas and its lavas have a strong centripetal dip and funnel structure not evident at kilauea or mauna loa the differences can be attributed to the different erosion depths and the time integrated subsidence profile of kilauea is also a stepped funnel having its apex at halemaumau koolau caldera is the focus of an extraordinarily intense dike complex and an intriguing feature is the great diminution in dike concentration into the caldera it is thought that dike injection in any part of the complex generally continued until it reached 50 to 65 in outer parts of the caldera the complex was maintained at or rebuilt to this value despite subsidence in the center of the caldera where the positive bouguer anomaly is centered subsidence evidently greatly outpaced the capacity of dike injections to rebuild the complex assuming the same dike injection rate as kilauea and mauna loa yields a volumetric subsidence rate in koolau caldera exceeding 1 km3 per century hawaiian calderas are much more dynamic than calderas of silicic volcanoes shaped by frequent small events instead of a few great ones the temporal and volumetric correspondence of historical subsidence events with eruptions is poor and this and the high subsidence rates argue for a caldera forming mechanism that consumes the subsided rocks it is suggested that subsidence is caused by the great localized excess load of intrusive rocks carrying the center of the volcano into the thermally weakened lithosphere above the hawaiian hot spot it is envisaged that under steady state conditions the magma chamber rises as the injection of intrusions causes the level of neutral buoyancy at which the chamber is located to ascend and keeps pace with subsidence

internal architecture of la reunion indian ocean inferred from geophysical data the internal structure of la reunion indian ocean is investigated using gravity magnetic and electromagnetic data a general model of the island is derived providing new information on the individual volcanoes as well as on the growth of the island as a whole the reconstructed topography of the island at 780 ka brunhes matuyama magnetic reversal shows a paleo island organized around a huge piton des neiges volcano and a smaller les alizes volcano both having large offshore extensions and dense hypovolcanic complexes during the brunhes period the piton des neiges has growth predominantly in its central and west southwest areas and to a less extent to the north the construction of piton de la fournaise at the east has caused the other major change in the topography of the island this latter volcano the youngest and the smallest of the three identified volcanoes of la reunion only achieves a large thickness beneath its central and southern zones the resistivity and magnetic patterns of its central area support the presence of a hydrothermal system in its western older part shallow highly resistive and dense layers are interpreted as piles of thick lava flows filling paleo depressions in the same area a deep dense body may represent the hypovolcanic intrusive complex from the early stages of piton de la fournaise piton des neiges is by far the largest volcano of la reunion the resistivity data show that its products are more altered than those of piton de la fournaise a hydrothermal alteration is particularly developed around the dense central hypovolcanic intrusive complex this latter characterized by resistive and dense terrains has a lateral extent that coincides with that of the overlying topographic depressions the three central cirques the bebour belouve depression and at its eastern tip the plaine des palmistes a genetic link between the morphology of the volcano and a possible subsidence of the dense complex is investigated but no definitive conclusion is reached the offshore part and the hypovolcanic complex of les alizes volcano have been described by previous works here we identified the reversely magnetized terrains beneath piton de la fournaise with at least part of the construction of les alizes volcano but no geophysical contrast marking at depth the boundary between piton de la fournaise and les alizes is recognized suggesting that the products of both volcanoes are lithologically similar at the scale of our work

observations of the subtropical mode water evolution from the kuroshio extension system study abstract properties and seasonal evolution of north pacific ocean subtropical mode water stmw within and south of the kuroshio extension recirculation gyre are analyzed from profiling float data and additional hydrographic and shipboard adcp measurements taken during 2004 the presence of an enhanced recirculation gyre and relatively low mesoscale eddy variability rendered this year favorable for the formation of stmw within the recirculation gyre stmw formed from late winter convection that reached depths greater than 450 m near the center of the gyre the lower boundary of stmw corresponding to 25 5 kg m 3 was set by the maximum depth of the late winter mixed layer properties within the deep portions of the stmw layer remained largely unchanged as the season progressed in contrast the upper boundary of the stmw layer eroded steadily as the seasonal thermocline deepened from late april to august vertical eddy diffusivity responsible for this erosion was estimated from a budget analysis of

formation and subduction of central mode water based on profiling float data 2003 08 abstract temperature and salinity data from argo profiling floats in the north pacific during 2003 08 have been analyzed to study the structure of winter mixed layer north of the kuroshio extension and the subsurface potential vorticity distribution in the subtropical gyre in relation to the formation and subduction of the central mode water cmw in late winter two zonally elongated bands of deep mixed layer extend at 33 39 and 39 43 n from the east coast of japan to 160 w these correspond to the formation region of the lighter variety of cmw l cmw and that of the denser variety of cmw d cmw and the recently identified transition region mode water trmw respectively in the western part of the l cmw and d cmw trmw formation regions west of 170 e the winter mixed layer becomes deeper and lighter to the east i e to the downstream as a result the formed mode water is reentrained into the mixed layer in the farther east in the following winter and modified to the lighter water and is thus

reemergence areas of winter sea surface temperature anomalies in the world s oceans 1 using datasets of sea surface temperature sst surface heat flux upper ocean thermal data and climatological temperature and salinity profiles we try to detect reemergence areas of winter sst anomalies in the world s oceans and describe characteristics of these areas in terms of mixed layer depth mld annual mean heat flux and properties of waters formed in winter mixed layer eventually seven reemergence areas are found four in the northern hemisphere and three in the southern hemisphere all areas have a large seasonal variation of mld and are the regions where annual mean heat fluxes are relatively small except for two regions in the northern hemisphere in the viewpoint of water properties it is found that these areas correspond to the mode water formation regions subtropical mode waters of the indian ocean south pacific south atlantic north pacific and north atlantic and north pacific central and north atlantic subpolar mode waters

escape of unradiogenic osmium during sub aerial lava degassing evidence from fumarolic deposits piton de la fournaise reunion island abstract this study presents new re os isotope and elemental data in gas condensates and corresponding lavas in order to examine the geochemical behavior of these two elements during magma degassing at piton de la fournaise reunion island gas sublimates formed between 2007 and 2011 at temperature ranging from 400 to ca 100 c include na k sulfate aphthitalite na sulfate thenardite ca cu sulfate e g gypsum ca mg al fe fluoride e g ralstonite and native sulfur the high temperature deposits show trace element typical of volcanic gas with high enrichment in re 24 to 79 ppb almost two order of magnitude higher than the corresponding lavas but with os abundances similar to those of the lavas 14 132 ppt in contrast the os contents of the low temperature fluoride deposits 13 77 ppb are higher than any of the other condensates the fluorides are also enriched in re albeit to lesser extent than os 2 9 15 3 ppb based on high temperature samples the fluid melt partition coefficients estimated for re and os are 100 80 and 1 2 respectively considering 1 of fluid loss these partition coefficients translate into emanation coefficients of 0 50 0 17 0 65 for re and 0 01 0 0 03 for os these results indicate that re unlike os is highly volatile at piton de la fournaise osmium isotopic compositions of samples collected at medium and low temperature 187 os 188 os between 0 130 0 135 and plot within the range of the april 2007 lava flow and the historical lavas of piton de la fournaise i e 0 130 0 137 however the highest temperature condensates na k sulfates with t of 384 400 c yield lower 187 os 188 os ratios i e 0 124 0 129 within the field of mantle signal such unradiogenic compositions are best explained if old mantle sulfides occur in lavas and contribute to volcanic gases within the general frame of osmium mantle geochemistry loss of unradiogenic os during magmas degassing could help to explain osmium isotope disequilibrium between lavas and melting residues

remote reemergence areas of winter sea surface temperature anomalies in the north pacific 1 taking account of water movement we detect pairs of formation and reemergence areas of winter sea surface temperature sst anomalies in the north pacific by analyzing various kinds of data sets two pairs of formation and reemergence areas are detected one pair is located in the northern subtropical gyre and water properties in both the formation and reemergence areas well correspond to those of north pacific central mode water npcmw another pair is found in the western to central part of the subtropical gyre the formation area corresponds well to that of north pacific subtropical water npstmw but the reemergence area is remote from the formation area examinations on upper ocean current field trace of temperature anomalies in the subsurface and change of water properties from the formation area to the reemergence area clearly indicate that waters with winter sst anomalies move from the formation area to the reemergence area in a year

calderas landslides and paleo canyons on piton de la fournaise volcano la reunion island indian ocean based upon a re interpretation of previous data and a new field campaign a structural evolution is proposed for the early history of piton de la fournaise volcano from 500 000 to 50 000 years conceptually it is shown that the formation of a caldera in which lava flows are contained inside the caldera depression gives time for erosion to excavate deep canyons on the external slopes of the volcano for example the riviere des remparts the riviere langevin and the riviere de l est canyons on piton de la fournaise volcano these canyons are infilled when lavas filling the caldera and overflowing its rim are able again to flow on the external slopes of the volcano in the past this excavating infilling process has occurred twice following the formation of the riviere des remparts and morne langevin calderas the formation of the third caldera the plaine des sables caldera was followed by the excavation of the current canyons in addition to this process two large landslides have been documented in the field the first which happened about 300 000 years ago is apparently the first episode of the break up of piton de la fournaise volcano predating the formation of the four large calderas the second landslide which occurred 150 000 years ago and is considered to be less extensive has carried away the entire southern flank of the riviere des remparts caldera

effects of mesoscale eddies on subduction and distribution of subtropical mode water in an eddy resolving ogcm of the western north pacific abstract the effects of mesoscale eddies on the subduction and distribution of the north pacific subtropical mode water stmw are investigated using an eddy resolving ocean general circulation model ogcm first the subduction rate is calculated and the contribution of eddies to the subduction of stmw is estimated it is found that eddy subduction significantly contributes to the total subduction of stmw second eddy thickness transport and diapycnal flux are directly diagnosed to investigate the large scale eddy induced transport process of stmw the large southward eddy thickness transport in the stmw core density is consistent with eddy subduction the eddy transport on the isopycnal surface of stmw is directed down the thickness gradient and traverses the mean flow the meridional eddy transport streamfunction indicates two eddy circulation cells south of 30 n associated with the circulation of stmw these cells flatten density surfaces similar to the effect of the gent and mcwilliams gm schem

a mechanism for the recurrence of wintertime midlatitude sst anomalies abstract in the early 1970s namias and born speculated that ocean temperature anomalies created over the deep mixed layer in winter could be preserved in the summer thermocline and reappear at the surface in the following fall or winter this hypothesis is examined using upper ocean temperature observations and simulations with a mixed layer model the data were collected at six ocean weather stations in the north atlantic and north pacific concurrent and lead lag correlations are used to investigate temperature variations associated with the seasonal cycle in both the observations and the model simulations concurrent correlations between the surface and subsurface temperature anomalies in both the data and the model indicate that the penetration of temperature anomalies into the ocean is closely tied to the seasonal cycle in mixed layer depth high correlations extend to relatively deep shallow depths in winter summer lead lag correlations in both the data and the model at some of the stations

effect of mesoscale eddies on subtropical mode water variability from the kuroshio extension system study kess abstract forty eight profiling floats have been deployed in the kuroshio extension ke region since may 2004 as part of the kuroshio extension system study kess project by combining the float temperature salinity measurements with satellite altimetry data this study investigates the role played by mesoscale eddies in controlling the property changes in north pacific subtropical mode water stmw following a 3 yr period of low eddy activity in 2002 04 the ke showed a transition to a high eddy kinetic energy state in 2005 this transition is the result of delayed oceanic response to the 2002 shift in the basin scale surface wind forcing in connection with the pacific decadal oscillation the high eddy kinetic energy state of the ke is characterized by successive shedding of strong cold core rings into the recirculation gyre resulting from the interaction of the ke jet with the shatsky rise or the preexisting cutoff rings by transporting northern origin high potential vorticity pv ke water into t

formation region of north pacific subtropical mode water in the late winter of 2003 1 the mixed layer structure and properties in the formation region of the north pacific subtropical mode water stmw east of japan in the late winter of 2003 were examined using temperature and salinity data primarily from argo profiling floats the formation region extends south of the kuroshio extension between 30n and 35n and as far east as 175e it is characterized by a mixed layer with depths greater than 200db and meridionally uniform temperature between 16 5c and 18 2c associated with a mixed layer front at its southern edge the mixed layer in the formation region becomes colder fresher and denser toward the east leading to the formation of warmer saltier and lighter stmw in the western part of the region the spatial variation of the mixed layer depth in the formation region corresponds well with the underlying permanent thermocline depth index terms 4223 oceanography general descriptive and regional oceanography 4283 oceanography general water masses 4572 oceanography physical upper ocean processes citation oka e and t suga formation region of north pacific subtropical mode water in the late winter of 2003 geophys res lett 30 23 2205 doi 10 1029 2003gl018581 2003

the salazie cirque ignimbrite piton des neiges volcano reunion island chronostratigraphy description and significance of lithic fragments and eruptive mechanisms abstract an ignimbrite sequence in the salazie cirque is characterized by a pyroclastic flow unit composed of basal breccias layers a1 and a2 a welded tuff layer b and an upper ashy layer layer c this eruption began a new eruptive cycle in the piton des neiges volcanic complex the second stage of a differentiation series characterized by the emission of undersaturated lava the paleogeography of the volcano s eastern flank is reconstituted at about 150 000 y b p from the distribution of ignimbrite deposits that partially fill two large erosion cirques the salazie protocirque and the marsouins protocirque the total volume of these deposits is only a few km3 and no evidence indicates a related caldera collapse basal breccias are composed of 1 juvenile magmatic clasts yielding a mugearitic composition and normative nepheline and 2 xenoliths representative of the different units in a 1500 2000 m thick volcanic pile which include differentiated lavas first stage of differentiated series basaltic lava flows and plutonic rocks oceanite series the appearance of hydrothermal parageneses only in subsurface facies emphasizes the striking role of volcanotectonic structures in mineralized fluid circulation the common features of magma mixing abnormal mineralogical associations reverse zonings disequilibrium features in layer b are interpreted as resulting from the assimilation of highly differentiated foreign rocks by the mugearite magma during and after the course of the eruption the salazie cirque ignimbrite is of the overflowing type non plinian characterized by high emplacement temperatures a model is proposed for its formation 1 rising of a mugearitic eruption column 2 vesiculation of magma at shallow depth beneath the plug 3 increase of magma pressure and eruption of a mixture of gas magmatic blocks and xenoliths down slope deposition of fines depleted heterogenous lithic rich flow layer a1 4 explosion flashing of the gas rich upper part of the magma column and eruption of the main welded tuff unit with segregation of a basal non fines depleted lithic concentration zone during flow layer a2 and elutriation of an ash cloud fall layer c 5 effusive phase

sheet intrusions and deformation of piton des neiges and their implication for the volcano tectonics of la reunion abstract to understand the volcano tectonic history of piton des neiges the dormant volcano of la reunion we measured in the field the orientation of sheeted intrusions and deformation structures and interpreted the two datasets separately with a paleostress inversion results show that the multiple proposed rift zones may be simplified into three trends 1 a n30 e 5 km wide linear rift zone running to the south of the edifice active in the shield building 2 48 0 43 ma and terminal stages 190 22 ka 2 a curved n110 to n160 e rift zone widening from 5 km to 10 km toward the nw flank essentially active during the early emerged shield building 1 3 ma and 3 two sill zones 1 km thick in total in the most internal parts of the volcano active in the shield building and terminal stages in parallel deformation structures reveal that the tectonics of the edifice consisted in three end member stress regimes sharing common stress axes 1 nw se extension affecting in priority the south of the edifice near the n30 e rift zone 2 nne ssw extension on the northern half of the volcano near the n110 160 e rift zone 3 compression occurring near the sill zones with a ne sw or nw se maximum principal stress these three stress regimes are spatially correlated and mechanically compatible with the injection trends combined together our data show that the emerged piton des neiges underwent sector spreading delimited by perpendicular rift zones as observed on piton de la fournaise the active volcano of la reunion analogue experiments attribute such sector spreading to brittle edifices built on a weaker substratum we therefore conclude that la reunion volcanoes are both brittle as opposed to hawaiian volcanoes or mount etna whose radial spreading is usually attributed to a ductile body within the edifices

correlating hydrothermal system dynamics and eruptive activity a case study of piton de la fournaise volcano la reunion abstract piton de la fournaise volcano la reunion island is a basaltic shield volcano which underwent an intense cycle of eruptive activity between 1998 and 2008 self potential and other geophysical investigations of the volcano have shown the existence of a well established hydrothermal system within the summit cone the present study investigates the relationship between changes in the hydrothermal system and eruptive activity at the summit cone of piton de la fournaise here we consider the depth of the hydrothermal activity section to be the area where the hydrothermal flow is the most intense along its path ten complete loop self potential surveys have been analyzed through multi scale wavelet tomography mwt to characterize depth variations of the hydrothermal system between 1993 and 2008 our mwt models strongly support the existence of six main hydrothermal flow pathways associated with the main edifice structure each of these pathways is part of the main hydrothermal system and is connected to the main hydrothermal reservoir at depth in both 2006 and 2008 around dolomieu crater based on our results the hydrothermal activity sections are located between 2300 and 2500 m a s l which correlate well with the elevation of the observed fumarole belt within the post 2007 collapse crater wall our results show that the depths of the local hydrothermal activity sections change substantially over the investigated period vertical displacement of the main potential generation area associated with these hydrothermal activity sections is observed on the order of several hundred meters at the transition between the period of quiescence 1993 1997 and the resumption of eruptive activity in 1998 and 2007 respectively from 1999 to march 2008 the hydrothermal system was consistently located at relatively shallow depths by quantitatively determining the vertical displacement of hydrothermal fluids over 16 years we identify a significant link between hydrothermal system and magmatic activity hydrothermal system depth below the surface is an indicator of the activity level pressurization depressurization of the volcano within the shallow magmatic systems thus when used in conjunction with long term volcano monitoring this approach can contribute substantially to detection of the precursory signals of changes in volcanic activity

morphology of piton de la fournaise basaltic shield volcano la reunion island characterization and implication in the volcano evolution the topography of piton de la fournaise volcano pdf differs from the classic view of basaltic shield volcanoes as it is characterized by 1 several steep slope zones on its flanks and 2 a large u shaped caldera the enclos grand brule structure egbs most of these structures were previously interpreted as the scars of lateral landslides the deposits of which cover the submarine flanks of pdf we carried out a detailed analysis of the morphology of pdf which reveals that the steep slope zones form two independent circumferential structures that continue into the caldera the development of circumferential steep slopes on volcano flanks may have several origins constructive destructive and deformation processes we interpret those processes acting on pdf as caused by the spreading of the volcanic edifice above a weak hydrothermal core leading to outward displacements and a summit extensive stress field the continuity of the steep slope on both sides of the egbs escarpments suggests that this structure was not caused by a 4 5 ka old giant landslide as it is usually proposed but is due to a mainly vertical collapse the recent debris avalanche deposits east of the island indicate that this event likely destabilized part of the submarine flank we propose that the collapse of the grand brule the lower half of the egbs was due to the downward drag related to the dense intrusive complex of the alizes volcano which is located 1 km below the grand brule the collapse of the enclos is interpreted as the consequence of the deformation of the hydrothermal system of the pre enclos volcano although the continuity of the geological and morphological structures between the enclos and the grand brule suggests a narrow link between these two collapse events their chronology and relationship are still uncertain finally we hypothesize that the persistence of the ne and se rift zones during the last 150 ka despite the large changes of the topography related to the recurrent flank destabilizations is linked to a deep sources which can be either underlying crustal faults or the continuous downward subsidence of the alizes intrusive complex

the subtropical mode water circulation in the north pacific abstract historical hydrographic data are analyzed to investigate the seasonal variation of the subsurace pycnostad in the northwestern part of the north pacific and to relate it to the circulation of subtropical mode water stmw a gyre scale scheme of stmw circulation is deduced based on its observed climatological formation area and climatological geostrophic flow fields the deduced circulation is supported by climatological features of the subsurface hydrography presented separately for the non large meander period and the large meander period of the kuroshio zonal and meridional sections and isopycnal maps of potential vorticity and apparent oxygen utilization the kuroshio countercurrent composing the kuroshio recirculation system advects stmw formed in the wintertime thick mixed layer immediately off the kuroshio and the kuroshio extension during the non large meander period the recirculation system has a single anticyclonic gyre centered near 30 n 137 e and advects stmw formed off the kurosh

subtropical countercurrent in an idealized ocean gcm abstract in the north pacific there is a shallow eastward current called the subtropical countercurrent which flows across the central subtropical gyre the present article studies the generation mechanism of the subtropical countercurrent reproduced in an ocean general circulation model gcm with a simple geometry driven by surface wind stress and surface buoyancy forcing in the ocean gcm the deep mixed layer occurs in the northern part of subtropical gyre and shoals abruptly in the central subtropical gyre the mixed layer front the narrow transition zone of the mixed layer depth slants from the western central subtropical gyre to the northeast and the low potential vorticity fluid is formed at the intersection of the mixed layer front and the outcrop line since the surface density is almost zonally uniform and the mixed layer front slants northeastward the minimum potential vorticity fluids on denser isopycnals are formed in the northeastern region while those on lighter isopycnals are form