spatiotemporal variability of west african rainfall part ii associated surface and airmass characteristics abstract in the second part of this study the variability of surface and airmass characteristics associated with west african rainfall fields is investigated ship observations over the tropical atlantic and rawinsonde records over west africa are used for the months of july august and september from 1948 to 1978 a composite analysis based on the sign of monthly rainfall departures north of 10 n indicates that the sahelian drought may result from a more southern than normal position of the intertropical convergence zone itcz over the tropical atlantic a composite analysis into four monthly rainfall types is performed by using rainfall information north and south of 10 n it shows two types of sahelian drought the first is linked to an enhanced surface atmospheric circulation over the northern tropical atlantic higher sea surface temperatures in the gulf of guinea a convergent moisture flux anomaly south of 10 n along the western african coast and a lower larger water content in the monsoon fl

el nino la nina and sahel precipitation during the middle holocene simulations with a synchronously coupled atmo sphere land ocean sea ice model indicate el nino la nina like events in the eastern tropical pacific ocean during the middle holocene 6 ka with similar intensities and frequencies as at present 0 ka july august september sahel precipitation shifts northward as expected from proxy data for present day interannual decadal variability of western sahel precipi tation is correlated with both pacific el nino la nina sea sur face temperature sst anomalies and the atlantic dipole teleconnections at 6 ka between sahel precipitation and el nino la nina sst anomalies are absent with tropical atlantic ssts asserting a dominant influence these results illustrate potential problems with using present day teleconnection patterns in interpreting past climate variability from proxy data

detailed geomorphology and neotectonics of the endeavour segment juan de fuca ridge new results from seabeam swath mapping a 36 km wide corridor of seabeam bathymetry was collected on the juan de fuca ridge north of 48 00 n in may 1983 these data merged with previous seabeam bathymetric compilations for the cobb offset as well as with sea marc i and sea marc ii side scan sonar seismic reflection magnetometer and deep tow photography data have been used to identify morphological variations along the spreading center of the endeavour segment of the juan de fuca ridge between the cobb offset and sovanco fracture zone the endeavour segment consists of two ridge sections separated by the 13 km wide endeavour offset which formed when spreading jumped from middle valley to west valley 200 000 yr ago the locus of spreading occurs along the entire endeavour segment as a narrow 1 2 km wide shallow 10 30 m deep inner rift which is superimposed upon larger structures that range from broad 5 10 km wide deep as much as 3 000 m median valleys to narrow 5 km wide shallow 2 100 m volcanic ridges an abrupt discontinuity in the tectonic fabric occurs at about 48 05 n south of this latitude spreading occurs within an axial high endeavour ridge which gradually deepens to the south south endeavour valley as it approaches the zone of overlap with the propagating rift at the cobb offset north of 48 05 n the axis is characterized by broad deep median valleys north endeavour valley and west valley these large scale morphological variations reflect a complex interplay of the spreading center with seamount volcanism waning magma supply at the distal ends of ridge axes thermal contrasts across ridge offsets propagating rifts and an unstable triple junction the pronounced axial deep of west valley is interpreted as reflecting the youth of the structure 200 000 yr and may be primarily due to collapse with little extension and magmatism the rift axis jump to west valley fortuitously isolated endeavour seamount the youngest edifice in the heck seamount chain from the pacific plate and trapped it between a pair of overlapping spreading centers at the newly formed endeavour offset development of a remarkably simple overlapping conjugate rift pair at endeavour offset has resulted in spite of the complex and highly variable crustal thickness magma supply and pre existing structural grain rifting in south west valley has caused pre existing topography generated at the endeavour ridge to be destroyed by subsidence and burial thereby creating the apparent discontinuity in morphology north of 48 05 n

north northeast brazil rainfall and its decadal scale relationship to wind stress and sea surface temperature in this study low frequency variability of rainfall data for the north part of northeast nne brazil is examined the time scale dependence of nne brazil rainfall and the associated sea surface temperature sst field is also described area averaged monthly rainfall data for the nne brazil from 1911 to 1989 exhibits not only a distinct interannual behavior but also significant decadal variability examination of a 30 year data set of tropical atlantic sst and wind stress show similar behavior it was found that the relationships between sst nne brazil rainfall and meridional pseudo wind stress on the interdecadal time scale are very similar to those on the interannual and seasonal time scale in both time scales sst in the tropical atlantic are in phase with the meridional displacement of the intertropical convergence zone and associated meridional pseudo wind stress and out of phase with rainfall

geology of a vigorous hydrothermal system on the endeavour segment juan de fuca ridge a high precision high resolution geologic map explicitly documents relationships between tectonic features and large steep sided sulfide sulfate silica deposits in the vigorously venting endeavour hydrothermal field near the northern end of the juan de fuca ridge water depth in the vent field varies from 2220 to 2200 m location of the most massive sulfide structures appears to be controlled by intersections of ridge parallel normal faults and other fracture fissure sets that trend oblique to and perpendicular to the overall structural fabric of the axial valley the fractured basaltic substrate is primarily composed of well weathered pillow and lobate flows as presently mapped the field is about 200 by 400 m on a side and contains at least 15 large 1000 m3 sulfide edifices and many tens of smaller commonly inactive sulfide structures the larger sulfide structures are also the most vigorously venting features in the field they are commonly more than 30 m in diameter and up to 20 m in height actively venting sulfide structures in the northern portion of the field stand higher and are more massive than active structures in the southern portion of the field which tend to be slightly to distinctly smaller maximum venting temperatures of 375 c are associated with the smaller structures in the southeastern portion of the field highest temperature venting fluids from the more massive structures in the northern portion of the field are consistently 20 30 c lower hydrothermal output from individual active sulfide features varies from no flow in the lower third of the edifice to vigorous output from fracture controlled black smoker activity near the top of the structures a different type of high temperature venting takes place from the upper sides of the structures in the form of overflow from fully exposed quiescent pools of buoyant 350 c vent water trapped beneath overhanging sulfide sulfate silica ledges or flanges these flanges are attached to the upper outer walls of the large sulfide edifices two types of diffuse venting in the endeavour field include a lower temperature 8 15 c output through colonies of large tubeworms and 25 50 c vent fluid that seems to percolate through the tops of overhanging flanges the large size and steep walled nature of the these structures evidently results from sustained venting in a mature hydrothermal system coupled with dual mineral depositional mechanisms involving vertical growth by accumulation of chimney sulfide debris and lateral growth by means of flange development

high pressure transformations in mgal2o4 x ray diffraction and transmission electron microscopy on laser heated diamond cell samples show that with increasing pressure mgal2o4 spinel transforms first to al2o3 corundum mgo periclase then to the cafe2o4 structured phase and finally to a new phase having the cati2o4 structure above 40 gpa the cafe2o4 and the cati2o4 structures are closely related and have almost the same densities and bulk moduli transformation from the cafe2o4 to the cati2o4 phase would be expected to take place in oceanic crust that is subducted deep into the lower mantle

discovery of hydrothermal vent fields on alarcon rise and in southern pescadero basin gulf of california author s paduan jb zierenberg ra clague da spelz rm caress dw troni g thomas h glessner j lilley md lorenson t lupton j neumann f santa rosa del rio ma wheat cg abstract 2018 the authors hydrothermal vent fields located in the gap between known sites in guaymas basin and 21 n on the east pacific rise were discovered on the alarcon rise and in southern pescadero basin the alarcon rise spreading segment was mapped at 1 m resolution by an autonomous underwater vehicle individual chimneys were identified using the bathymetric data vent fields were interpreted as active from temperature anomalies in water column data and observed and sampled during remotely operated vehicle dives the ja sit pericu and meyibo active fields are near the eruptive fissure of an extensive young lava flow vent fluids up to 360 c from meyibo have compositions similar to northern east pacific rise vents the tzab ek field is 850 m west of the volcanic axis and active chimneys rise up to 33 m above a broad sulfide mound the inactive field is 10 km north northeast along the rift axis and most sulfide chimneys are enriched in zn and associated elements that are transported at lower temperature compared to the more cu rich active fields in southern pescadero basin the auka field is on the margin of a sediment filled graben at 3 670 m depth discharging fluids are clear contain hydrocarbons and have neutral ph elevated salinity and temperatures up to 291 c they have deposited massive mounds of calcite with minor sulfide the fluids are compositionally similar to those in guaymas basin produced by high temperature basalt seawater interaction followed by reaction with sediment the paucity of sulfide minerals suggests subsurface deposition of metals

volcanic episodicity and a non steady state rift valley along northeast pacific spreading centers evidence from sea marc i sea marc i side looking sonar images and sea beam bathymetry along a 400 km stretch of the juan de fuca ridge crest provide evidence that excessive extrusive volcanism periodically builds a crestal ridge along the axis of seafloor spreading an elongate summit depression esd or rift valley is commonly observed in the spine of this crestal ridge the crestal ridge volcanic landform has a distinctive shape that is recognized in bathymetric contours both along the spreading axis and at least up to 30 km away from the axis the landform has a plan form shape that resembles a side view of an archer s bow with the long dimension of the bow form parallel to the strike of the ridge in cross section the bow form is flat on top and has steep flanks these bow form shapes can be explained by magma that rises into the crust at a discrete center and flows laterally into belts of ridge parallel dikes similar to icelandic fissure eruptions both the variable dimensions of the esd along axis of the juan de fuca ridge and the relationship among volcanic flow morphologies within and beyond the esd suggest the four different segments of the juan de fuca ridge presented in detail here display different stages in a cycle of oceanic crust accretion this cycle includes episodes in which there is 1 extrusive volcanic construction which widen the crestal ridge prior to the collapse of the summit depression 2 collapse within the summit region of the crestal ridge to form an esd during a phase of volcanic inactivity and 3 renewed magmatism in the esd as its floor widens by extension and brittle fracture of the upper crust this episodic model implies that the width of the young seafloor affected by volcanic extrusion or dominated by tectonic stretching varies through time

expansion and contraction of the sahara desert from 1980 to 1990 data from polar orbiting meteorological satellites have been used to determine the extent of the sahara desert and to document its interannual variation from 1980 to 1990 the sahara desert ranged from 8 633 000 square kilometers in 1980 to 9 982 000 square kilometers in 1984 the greatest annual north south latitudinal movement of the southern saharan boundary was 110 kilometers from 1984 to 1985 and resulted in a decrease in desert area of 724 000 square kilometers

the southern ocean as a source region for tropical atlantic variability abstract a coupled model of intermediate complexity is used to examine the effect of the southern ocean on tropical atlantic variability model experiments with positive surface temperature freshwater and wind forcings in the southern ocean show an increased advection of polar water into the atlantic basin resulting in anomalously cold freshwater in the eastern atlantic ocean at 25 s with time these anomalies spread northward and away from the coast into the tropical thermocline the magnitude of the response increases linearly with the strength of the forcing the results of these experiments show that southeastern atlantic ocean temperature and salinity are particularly sensitive to changes in the southern ocean these findings suggest that this link could be the oceanic branch of a mode of variability linking the southern and tropical atlantic oceans a possible mechanism for the as yet unexplained decadal mode of tropical atlantic variability

teleconnections of the southern oscillation in the tropical atlantic sector in the osu coupled upper ocean atmosphere gcm abstract the oregon state university coupled upper ocean atmosphere gcm has been shown to qualitatively simulate the southern oscillation a composite analysis of the warm and cold events simulated in this 23 year integration has been performed during the low phase of the southern oscillation when warm anomalies occur in the eastern pacific the model simulates for the atlantic region during march may 1 a deficit of precipitation over the tropical south american continent 2 caribbean and gulf of mexico sea level pressure and sea surface temperature are in phase with the eastern pacific anomalies while those east of the nordeste region are out of phase and 3 northeast trade winds are anomalously weak and southwest trade winds are anomalously strong as inferred from surface current anomalies the anomalies in the oceanic processes are induced by perturbations in the atmospheric circulation over the atlantic and are coupled to changes in the walker circulation during the high phase of the simulate

mineral assemblages of basalt in the lower mantle high pressure x ray diffraction and analytical transmission electron microscopy of laser heated diamond cell samples show temperature dependent phase assemblages in mid ocean ridge basalt morb at lower mantle pressures the assemblage observed below 2250 k consists of only stishovite plus an orthorhombic perovskite phase space group pbnm or cmcm having a composition close to that of morb but slightly depleted in sio2 with increasing temperature the assemblage transforms to orthorhombic and cubic perovskites plus stishovite and then at 3250 k to orthorhombic and cubic perovskites plus stishovite and the cafe2o4 phase all of these assemblages could be present in slabs that have penetrated deep into the mantle and can therefore contribute to the seismological heterogeneity of the lower mantle

changing brine inputs into hydrothermal fluids southern cleft segment juan de fuca ridge author s wheat cg zierenberg ra paduan jb caress dw clague da chadwick ww abstract 2020 the authors in 2016 temperature recorders were recovered temperatures were measured and fluid samples were collected from vent 1 a high temperature 338 c hydrothermal discharge site on the southern cleft segment of the juan de fuca ridge coupled with previous sampling efforts this collection represents a 32 year record of discharge from a single chimney structure the longest record to date remarkably the fluid has remained brine dominated for more than three decades this brine formed during phase separation and segregation prior to initial observations in 1984 although the chloride concentration of the discharging fluid has decreased with time the fluid temperature has remained nearly constant for at least 3 3 years and probably for 15 or even 22 years compositions of the discharging fluids are consistent with inputs from a deep sourced brine which was last equilibrated at g400 c at a depth consistent with the base of the sheeted dikes and the brittle ductile transition this brine mixed diffusion or dispersion with a likely non phase separated hydrothermal fluid prior to discharge a survey of hydrothermal endmember fluids with chlorinities in excess of 700 mmol kg shows with the exception of fe a single trend between major ion concentrations and chlorinity even though data are from a range of crustal compositions spreading rates and water and magma depths calculated deep sourced brines from hydrothermal fluid data are similar to data based on fluid inclusions and estimates of brine assimilation in magmas a better understanding of brines is required given their potential duration of discharge and capacity for mobilizing metals

seismic evidence that black smoker heat flux is influenced by localized magma replenishment and associated increases in crustal permeability hydrothermal circulation at mid ocean ridges is responsible for 25 of earth s heat flux and controls the thermal and chemical evolution of young oceanic crust the heat flux of black smoker hydrothermal systems is thought to be primarily controlled by localized magma supply and crustal permeability nevertheless magma chamber characteristics and the nature of crustal permeability beneath such systems remains unclear here we apply three dimensional full waveform inversion to seismic data from the hydrothermally active endeavour segment of the juan de fuca ridge to image the upper crust in high resolution we resolve velocity variations directly above the axial magma chamber that correlate with variations in seismicity black smoker heat flux and the depth of the axial magmatic system we conclude that localized magma recharge to the axial magma lens along with induced seismogenic cracking and increased permeability influences black smoker heat flux

segmentation of mid ocean ridges attributed to oblique mantle divergence mantle flow beneath mid ocean ridges was thought to respond passively to plate motions analysis of seismic data from ridges reveals a skew between the directions of plate motion and mantle flow implying mantle flow may stress the plates

seismic structure of the endeavour segment juan de fuca ridge correlations with seismicity and hydrothermal activity 1 multichannel seismic reflection data collected in july 2002 at the endeavour segment juan de fuca ridge show a midcrustal reflector underlying all of the known high temperature hydrothermal vent fields in this area on the basis of the character and geometry of this reflection its similarity to events at other spreading centers and its polarity we identify this as a reflection from one or more crustal magma bodies rather than from a hydrothermal cracking front interface the endeavour magma chamber reflector is found under the central topographically shallow section of the segment at two way traveltime twtt values of 0 9 1 4 s 2 1 3 3 km below the seafloor it extends approximately 24 km along axis and is shallowest beneath the center of the segment and deepens toward the segment ends on cross axis lines the axial magma chamber amc reflector is only 0 4 1 2 km wide and appears to dip 8 36 to the east while a magma chamber underlies all known endeavour high temperature hydrothermal vent fields amc depth is not a dominant factor in determining vent fluid properties the stacked and migrated seismic lines also show a strong layer 2a event at twtt values of 0 30 0 09 s 380 120 m below the seafloor on the along axis line and 0 38 0 09 s 500 110 m on the cross axis lines a weak moho reflection is observed in a few locations at twtt values of 1 9 2 4 s below the seafloor by projecting hypocenters of well located microseismicity in this region onto the seismic sections we find that most axial earthquakes are concentrated just above the magma chamber and distributed diffusely within this zone indicating thermal related cracking the presence of a partially molten crustal magma chamber argues against prior hypotheses that hydrothermal heat extraction at this intermediate spreading ridge is primarily driven by propagation of a cracking front down into a frozen magma chamber and indicates that magmatic heat plays a significant role in the hydrothermal system morphological and hydrothermal differences between the intermediate spreading endeavour and fast spreading ridges are attributable to the greater depth of the endeavour amc and the corresponding possibility of axial faulting

large massive sulfide deposits in a newly discovered active hydrothermal system the high rise field endeavour segment juan de fuca ridge submersible observations and side scan sonar imagery delineate ten large venting sulfide sulfate silica structures located on an uplifted block of the axial valley floor on the central portion of the endeavour segment juan de fuca ridge this newly discovered vent field lies 1 8 km north of the main endeavour vent field the axial valley a horst and faults controlling sulfide deposition trend parallel to the ridge axis the horst rises 10 to 15 m above the valley floor and offsets a lava lake which extended 500 m in width across the valley the largest sulfide structures are perched along the major horst bounding faults one structure is the tallest single edifice 45 m reported in the pacific ocean smaller less active and inactive sulfide edifices are associated with faulted remnants of the lava lake surface fissured pressure ridges and at the edge of collapse pits based on the relationship between faults and sulfide structures the morphology of sulfide edifices and their venting characteristics we infer that sulfide structures grow vertically by chimney accumulation and laterally by flange development late stage deposition of amorphous silica toughens the large deposits and may be responsible for the unusual size and aspect ratio of endeavour edifices

hydrothermal sulfide accumulation along the endeavour segment juan de fuca ridge highlights high resolution auv bathymetry is used to identify hydrothermal sulfides on the seafloor 1 2 mt of massive sulfide is identified along 24 km of mid ocean ridge length massive sulfide accumulated at a rate of 400 t yr efficiency of sulfide deposition is 5 of total mobilized metals and reduced sulfur current global seafloor sulfide estimates likely underestimate amount present by about a factor of four abstract hydrothermal sulfide deposits that form on the seafloor are often located by the detection of hydrothermal plumes in the water column followed by exploration with deep towed cameras side scan sonar imaging and finally by visual surveys using remotely operated vehicle or occupied submersible hydrothermal plume detection however is ineffective for finding hydrothermally inactive sulfide deposits which may represent a significant amount of the total sulfide accumulation on the seafloor even in hydrothermally active settings here we present results from recent high resolution autonomous underwater vehicle based mapping of the hydrothermally active endeavour segment of the juan de fuca ridge in the northeast pacific ocean analysis of the ridge bathymetry resulted in the location of 581 individual sulfide deposits along 24 km of ridge length hydrothermal deposits were distinguished from volcanic and tectonic features based on the characteristics of their surface morphology such as shape and slope angles volume calculations for each deposit results in a total volume of 372 500 m3 of hydrothermal sulfide sulfate silica material for an equivalent mass of 1 2 mt of hydrothermal material on the seafloor within the ridge s axial valley assuming a density of 3 1 g cm3 much of this total volume is from previously undocumented inactive deposits outside the main active vent fields based on minimum ages of sulfide deposition the deposits accumulated at a maximum rate of 400 t yr with a depositional efficiency proportion of hydrothermal material that accumulates on the seafloor to the total amount hydrothermally mobilized and transported to the seafloor of 5 the calculated sulfide tonnage represents a four fold increase over previous sulfide estimates for the endeavour segment that were based largely on accumulations from within the active fields these results suggest that recent global seafloor sulfide resource estimates which were based mostly on the sizes and distribution of hydrothermally active deposits may be similarly underestimating the amount of sulfide along the global submarine neovolcanic zones

a seismic swarm and regional hydrothermal and hydrologic perturbations the northern endeavour segment february 2005 the february 2005 swarm at the overlapping spreading center osc on the northern end of the endeavour segment is the first swarm on the juan de fuca ridge recorded on a local seafloor seismic network the swarm included several larger earthquakes and caused triggered seismicity and a hydrothermal response in the endeavour vent fields as well as regional scale hydrologic pressure perturbations the spatial and temporal pattern of over 6000 earthquakes recorded during this seismic sequence is complex small magnitude events dominate and seismicity rates wax and wane indicating a magmatic process the main swarm initiates at the northern end of the endeavour ridge however most of the moment release including six strike slip events occurs in the southwest endeavour valley where the swarm epicenters generally migrate south the main swarm is contemporaneous with a hydrologic pressure response at four sealed seafloor boreholes 25 105 km away we infer that the seismic sequence is driven by a largely aseismic magma intrusion at the northern endeavour axis resulting stress changes trigger slip on tectonic faults and possibly dike propagation at the opposing limb of the endeavour osc in the southwest endeavour valley consistent with the eventual decapitation of the endeavour by the west valley segment furthermore 2 5 days after the start of the main swarm seismicity is triggered beneath the endeavour vent fields and temperature increases at a diffuse vent in the mothra field we infer that this delayed response is due to a hydrologic pressure pulse that diffuses away from the main magma intrusion

spatial and temporal evolution of magmatic systems beneath the endeavour segment juan de fuca ridge tectonic and petrologic constraints major and trace element data for a suite of lavas from fifty six dredges and alven dives on the ridge axis and adjacent abyssal hills have been used to investigate the geometry and evolution of magmatic systems beneath the endeavour segment juan de fuca ridge the morphology of the endeavour segment between the northward propagating cobb offset and the recently formed 0 2 m y endeavour offset is dominated by a shallow rifted elongate crestal volcano endeavour ridge that deepens along strike into a broad deep basin at each offset a set of ridges interpreted to be previous crestal volcanoes rifted apart by spreading flank the endeavour ridge and chronicle the dueling propagator history of the cobb offset the tectonic evidence strongly suggests that a large portion of the endeavour segment may be a failing rift segment at this time lavas from the current axis of the endeavour segment are moderately fractionated mgo 6 8 5 wt and have generally higher sio2 al2o3 na2o and k2o and lower feo man lavas from south of the cobb offset soco lavas incompatible trace element abundances and ratios indicate the endeavour lavas are primarily enriched e morbs and t morbs e g zr nb 7 24 zr y 2 5 5 9 and ba tio2 6 64 in contrast with the soco lavas which are more depleted in character thus the 30 km wide cobb offset appears to mark a major geochemical boundary beneath the juan de fuca ridge in contrast with the endeavour segment axial lavas samples from adjacent abyssal hills are more similar to the soco lavas in their major and trace element characteristics these observations suggest that the parental magmas of the endeavour segment exhibit temporal variability with more enriched material arriving only recently beneath the ridge axis pronounced compositional variability is observed at small spatial scales within the endeavour segment axial lavas which does not correlate with axial morphology this variability is interpreted to reflect ubiquitous small scale mantle heterogeneity and poor mixing of multiple parental magmas during migration from the melt region or within axial magma chambers highly enriched samples zr nb 10 are localized near the summit region of endeavour ridge whereas slightly less enriched samples occur along the length of the axis and on the older flanking ridges recent enrichment may result from diminishing extents of partial melting of a heterogeneous source in response to tectonic reconfiguration causing more fusible enriched domains to dominate the chemical signature of melts produced small scale heterogeneity along strike seems incompatible with models of centralized upwelling of melts beneath the summit region of the ridge axis with shallow lateral injection of melts to distal ends of the segment unless these spatial variations actually reflect temporal variations in the source composition and collapse of the shallow magmatic systems toward the summit region as rift failure has progressed

phase transition and density of subducted morb crust in the lower mantle abstract phase relations mineral chemistry and density of a natural mid oceanic ridge basalt morb composition were investigated up to 134 gpa and 2300 k by a combination of in situ x ray diffraction measurements and chemical analyses using transmission electron microscope tem results demonstrate that the morb composition consists of mgsio 3 rich perovskite stishovite casio 3 perovskite and cafe 2 o 4 type al phase in the upper part of the lower mantle the most abundant mineral of mgsio 3 rich perovskite undergoes phase transition to a cairo 3 type post perovskite phase above 110 gpa and 2500 k post perovskite phase is similar in composition to perovskite except considerably high na 2 o content stishovite transforms to cacl 2 type sio 2 phase above 62 gpa and 2000 k and further to pbo 2 type phase above 110 gpa pbo 2 type sio 2 phase includes large amount of al 2 o 3 which significantly expands its stability relative to cacl 2 type phase phase transition of casio 3 perovskite from tetragonal to cubic was also observed with increasing temperature cafe 2 o 4 type al phase is stable to the bottom of the mantle the density of morb crust was calculated using volume data combining with measured chemical compositions and calculated mineral proportions the former morb crust is denser than the average lower mantle at all depths greater than 720 km contrary to earlier predictions the subducted basaltic crust may have accumulated at the base of the mantle

termination of a 6 year ridge spreading event observed using a seafloor seismic network on the endeavour segment juan de fuca ridge 1 we present automatically determined epicenters and magnitudes for 36 523 earthquakes recorded along the endeavour segment between august 2003 and october 2006 using a local ocean bottom seismometer obs network the catalog is dominated by two swarm sequences in january and february 2005 in the vicinity of the endeavour overlapping spreading center which included earthquakes in west valley the northern portion of the endeavour segment southwest endeavour valley and the endeavour vent fields these swarms are attributed to volcanism including a dike intrusion on the northern endeavour in february 2005 and smaller diking events on the propagating tip of the west valley segment in both swarms the dike on the northern endeavour propagated to the south which is inconsistent with magma sourced from the axial magma chamber beneath the elevated central portion of the segment following the swarms seismic activity on the endeavour segment decreased on average to 15 of pre swarm values and almost ceased at the segment ends we infer that a 6 year non eruptive event that started with a swarm in 1999 and finished with the 2005 swarms ruptured the entire segment and relieved plate spreading stresses the inferred coupling between the 1999 and 2005 events the observation of extensive precursory activity prior to the 2005 swarms and the interaction between seismically active regions during the swarms is consistent with static triggering with delays influenced by viscoelastic relaxation hydraulic diffusion and magma withdrawal and replenishment

seismic evidence for a 920 km discontinuity in the mantle analysis of hundreds of seismograms from a seismic array in the japanese islands reveals a seismic discontinuity at a depth of about 920 km in the mantle beneath the tonga subduction zone this discontinuity is also evident beneath subduction zones in the japan and flores seas the discontinuity probably represents either a change in mantle chemical composition or the signature of the subducted slab s garnet layer

near axis crustal structure and thickness of the endeavour segment juan de fuca ridge a model of crustal thickness and lower crustal velocities is obtained for crustal ages of 0 1 1 2 ma on the endeavour segment of the juan de fuca ridge by inverting travel times of crustal paths and non ridge crossing wide angle moho reflections obtained from a three dimensional tomographic experiment the crust is thicker by 0 5 1 km beneath a 200 m high plateau that extends across the segment center this feature is consistent with the influence of the proposed heckle melt anomaly on the spreading center the history of ridge propagation on the cobb overlapping spreading center may also have influenced the formation of the plateau the sharp boundaries of the plateau and crustal thickness anomaly suggest that melt transport is predominantly upward in the crust lower crustal velocities are lower at the ends of the segment likely due to increased hydrothermal alteration in regions influenced by overlapping spreading centers and possibly increased magmatic differentiation

geology and hydrothermal evolution of the mothra hydrothermal field endeavour segment juan de fuca ridge detailed characterization of the mothra hydrothermal field the most southern and spatially extensive field on the endeavour segment of the juan de fuca ridge provides new insights into its geologic and hydrothermal development meter scale bathymetry side scan sonar imagery and direct dive observations show that mothra is composed of six actively venting sulfide clusters spaced 40 200 m apart chimneys within each cluster have similar morphology and venting characteristics and all clusters host a combination of active and extinct sulfide structures black smoker chimneys venting fluids above 300 c are rare while more common lower temperature diffusely venting chimneys support dense colonies of macrofauna and bacterial mat hydrothermal sediment and extinct sulfide debris cover 10 15 m of the seafloor surrounding each vent cluster obscuring the underlying basaltic substrate of light to moderately sedimented pillow lobate sheet and chaotic flows basalt talus and collapse terrain extinct sulfide chimneys and debris between the clusters indicate that hydrothermal flow was once more widespread and that it has shifted spatially over time the most prominent structural features in the axial valley at mothra are regional 020 trending faults and fissures and north south trending collapse basins the location of actively venting clusters within the field is controlled by 1 localization of fluid upflow along the western boundary fault zone and diversion of these fluids by antithetic faults to feed vent clusters near the western valley wall and 2 tapping of residual magmatic heat in the central part of the axial valley which drives flow beneath vent clusters directly adjacent to the collapse basins 70 90 m east of the western valley wall these processes form the basis for a model of axial valley and hydrothermal system development at mothra in which the field is initiated by an eruptive diking episode and sustained through intense microseismicity and non eruptive diking events

the west african monsoon dynamics part iii the quasi biweekly zonal dipole abstract this paper presents an investigation of the mechanisms giving rise to the main intraseasonal mode of convection in the african monsoon during northern summer here identified as the quasi biweekly zonal dipole qbzd the qbzd is primarily characterized by a quasi stationary zonal dipole of convection whose dimension is larger than the west african monsoon domain with its two poles centered along the guinean coast and between 30 and 60 w in the equatorial atlantic the qbzd dynamical processes within the atlantic africa domain are examined in some detail the qbzd has a dipole pattern associated with a walker type circulation in the near equatorial zonal plane it is controlled both by equatorial atmospheric dynamics through a kelvin wave like disturbance propagating eastward between its two poles and by land surface processes over africa inducing combined fluctuations in surface temperatures surface pressure and low level zonal winds off the coast of west africa when convection is at a min

paving the seafloor volcanic emplacement processes during the 2005 2006 eruptions at the fast spreading east pacific rise 9 50 n 1 the 2005 2006 eruptions near 9 50 n at the east pacific rise epr marked the first observed repeat eruption at a mid ocean ridge and provided a unique opportunity to deduce the emplacement dynamics of submarine lava flows since these new flows were documented in april 2006 a total of 40 deep towed imaging surveys have been conducted with the woods hole oceanographic institution s whoi towcam system more than 60 000 digital color images and high resolution bathymetric profiles of the 2005 2006 flows from the towcam surveys were analyzed for lava flow morphology and for the presence of kipukas collapse features faults and fissures we use these data to quantify the spatial distributions of lava flow surface morphologies and to investigate how they relate to the physical characteristics of the ridge crest such as seafloor slope and inferred dynamics of flow emplacement we conclude that lava effusion rate was the dominant factor controlling the observed morphological variations in the 2005 2006 flows we also show that effusion rates were higher than in previously studied eruptions at this site and varied systematically along the length of the eruptive fissure this is the first well documented study in which variations in seafloor lava morphology can be directly related to a well documented ridge crest eruption where effusion rate varied significantly

geology of the alarcon rise southern gulf of california author s clague da caress dw dreyer bm lundsten l paduan jb portner ra spelz madero r bowles ja castillo pr guardado france r le saout m martin jf santa rosa del rio ma zierenberg ra abstract 2018 the authors meter scale auv bathymetric mapping and rov sampling of the entire 47 km long alarcon rise between the pescadero and tamayo transforms show that the shallowest inflated portion of the segment hosts all four active hydrothermal vent fields and the youngest hottest and highest effusion rate lava flows this shallowest inflated part is located 1 3 of the way between the tamayo and pescadero transforms and is paved by a 16 km2 channelized flow that erupted from 9 km of en echelon fissures and is larger than historic flows on the east pacific rise or on the gorda and juan de fuca ridges starting 5 km south of the pescadero transform 6 5 km of the alarcon rise is characterized by faulted ridges and domes of fractionated lavas ranging from basaltic andesite to rhyolite with up to 77 3 wt sio2 these are the first known rhyolites from the submarine global mid ocean ridge system silicic lavas range from g11 7 ka to as young as 1 1 ka a basalt to basaltic andesite sequence and an andesite to dacite to rhyolite sequence are consistent with crystal fractionation but some intermediate basaltic andesite and andesite formed by mixing basalt with dacite or rhyolite magmatism occurred along the bounding tamayo and pescadero transforms as extensive channelized flows the flows erupted from ring faults surrounding uplifted sediment hills inferred to overlie sills the transforms are transtensional to accommodate magma migration from the adjacent alarcon rise

sulfide geochronology along the endeavour segment of the juan de fuca ridge 1 forty nine hydrothermal sulfide sulfate rock samples from the endeavour segment of the juan de fuca ridge northeastern pacific ocean were dated by measuring the decay of 226ra half life of 1600 years in hydrothermal barite to provide a history of hydrothermal venting at the site over the past 6000 years this dating method is effective for samples ranging in age from 200 to 20 000 years old and effectively bridges an age gap between shorter and longer lived u series dating techniques for hydrothermal deposits results show that hydrothermal venting at the active high rise sasquatch and main endeavour fields began at least 850 1450 and 2300 years ago respectively barite ages of other inactive deposits on the axial valley floor are between 1200 and 2200 years old indicating past widespread hydrothermal venting outside of the currently active vent fields samples from the half graben on the eastern slope of the axial valley range in age from 1700 to 2925 years and a single sample from outside the axial valley near the westernmost valley fault scarp is 5850 205 years old the spatial relationship between hydrothermal venting and normal faulting suggests a temporal relationship with progressive younging of sulfide deposits from the edges of the axial valley toward the center of the rift these relationships are consistent with the inward migration of normal faulting toward the center of the valley over time and a minimum age of onset of hydrothermal activity in this region of 5850 years

establishing a new era of submarine volcanic observatories cabling axial seamount and the endeavour segment of the juan de fuca ridge abstract at least 70 of the volcanism on earth occurs along the 65 000 km network of mid ocean ridge mor spreading centers within these dynamic environments the highest fluxes of heat chemicals and biological material from the lithosphere to the hydrosphere occur during volcanic eruptions however because underwater volcanoes are difficult and expensive to access researchers are rarely if ever in the right place at the right time to characterize these events therefore our knowledge is limited about the linkages among hydrothermal chemical and biological processes during seafloor formation and crustal evolution to make significant advancements in understanding the evolution of mor environments the united states and canada have invested in the first plate scale submarine cabled observatory linked through the global internet spanning the juan de fuca tectonic plate these two networks include 1700 km of cable and 14 subsea terminals that provide 8 10 kw power and 10 gbs communications to hundreds of instruments on the seafloor and throughout the overlying water column resulting in a 24 7 365 presence in the oceans data and imagery are available in real to near real time the initial experimental sites for monitoring volcanic processes include the mor volcanoes called axial seamount and the endeavour segment that are located on the juan de fuca ridge axial a hot spot influenced seamount is the most robust volcano along the ridge rising nearly 1400 m above the surrounding seafloor and it has erupted twice in the last 15 years in contrast the endeavour segment is characterized by more subdued topography with a well defined axial rift and it hosts one of the most intensely venting hydrothermal systems known a non eruptive spreading event lasting 6 years was documented at endeavour between 1999 and 2005 hydrothermal venting intensity chemistry and temperature as well as associated biological communities at both sites were significantly perturbed by the magmatic and intrusive events this paper presents the similarities and differences between the axial and endeavour volcanic systems and identifies reasons why they are ideal candidates for comparative studies the u s has made a 25 year commitment for sustained observations using the cabled infrastructure the highly expandable nature of submarine optical networking will allow for the future addition of novel experiments that utilize ever evolving advancements in computer sciences robotics genomics and sensor miniaturization comprehensive modeling of the myriad processes involved will continue to assimilate and integrate growing databases yielding a new understanding of integrated processes that create the seafloor in the global ocean basins

a quasi biennial oscillation in the equatorial atlantic ocean a quasi biennial oscillation with features characteristic of equatorial coupled modes is found in the equatorial atlantic ocean the origins of the phenomenon are investigated by using a coupled atmosphere ocean general circulation model in which the fully coupled domain is restricted to the tropical atlantic 20 s 30 n the simulations capture a similar oscillation regardless of whether sea surface temperatures prescribed outside the coupled domain correspond to the observed or the climatology i e lack year to year variability it is concluded that the equatorial atlantic hosts a quasi biennial oscillation generated by coupled atmosphere ocean interactions internal to the basin with little influence from remote events even from el nino southern oscillation

interannual variability in the tropical atlantic the anomalous meteorological and oceanic conditions in the tropical atlantic basin during 1983 84 can be seen as an extreme example of a common historical pattern here we present time series of rainfall in subsaharan west africa swa and north east brazil neb which show that departures in 1983 84 were severe but not unprecedented the associated sea surface temperature sst anomaly patterns included one that had previously been shown to accompany earlier years of extreme rainfall in swa and neb and others that contained key elements of that pattern the pattern concerned was recently shown to be a major mode of tropical atlantic sst variation

a self sustaining climate mode in the tropical atlantic 1995 97 observations and modelling an anomalous climatic event in the tropical atlantic starting in autumn 1995 and continuing into the autumn of 1997 is investigated using remotely sensed datasets and a reduced gravity model it is shown that this event contained both a warm and a cold phase each of roughly 12 months duration the propagating signal within each phase is identified in the sea surface height and temperature signatures as both equatorial and off equatorial waves in the ocean with coupling to atmospheric convection the strength geographical pattern and timing of the anomalous period is consistent with it being the first observed example of a coupled mode of interaction between the ocean and atmosphere found in two previous coupled ocean atmosphere models of the tropics this interpretation means that the two phased climate event which we will call the equatorial atlantic oscillation eao was associated with purely internal atmospheric and oceanographic variability within the atlantic basin and hence was independent of the pacific climate it can also be inferred from long term monthly sea surface temperature and sea level pressure datasets that there were potentially several previous eao events during the past century

eruptive and tectonic history of the endeavour segment juan de fuca ridge based on auv mapping data and lava flow ages high resolution bathymetric surveys from autonomous underwater vehicles abe and d allan b were merged to create a coregistered map of 71 7 km2 of the endeavour segment of the juan de fuca ridge radiocarbon dating of foraminifera in cores from three dives of remotely operated vehicle doc ricketts provide minimum eruption ages for 40 lava flows that are combined with the bathymetric data to outline the eruptive and tectonic history the ages range from modern to 10 700 marine calibrated years before present yr bp during a robust magmatic phase from 10 700 yr bp to 4300 yr bp flows erupted from an axial high and many flowed 5 km down the flanks some partly buried adjacent valleys axial magma chambers amcs may have been wider than today to supply dike intrusions over a 2 km wide axial zone summit seamount formed by 4770 yr bp and was subsequently dismembered during a period of extension with little volcanism starting 4300 yr bp this tectonic phase with only rare volcanic eruptions lasted until 2300 yr bp and may have resulted in near solidification of the amcs the axial graben formed by crustal extension during this period of low magmatic activity infrequent eruptions occurred on the flanks between 2620 1760 yr bp and within the axial graben since 1750 yr bp this most recent phase of limited volcanic and intense hydrothermal activity that began 2300 yr bp defines a hydrothermal phase of ridge development that coincides with the present day 1 km wide amcs and overlying hydrothermal vent fields

equation of state of al bearing stishovite to 40 gpa at 300 k the compression behavior of al bearing stishovite was investigated by powder x ray diffraction up to 40 gpa with the bl13a beamline at the photon factory kek japan a reliable equation of state for stishovite was obtained using a diamond anvil cell coupled with a yttrium aluminum garnet yag laser heating a sample containing 2 1 wt al 2 o 3 was heated using a yag laser at each pressure increment to relax deviatoric stress x ray diffraction measurements were carried out at 300 k using the angle dispersive technique a least squares refinement of the data yielded equation of state parameters where the bulk modulus k o 282 2 gpa when the first pressure derivative of the bulk modulus k o was fixed at 4 the effect of al is to decrease slightly the bulk modulus of stishovite and increase the density of the subducted oceanic crust the enhanced compressibility of al bearing stishovite certainly has geophysical and geochemical implications for the fate of the subducted slab as this mineral is the main constituent of subducted mid oceanic ridge basalt morb in the earth s mantle

geology and venting characteristics of the mothra hydrothermal field endeavour segment juan de fuca ridge the 500 m long mothra hydrothermal field is the largest venting site in areal extent on the endeavour segment of the juan de fuca ridge within this field there are at least five actively venting sulfide complexes spaced 40 to 200 m apart along a trend of 020 the clusters are composed of multiple steep sided pinnacles that rise up to 20 m above the seafloor most of the sulfide structures are awash in diffusely venting fluids 30 200 c that support rich and diverse macrofaunal and microbial communities isolated black smoker chimneys vent 302 c fluids the linear arrays of sulfide complexes delineate a zone of active faulting near the western wall of the axial valley this fault network serves as the conduit for the rising fluids

unusual conditions in the tropical atlantic ocean in 1984 au cours de la 1ere moitie de 1984 les conditions oceaniques et atmospheriques dans l atlantique tropical ont ete a bien des egards similaires a celles dans le pacifique au cours de l el nino l ocean superieur a ete inhabituellement chaud dans sa partie orientale les pluies ont ete abondantes sur des regions normalement arides au sud de l equateur et l upwelling cotier a ete contrarie dans une region sud ouest africain ou il constitue un phenomene saisonnier le changement de tsm sst qui apparait lorsque d inhabituels courants portant a l est au sud de l equateur transportent de l eau chaude de surface vers l afrique contribue au changement des conditions atmospheriques ces dernieres en retour contribuent au changement de la circulation oceanique et de la tsm