numerical simulations of marine hydrothermal plumes for europa and other icy worlds abstract the liquid water interiors of europa and other icy moons of the outer solar system are likely to be driven by geothermal heating from the sea floor leading to the development of buoyant hydrothermal plumes these plumes potentially control icy surface geomorphology and are of interest to astrobiologists we have performed a series of simulations of these plumes using the mit gcm ocean circulation model we assume here that europa s ocean is deep of order 100 km and unstratified and that plume buoyancy is controlled by temperature not composition our experiments explore a limited region of parameter space with ocean depth h ranging from 50 to 100 km deep source heat flux q between 0 1 and 10 gw and coriolis parameter f corresponding to europa latitudes between 9 and 47 as predicted by earlier work the plumes in our simulations form narrow cylindrical chimneys a few km across under the influence of the coriolis effect these plumes broaden over time until they become baroclinically unstable breaking up into cone shaped eddies when they become 10 35 km in diameter the shed eddies are of a similar size large scale currents in the region of the plume range between 1 and 5 cm s temperature anomalies in the plume far from the seafloor are tiny varying between 10 and 180 k variations in plume size shape speed and temperature are in excellent agreement with previous laboratory tank experiments and in rough agreement with theoretical predictions plume dynamics and geometry are controlled by a natural rossby number which depends strongly on depth h and coriolis parameter f but only weakly on source heat flux q however some specific theoretical predictions are not borne out by these simulations this may occur because the plumes are reingesting their own emissions a process not considered in our earlier theory

the importance of hydrothermal venting to water column secondary production in the northeast pacific abstract the purpose of this study is to show that seafloor hydrothermal venting in the open northeast pacific ocean has a marked impact on secondary biomass and production within the overlying water column specifically we use net tows and concurrently measured acoustic backscatter data collected over six summers to examine the effects of hydrothermal venting from the endeavour segment of juan de fuca ridge on macro zooplankton biomass and production throughout the entire 2000 m depth range previous research shows that ontogenetic diapausing migrators and their predators from the upper ocean aggregate above the neutrally buoyant plumes in summer and resume feeding on plume and bottom upwelled particles resulting in increased zooplankton reproductive output to the upper ocean within the limitations of our sampling methodology net tows reveal a statistically significant exponential decline in total water column biomass with increasing lateral distance from the vent fields the acoustic backscatter data show a similar decline but only below 800 m depth near surface biomass was highly variable throughout the region but values near vents consistently ranged higher than summer values found elsewhere in the offshore northeast pacific water column biomass was similar in magnitude above and below 800 m depth throughout the region because epiplume biomass can be advected a considerable distance from vent fields biomass enhancement of the water column from hydrothermal venting may extend considerable distances to the west and northwest of the vent sites in the prevailing directions of the subsurface flow based on the extensive acoustic doppler current profiler adcp data collected and the strong correlation between zooplankton production derived from net sample biomass and acoustic backscatter intensity we estimate that daily macro zooplankton production in the upper 400 m of the water column within 10 km of the vent fields averages approximately 16 of photosynthetic primary production the z ratio whereas the total water column zooplankton production averages 26 of surface primary production local grazing rate estimates metabolic constraints and other open ocean studies suggest that the z ratio should be no higher than 5 which it is at off axis background sites in the study region this finding indicates that nutrient sources other than upper ocean primary production fuel both upper and deep ocean zooplankton biomass and growth near the endeavour ridge hydrothermal vents

middepth flow near hydrothermal venting sites on the southern juan de fuca ridge long term moored current measurements around the southern end of the juan de fuca ridge show that flow parallels the ridge and may be intensified by its presence the mean flow is northward on the west side and southward on the east thus plumes emanating from the ridge probably are advected along the ridge before flowing away from the ridge other topographic features may be responsible for westward or eastward turning along the west side of the ridge we hypothesize a regional flow pattern with southward flow north of axial seamount northward flow south of axial and a convergence somewhere south of axial resulting in westward flow along the east side the flow may be southward everywhere above the ridge crest the long term mean flow is weaker and more variable and it includes tidal inertial and a dominant broadband 4 day oscillation this 4 day energy is maximum in the rift valley where the inertial is least decreases upward and away from the ridge and is more energetic in winter this oscillation is predominately a clockwise rotating feature which along with the presence of inertial oscillations suggests that it may be storm induced from the sea surface these observations along with those by cannon and pashinski 1990 suggest northward propagation flow variations across the ridge lasting one to a few months have mean speeds of 1 3 cm s and are capable of transporting plumes off axis to the west or east into the northerly or southerly flows respectively

the 5 september 2012 nicoya costa rica mw 7 6 earthquake rupture process from joint inversion of high rate gps strong motion and teleseismic p wave data and its relationship to adjacent plate boundary interface properties 1 on 5 september 2012 a large thrust earthquake mw 7 6 ruptured a densely instrumented seismic gap on the shallow dipping plate boundary beneath the nicoya peninsula costa rica ground motion recordings directly above the rupture zone provide a unique opportunity to study the detailed source process of a large shallow megathrust earthquake using very nearby land observations hypocenter relocation using local seismic network data indicates that the event initiated with small emergent seismic waves from a hypocenter 10 km offshore 13 km deep on the megathrust a joint finite fault inversion using high rate gps strong motion ground velocity recordings gps static offsets and teleseismic p waves reveals that the primary slip zone slip 1 m is located beneath the peninsula the rupture propagated downdip from the hypocenter with a rupture velocity of 3 0 km s the primary slip zone extends 70 km along strike and 30 km along dip with an average slip of 2 m the associated static stress drop is 3 mpa the seismic moment is 3 5 1020 nm giving mw 7 6 the coseismic large slip patch directly overlaps an onshore interseismic locked region indicated by geodetic observations and extends downdip to the intersection with the upper plate moho at deeper depths below the upper plate moho seismic tremor and low frequency earthquakes have been observed most tremor locates in adjacent areas of the megathrust that have little coseismic slip a region of prior slow slip deformation to the southeast also has no significant coseismic slip or aftershocks an offshore locked patch indicated by geodetic observations does not appear to have experienced coseismic slip and aftershocks do not overlap this region allowing the potential for a comparable size rupture offshore in the future

the distribution of geothermal fields on the juan de fuca ridge near bottom water temperatures were mapped along 400 km of the strike of the juan de fuca ridge as part of a combined sea marc seabeam experiment to image the variability of morphology and structure along a spreading center segment the water temperature data collected by a continuously towed thermistor chain in addition to salinity data indicate that there are four geothermal areas spaced at distances of 100 km from each other south of the cobb propagator and one field just to the north of the propagator on the endeavor ridge segment each thermal region is located above a morphological dome on the spreading center these domes are an average of 100 200 m shallower than the rest of the axis the structure of bottom water suggests that the geothermal regions are on average 20 km long and that the heat from these fields raises the temperature in the water column by a minimum of 0 06 sup 0 c up to 300 m above the bottom two simple models are used to estimate the heat flux associated with these features

constraints on the energy and chemical balances of the modern tag and ancient cyprus seafloor sulfide deposits the size chemical composition energy flux and fluid composition of the tag hydrothermal sulfide deposit at the mid atlantic ridge and the size chemical composition and reaction zone characteristics of the skouriotissa volcanogenic massive sulfide deposit of the troodos ophiolite in cyprus are used to examine the energy requirements and chemical balances associated with the generation of a large volcanogenic massive sulfide deposit we conclude that formation of large sulfide deposits from oceanic hydrothermal systems is a geologically rapid process and occurs on timescales of hundreds of years with episodes of activity as short as a few tens of years separated by thousands of years of inactivity about 2 1019 j of energy supplied at high temperature is required to form a deposit the size of the tag mound metals up to 4 times the mass of any element present in the sulfide deposit are leached out of relatively small reaction zones most likely at the base of the sheeted dikes chemical balance can be struck for all elements except sulfur with a reaction zone 1 2 km3 in volume from which a small proportion of iron and large proportions of copper zinc and manganese are removed a sulfur balance requires that a significant fraction of sulfur be derived from reduction of seawater sulfate as suggested by stable isotope analyses we argue that the principal source of energy that drives hydrothermal circulation is latent heat of crystallization of magma close to the top of the plutonic section furthermore we speculate that activity of the tag hydrothermal system is related to periods of more rapid magma supply from the mantle at magma supply rates similar to those observed in volcanoes in hawaii and iceland

estimations of heat transfer from grotto s north tower a neptune observatory case study abstract the overall heat transfer through an active hydrothermal sulfide edifice in particular the north tower of grotto has been estimated at 80 mw or greater based on the following in situ measurements and assumptions 1 the heat transfer by diffuse flow is estimated at 33 380 mw based on extrapolating the acoustically mapped area to all sides of the north tower visible area 30 m 2 extrapolated area 100 m 2 and using the range of available spot measurements of temperature 6 23 c and vertical velocity 0 07 0 28 m s the lower number 33 mw is more likely but there is insufficient knowledge of the temporal and spatial variability of diffuse flow to be certain 2 the heat transfer by focused flow is estimated at 30 70 mw based on summing the estimated individual rates of heat transfer for 4 out of 7 documented black smokers and flanges 3 conductive heat transfer out of the mound is unknown but is likely to be much less than the advective heat transfer additionally the plume transport of heat is estimated at 20 40 mw based on the direct measurement of temperature within the plume at 5 25 m above the top of the edifice despite uncertainties the lower estimate of plume versus smoker heat transfer suggests that heat transfer is dominantly by diffuse flow furthermore not all plumes from individual smokers may merge even for so small an area as the north tower of grotto

seismic attenuation structure beneath the juan de fuca ridge from tomographic inversion of amplitudes tomographic inversion of seismic amplitudes for attenuation structure has been applied to first arrival amplitudes from a seismic refraction profile across the juan de fuca ridge to further constrain a velocity structure previously obtained by travel time velocity tomography and to obtain a model for attenuation variations within the shallow crust amplitude analysis has identified an additional boundary defined by a decrease in vertical velocity gradient at 2 0 km subbottom which was not required by the travel time data this transition zone likely represents the oceanic layer 2 layer 3 boundary the analysis also identified zones of relatively high attenuation qe 20 100 at subbottom depths of 0 3 2 0 km beneath the ridge axis which correlate well with low velocity zones velocities reduced by 0 4 km s in the velocity model the subaxial zone is interpreted as a region of increased fracture porosity and or permeability associated with axial hydrothermal circulation within the extrusives and sheeted dike complex

the first measurements of hydrothermal heat output at 9 50 n east pacific rise abstract despite the importance of the heat output of seafloor hydrothermal systems for the earth s energy budget hydrothermal heat output measurements have been very limited in this paper we report the first measurements of hydrothermal heat output at the ridge 2000 integrated study site on the east pacific rise we focused our work on the bio 9 complex situated at 9 50 n where there has been an extensive measurement and sampling program since 1991 this site is located along the eruptive fissure of the 1991 1992 event and the site of the 1995 earthquake swarm we made direct measurements of advective heat output at several individual vents and at one site of diffuse flow tica although these data do not describe the complete heat flux picture at this vent field the data yield a total hydrothermal heat output of 325 160 mw with 42 21 mw coming from high temperature vents along this 2 km segment of ridge this result assumes a diffuse flux similar to that measured at tica occurs at each high temperature vent site our initial measurements thus suggest that the heat output of the low temperature diffuse venting is approximately 10 times that of the high temperature vents but may also be one or two orders of magnitude greater

heat and salt fluxes in the atlantis ii deep red sea abstract the atlantis ii deep is located on the red sea axial rift it is a topographic depression enclosing a volume of about 17 km3 between 1 900 m and 2 200 m depth that contains layered brines of temperatures up to 66 c and salinities up to 270 previous geochemical investigations showed that the hot brines result from discharge of hydrothermal solutions that have exchanged heat and chemical components with the basaltic substratum the last investigation of the deep in 1992 showed that the brines occurred in four well mixed layers with the shallowest at a depth of 2 000 m the temperature and salinity profiles describe a transition zone from 2 000 m to 1 900 m above which red sea water occurs the distribution of temperature and salt in this transition zone appears to be controlled by the topography of the deep the hydrographic structure of the brine column has been documented in the literature for seven time intervals between 1966 and 1992 examination of these data shows that the system changed with time the evolution of the entire package of brines that fill the deep shows the following changes 1 the temperature of the brines increased 2 the salinity of the solutions increased 3 the two brine layers described in 1966 still existed in 1992 but new layers appeared above them 4 for 26 years almost all additional heat and salt supplied in the deep were confined to the depression and were not dispersed into the overlying seawater the fourth point indicates that a heat and salt balance for the deep can be calculated the calculation of the heat that entered into the system was divided into two components the temperature increase of the brines and the heat loss at the wall rock the latter component was negligible the rate of heat input to the deep was constant during the period considered and amounted to 0 54 109 w the salt input was also constant and equalled 250 350 kg s during the period 1966 1992 heat and salt were most probably supplied by a hydrothermal solution with an average range of flow rate temperature and salinity of 670 1000 kg s 195 310 c and 270 370 respectively

numerical simulation of mean currents and water property anomalies at endeavour ridge hydrothermal versus topographic forcing 1 numerical simulations based on realistic seafloor topography are used to examine near bottom currents in the region of the endeavour segment of juan de fuca ridge endeavour ridge in the northeast pacific results support earlier findings that hydrothermal venting within the 2200 m deep axial valley rather than topographically modified basin scale cross ridge geostrophic flow is responsible for the near steady northward currents observed within the confines of the valley although it does not generate deep flow within the valley the basin scale circulation determines the horizontal redistribution of the hydrothermal plumes once they rise above the ridge crest simulations of the near bottom temperature and salinity fields reveal that model runs that incorporate a deep westward background flow most closely reproduce the observed plume anomaly distributions above the ridge indicating that bottom currents in the region are predominantly westward counter to the prevailing southeasterly flow of the wind driven california current in the upper half of the water column

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

microbial ammonia oxidation and enhanced nitrogen cycling in the endeavour hydrothermal plume abstract ammonium was injected from the subseafloor hydrothermal system at the endeavour segment juan de fuca ridge into the deep sea water column resulting in an nh 4 rich 177 nm neutrally buoyant hydrothermal plume this nh 4 was quickly removed by both autotrophic ammonia oxidation and assimilation the former accounted for at least 93 of total net nh 4 removal with its maximum rate in the neutrally buoyant plume 53 nm d 1 up to 10 fold that in background deep water ammonia oxidation in this plume potentially added 26 130 mg no 3 m 2 d 1 into the deep sea water column this oxidation process was heavily influenced by the presence of organic rich particles with which ammonia oxidizing bacteria aob were often associated 40 68 aob contributed up to 10 8 of the total microbial communities within the plume and might constitute a novel lineage of proteobacterial aob based on 16s rrna and amoa phylogenetic analyses meanwhile nh 4 assimilation rates were also substantially enhanced within the neutrally buoyant plume 26 4 nm d 1 and accounted for at least 47 of total net nh 4 removal rates the combined nh 4 oxidation and assimilation rates always exceeded total net removal rates suggesting active in situ nh 4 regeneration rates of at least an order of magnitude greater than the particulate nitrogen flux from the euphotic zone ammonia oxidation is responsible for nh 4 turnover of 0 7 13 days and is probably the predominant in situ organic carbon production process 0 6 13 mg c m 2 d 1 at early stages of endeavour neutrally buoyant plumes

hydrothermal cooling of midocean ridge axes do measured and modeled heat fluxes agree abstract accurate estimates of near axis hydrothermal cooling are keys to understanding many geological chemical and biological processes at midocean ridges at present however field observations h heat flux vent field are few and imprecise and model results h m heat flux km of axis vary widely depending on assumptions about crustal formation crustal composition and the cooling efficiency and spatial and temporal extent of hydrothermal circulation in this paper i compare all published vent field scale measurements of h to a range of published h m for slow 40 mm yr and fast 40 mm yr ridges to assess the present state of knowledge about the magnitude of near axis hydrothermal cooling h has been evaluated 28 times either by point measurements using inventories of individual sources 362 552 mw n 12 measurements or by integrated measurements using water column observations 876 1011 mw n 16 comparable estimates of mean vent field expected h at any spreading rate can be derived by merging h m from various models with the predicted global inventory of active vent sites these estimates range from 1 1 5 gw field and are approximately constant for all spreading rates this range agrees with the best constrained results of h from slow ridge vent fields 1669 1354 mw but is considerably higher than those from fast ridges 245 170 mw inferences about the spatial and temporal variability of h can be gained by defining the parameter l h h m equivalent to the axis length needed to continuously power a hydrothermal field the resultant large l values 13 333 km for slow ridges 1 32 km for fast demand punctuated cooling where for short periods h h m the percentage of time that a given vent field might be active can be speculatively estimated by dividing l by the length of axis the field cools examples of this percentage range from a low of 50 for some fast ridge fields

measuring fluid flow and heat output in seafloor hydrothermal environments we have designed built calibrated and tested new anemometer and turbine flow meter devices to measure fluid velocity at high and low temperature focused and diffuse discharge sites at oceanic spreading centers the devices perform at ocean floor depths and black smoker temperatures and can be used to measure flow rates ranging over two orders of magnitude flow velocity is determined from the rotation rate of the rotor blades or paddle assembly these devices have an open bearing design that eliminates clogging by particles or chemical precipitates as the fluid passes by the rotors the devices are compact and lightweight enough for deployment from either an occupied or remotely operated submersible the measured flow rates can be used in conjunction with vent temperature or geochemical measurements to obtain heat outputs or geochemical fluxes from both vent chimneys and diffuse flow regions the devices have been tested on 30 alvin dives on the juan de fuca ridge and 3 jason dives on the east pacific rise epr we report 102 new measurements over a wide range of discharge temperatures 5 363 c velocities 2 199 cm s and depths 1517 2511 m these include the first advective heat output measurements at the high rise vent field and the first direct fluid flow measurement at middle valley our data suggest that black smoker heat output at the main endeavor vent field may have declined since 1994 and that after the 2005 2006 eruption the high temperature advective flow at the epr 9 50 n field may have become more channelized predominately discharging through the bio 9 structure we also report first direct advective measurements in the lau basin and at the epr 9 39 5 n with flow meters that predated devices described in this work and were used in the process of their development we discuss potential error sources and how they may affect the accuracy of measurements by our and other devices in particular we use the turbulent plume theory to evaluate the effect of entrainment of ambient seawater

fluid accumulation along the costa rica subduction thrust and development of the seismogenic zone in 2011 we acquired an 11 55 km 3 d seismic reflection volume across the costa rica margin nw of the osa peninsula to accurately image the subduction thrust in 3 d to examine fault zone properties and to infer the hydrogeology that controls fluid accumulation along the thrust following processing to remove water column multiples noise and acquisition artifacts we constructed a 3 d seismic velocity model for kirchhoff prestack depth migration imaging images of the plate boundary thrust show high reflection amplitudes underneath the middle to lower slope that we attribute to fluid rich poorly drained portions of the subduction thrust at 5 km subseafloor beneath the upper slope the plate interface abruptly becomes weakly reflective which we interpret as a transition to a well drained subduction thrust mineral dehydration during diagenesis may also diminish at 5 km subseafloor to reduce fluid production and contribute to the downdip change from high to low amplitude there is also a layered fabric and systems of both thrust and normal faults within the overriding plate that form a plumbing system faults commonly have fault plane reflections and are presumably fluid charged the faults and layered fabric form three compartmentalized hydrogeologic zones 1 a shallow ne dipping zone beneath the slope 2 a steeply sw dipping zone beneath the shelf slope break and 3 a ne dipping zone beneath the shelf the more direct pathway in the middle zone drains the subduction thrust more efficiently and contributes to reduced fluid pressure elevates effective stress and creates greater potential for unstable coseismic slip

acoustic doppler current profiler observations of a mid ocean ridge hydrothermal plume on september 10 1987 an oceanic profiling package consisting of a guildline conductivity temperature depth probe ctd seatec transmissometer and rd instruments 150 khz acoustic doppler current profiler adcp was used to investigate the hydrothermal plume emanating from vent sites located at 2200 m depth on the endeavour segment of juan de fuca ridge in the northeast pacific ocean 47 58 n 129 06 w the ctd transmissometer profiles indicate that the plume immediately downstream of the vent site consisted of two 50 m thick particle laden effluent layers separated by a comparably thick zone of relatively particle free ambient fluid the lower 50 m of the water column appeared to be dominated by smaller concentrations of resuspended particles within a benthic boundary layer maximum temperature salinity and light attenuation anomalies within the main core of the plume at 1970 m depth were approximately 0 04 c 0 01 ppt and 0 05 m 1 respectively the adcp measurements represent the first attempt to acoustically map the current shear and backscatter amplitude associated with a mid ocean ridge hydrothermal plume repeated profiles by the adcp revealed that the top of the plume was a region of enhanced current shear with vertical changes in speed of the order of 0 05 m s 1 over depths of approximately 100 m the velocity measurements also indicate that the current reversed from northwest to southwest over the 1 hour observation period consistent with the predicted flow derived from moored current meters recovered immediately prior to the profiler survey contrary to expectation acoustic backscatter signals within the particle laden layers of the plume were lower than background reference levels while echo return amplitudes from the particle free layer were close to background levels time series plots of the amplitude data indicate that the basic layered structure of the plume is disrupted by billowlike features with vertical and horizontal wavelengths of 40 to 60 m and 20 to 40 m respectively we suggest that the weakened acoustic backscatter signals from the effluent layers is linked to a reduction in the numbers of zooplankton relative to the ambient waters

crustal structure of endeavour ridge segment juan de fuca ridge from a detailed seismic refraction survey a detailed seismic refraction survey was carried out over the endeavour segment of the juan de fuca ridge a medium rate spreading center which lies off western north america to investigate the creation and evolution of oceanic crust a bathymetric high and the presence of hydrothermal vents suggested that the study area was the most recent locus of spreading travel time and amplitude data from 15 in line air gun ocean bottom seismometer profiles were forward modeled using asymptotic ray theory to obtain two dimensional velocity models consisting of four primary layers which correlate well with classic models of oceanic crust significant lateral variations in thicknesses and velocities on the scale of a few to 10 km are superimposed on this basic velocity structure but they appear to be random rather than distributed symmetrically about the ridge we attribute them to fracturing which causes porosity changes hydrothermal circulation which fills voids and fractures with alteration products and variations in magmatic and or deformational processes at the spreading center layer 2a is found to have low 2 6 2 8 km s velocities to average 0 4 km in thickness with variations up to 0 2 km and to be bounded at its base by a sharp velocity increase to 4 8 km s along the axial ridge velocities 0 4 0 6 km s higher than average are interpreted for layers 2b and 2c but these values are confined to a 2 km wide zone centered below the ridge velocities along ridge parallel lines offset 10 km are normal indicating that maturation to off ridge structure has occurred within at most 0 3 ma layer 3 velocities decrease by 0 1 0 2 km s for arrivals traveling along and under the axial ridge perhaps caused by higher temperatures however we find no anomalously low velocities beneath the ridge indicating that no large crustal magma chamber exists on the basis of this study we conclude that magmatic accretion is a fully three dimensional process within ridge segments such as endeavour ridge

small scale deformation structures and physical properties related to convergence in japan trench slope sediments diatomaceous mudstones cored on dsdp legs 56 57 off japan contain numerous postdepositional veins healed fractures and microfaults beneath the forearc basin these secondary structures occur only at depths greater than 620 m miocene in a zone of normal faulting beneath the landward slope of the trench however they occur at depths as shallow as 250 m pliocene in hemipelagic sediments which are inferred to blanket tectonically accreted deposits the intensity of deformation in the trench slope deposits increases in a seaward direction bulk densities indicate that the trench slope deposits have not dewatered normally but are above 400 m more consolidated than sediment deposited on the deep sea terrace a decline in sediment bulk density with increasing depth suggests that open fractures may exist in situ perhaps induced and or maintained by excess pore pressure below about 550 m a normal pattern of increasing density with increasing depth may reflect fracture closure and consolidation under lithostatic load we suggest that tectonic stress related to plate convergence has been communicated to the sediment on the landward japan trench slope causing tectonic dewatering within the subduction complex and inducing pervasive but small scale veining fracturing and faulting in near surface 700 m hemipelagic deposits this deformation occurs in sediment which has not been subject to large tectonic movement and does not eradicate larger 50 100 m thick structural depositional units which are apparent on seismic reflection records the brecciation induced by deformation may reflect natural hydrofracturing under abnormally high pore pressures or opening of fractures formed earlier at shallower depths during the dewatering process the overpressures may in turn locally reduce the sediment shear strength at shallow levels in the sediment column and induce downslope mass sediment movement

compositional variability in the ascending fluxes from a hydrothermal plume 1 sequentially sampling sediment traps set with 33 day sampling intervals were deployed with current meters on three moorings in the northeast pacific ocean between july 1994 and may 1995 one mooring was deployed near the main vent field on endeavour ridge on axis site 47 57 0 n 129 05 7 w a second 3 km west of the main vent site west site and the third 43 km northeast of the main vent site east or background site ascending and descending particles were collected near 1600 and 2000 m depth well above and within the top of laterally spreading hydrothermal plumes the elemental composition of particles was used to evaluate their origins biogenic fluxes were indicated by elevated ca or si hydrothermal fluxes by elevated fe mn and cu and lithogenic fluxes by elevated ti we link temporal variability in both the ascending and descending particle composition and flux to variations in lateral transport of hydrothermal constituents and to seasonal drawdown of hydrothermal plume particles by biogenic material from the upper ocean the relatively low hydrothermal fe content of ascending material late in the experiment is thought to be due to uptake by descending biogenic material these results suggest that seasonal productivity and particle export from the ocean surface can modulate the hydrothermal flux of elements to the waters above and to the sediments below

fate of event hydrothermal plumes on the juan de fuca ridge current meter moorings were deployed in august 1993 at the megaplume site of the june 1993 seismic event on the coaxial segment of the juan de fuca ridge and were recovered in april 1994 record averaged flow was about 2 cm s across axis which is sufficiently large to advect plumes off axis in a few days during july two megaplumes 1800 m level disappeared between repeated ctd tows a deeper plume 2150 m had a relatively strong signature during august and extended southeastward in agreement with the observed flow a ctd tow along the east flank of axial volcano did not locate remnants of the megaplumes but did find deeper plumes which could have come from off axis advection from the coaxial sources followed by southward advection along the east side of ridge

a method for quantitatively estimating diffuse and discrete hydrothermal discharge submarine hydrothermal fluids discharge as undiluted high temperature jets and as diffuse highly diluted low temperature percolation estimates of the relative contribution of each discharge type which are important for the accurate determination of local and global hydrothermal budgets are difficult to obtain directly in this paper we describe a new method of using measurements of hydrothermal tracers such as fe mn fe heat and mn heat in high temperature fluids low temperature fluids and the neutrally buoyant plume to deduce the relative contribution of each discharge type we sampled vent fluids from the north cleft vent field on the juan de fuca ridge in 1988 1989 and 1991 and plume samples every year from 1986 to 1991 the tracers were on average 3 to 90 times greater in high temperature than in low temperature fluids with plume values intermediate a mixing model calculates that high temperature fluids contribute only 3 of the fluid mass flux but 90 of the hydrothermal fe and 60 of the hydrothermal mn to the overlying plume three years of extensive camera ctd sled tows through the vent field show that diffuse venting is restricted to a narrow fissure zone extending for 18 km along the axial strike linear plume theory applied to the temperature plumes detected when the sled crossed this zone yields a maximum likelihood estimate for the diffuse heat flux of8 9 104 w m for a total flux of 534 mw considering that diffuse venting is active along only one third of the fissure system for mean low and high temperature discharge of 25 c and 319 c respectively the discrete heat flux must be 266 mw to satisfy the mass flux partitioning if the north cleft vent field is globally representative the assumption that high temperature discharge dominates the mass flux in axial vent fields leads to an overestimation of the flux of many non conservative hydrothermal species by about an order of magnitude

the relationship between near axis hydrothermal cooling and the spreading rate of mid ocean ridges recent seismic observations and new crustal thermal models suggest that near axis 1 km from the spreading axis hydrothermal cooling increases linearly with increasing spreading rate we present here the first corroborating evidence from near axis hydrothermal plume observations comprehensive surveys of plume distributions have now been conducted on multi segment portions of ridge crest with full spreading rates from 20 to 150 km myr these surveys find thatph the fraction of ridge crest length overlain by hydrothermal plumes and full spreading rate us are related byph us where 0 004 myr km we recast this large scale spatial variability into long term temporal variability by postulating that any vent field site is hydrothermally active for an us fraction of geologic time we then use this relationship and the mean of eight studies that have determined instantaneous heat flux ha at vent field scales 75 45 mw km to estimate the time averaged heat flux ha asha us applying these scaled values ofha to the distribution of mid ocean ridge spreading rates yields a global near axis heat flux ha of 8 8 1011 w this value is 10 of the total oceanic hydrothermal heat flux and agrees with a crustal cooling model derived value of ha of 9 1011 w our estimate of ha implies a mean global3he heat ratio of 2 4 8 3 10 13 cm3 j stp and high temperature hydrothermal fluid flow of 1 8 1013 kg yr

hydrography and geochemistry of sea surface hydrothermal plumes resulting from hawaiian coastal volcanism the entry of lava into the ocean at kilauea volcano hawaii has produced extensive sea surface hydrothermal plumes studies at this site over 1989 1991 showed that particle rich chemically altered thermal plumes of 1 3 m thickness can be mapped for distances up to 2 km from the entries using an array of physical and chemical tracers particle concentrations in the plumes were as high as 65 mg l 1 and produced light attenuation anomalies exceeding 11 m 1 the maximum observed seawater temperature was 69 c the following extremes in plume chemical markers were measured h2 2 1 m dissolved mn 320 nm dissolved si 55 m co2 1 6 mm alkalinity 1 5 meq l 1 and ph 7 1 dissolved h2 linearly increased and co2 ph and total alkalinity decreased with increasing seawater temperature dissolved mn and si also increased with increasing temperature but were better correlated with each other indicating that heat is not conserved in the surface plumes heat and mass inputs to the ocean are comparable to those from mid ocean ridge crest vent fields

progressive change of pelagic clay microstructure during burial process examples from piston cores and odp cores abstract progressive change of microstructure in pelagic clays of different three orders of burial depths several centimeter several meters and several hundred meter depths was observed by measurement of anisotropy of magnetic susceptibility ams and observation by scanning electron microscope sem and thin section at depths of several centimeters void ratio 3 0 peds of several 10 m in diameter are linked by long and narrow connectors in edge face ef contact and pore sizes between the peds are large at depths of several meters void ratio 3 0 2 0 the shape of the connectors changes from narrow to broad the pore size between the peds decreases with burial depth at depths of several hundred meters void ratio

microearthquakes on the endeavour segment of the juan de fuca ridge 1 we report the results of a 55 day microearthquake experiment on the endeavour segment of juan de fuca ridge the network covered a 5 km section of the ridge axis centered on the main vent field and extended 15 km off axis on the west flank the ridge axis and flanks were seismically active and 1750 earthquakes were located with a minimum of five travel time picks including at least one s wave over half the earthquakes occurred in swarms and a waveform cross correlation technique was used to obtain relative locations on the western flank the hypocenters for four swarms at midcrustal depths are compatible with steeply dipping fault planes that strike at 035 050 n and oblique to the abyssal hills focal mechanisms determined from p wave first motions and p s amplitude ratios are predominantly strike slip with north south compression and appear to be affected by the reorganization of the explorer plate earthquakes beneath the ridge axis are concentrated in a band of intense seismicity at 1 5 3 5 km depth to the north of high rise vent field the seismicity defines a plane striking parallel to the ridge axis and dipping east at 70 and the earthquakes appear to extend beneath an axial reflector previously imaged at 2 3 km depth farther south the hypocenters are not compatible with a single fault plane focal mechanisms are characterized by subhorizontal tension axes oriented in all directions except parallel to the ridge and suggest a stress field that is about equally influenced by ridge spreading and hydrothermal cooling

long term monitoring of hydrothermal heat flux using moored temperature sensors cleft segment juan de fuca ridge heat flux from submarine vent fields can vary gradually on interannual scales or nearly instantaneously in response to volcano tectonic events in the underlying crust neither case is well documented because measurements of vent field scale heat flux are scarce we report here a new approach to hydrothermal plume monitoring sensitive to both progressive changes and hydrothermal events from june 1991 to may 1992 we moored 35 self contained temperature sensors and six current meters on seven moorings located in and around the plume from a vent field on the cleft segment juan de fuca ridge the hydrothermal plume was identified by a local temperature anomaly of 0 01 to 0 03 c in the lowermost 200 m of the water column plume heat flux defined as the net advection of this temperature anomaly averaged about 250 mw during the deployment this flux is less than previous estimates and thus supports speculation that the heat flux is declining after a sudden reinvigoration triggered by a seafloor rifting event in 1986

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

a 6 year time series of hydrothermal plumes over the cleft segment of the juan de fuca ridge the causal links between magmatic tectonic processes and hydrothermal venting along the mid ocean ridge axis will be difficult to discern without broadband observations of the temporal variability of hydrothermal activity i report here on temporal variability in the intensity and distribution of chronic and event plumes generated by continuous and episodic hydrothermal discharge at two vent fields on the cleft segment of the juan de fuca ridge hydrothermal temperature and light attenuation c anomalies were comprehensively mapped between 1 and 4 times per year during nine cruises from 1986 to 1991 intensity of and c in the chronic plume overlying the north cleft vent field showed more variability between years than within a given year both and c were exceptionally high in 1986 fell to moderate levels during 1988 to 1990 and declined to minimum values in 1991 the chronic plume over the south cleft vent field varied similarly superimposed on the chronic plumes were brief and intense releases of hydrothermal fluid that created event plumes distinguishable by their symmetry and exceptional rise height event plumes at the north cleft field ranged from the 1986 megaplume with a rise height exceeding 1000 m to smaller events in 1987 and 1989 with rise heights of 400 m chronic hydrothermal heat 660 277 1 mw and mass 204 86 1 g s 1 fluxes from the north cleft field were determined by assuming these fluxes were equivalent to the long term net advection of a mean cross section of the vertically integrated and c in the plume this 6 year history of hydrothermal plumes indicates that episodic discharge has been an important component of the recent hydrothermal flux and that the aggregate heat and mass fluxes declined significantly though not consistently between 1986 and 1991

effective strength of incoming sediments and its implications for plate boundary propagation nankai and costa rica as type examples of accreting vs erosive convergent margins abstract the location of the seaward tip of a subduction thrust controls material transfer at convergent plate margins and hence global mass balances at approximately half of those margins the material of the subducting plate is completely underthrust so that no accretion or even subduction erosion takes place along the remaining margins material is scraped off the subducting plate and added to the upper plate by frontal accretion we here examine the physical properties of subducting sediments off costa rica and nankai type examples for an erosional and an accretionary margin to investigate which parameters control the level where the frontal thrust cuts into the incoming sediment pile a series of rotary shear experiments to measure the frictional strength of the various lithologies entering the two subduction zones were carried out results include the following findings 1 at costa rica clay rich strata at the top of the incoming succession have the lowest strength res 0 19 while underlying calcareous ooze chalk and diatomite are strong up to res 0 43 peak 0 56 hence the entire sediment package is underthrust 2 off japan clay rich deposits within the lower shikoku basin inventory are weakest res 0 13 0 19 and favour the frontal proto thrust to migrate into one particular horizon between sandy competent turbidites below and ash bearing mud above 3 taking in situ data and earlier geotechnical testing into account it is suggested that mineralogical composition rather than pore pressure defines the position of the frontal thrust which locates in the weakest clay mineral rich up to 85 wt materials 4 smectite the dominant clay mineral phase at either margin shows rate strengthening and stable sliding in the frontal 50 km of the subduction thrust 0 0001 0 1 mm s 0 5 25 mpa effective normal stress 5 progressive illitization of smectite cannot explain seismogenesis because illite rich samples also show velocity strengthening at the conditions tested

hydrothermal plumes along the east pacific rise 8 40 to 11 50 n plume distribution and relationship to the apparent magmatic budget abstract the interactions between hydrothermal circulation and large scale geological and geophysical characteristics of the mid ocean ridge cannot be ascertained without large scale views of the pattern of hydrothermal venting such multi ridge segment surveys of venting are accomplished most efficiently by mapping the distribution and intensity of hydrothermal plumes in november 1991 we mapped hydrothermal temperature and light attenuation c anomalies above the east pacific rise epr continuously from 8 40 to 11 50 n a fast spreading ridge crest portion bisected by the clipperton transform fault plume distributions show a precise correlation with the distribution of active vents where video coverage of the axial caldera is exhaustive 9 09 54 n elsewhere in the study area the sketchy knowledge of vent locations gleaned from scattered camera tows predicts only poorly the large scale hydrothermal pattern revealed by our plume studies plumes were most intense between 9 42 and 9 54 n directly over a march april 1991 seafloor eruption these plumes had exceptionally high c ratios compared to the rest of the study area we suggest that the phase separated gas rich vent fluids discharging here fertilize an abundant population of bacteria hydrothermal plume distributions define three categories intense and continuous 8 48 8 58 n 9 29 10 01 n and 11 05 11 27 n weak and discontinuous 8 58 9 29 n and negligible the location of each category is virtually congruent with areas that are respectively magmatically robust magmatically weak and magmatically starved as inferred from previous measurements of axial bathymetric undulations cross axis inflation and magma chamber depth and continuity this congruency implies a fine scale spatial and temporal connection between magmatic fluctuations and hydrothermal venting we thus speculate that at least along this fast spreading section of the epr cyclic replenishment eruption and freezing of the thin axial melt lens exerts greater control over hydrothermal venting than the more enduring zones of crystal mush and hot rock we found intense and continuous plumes along 33 of the surveyed ridge crest an observation implying that any point on the ridge is on average hydrothermally active one third of the time combining this result with the 20 plume coverage found along the medium rate juan de fuca ridge suggests that superfast 150 mm yr spreading ridges should support vigorous venting along 50 of their length if spreading rate and along axis plume coverage are linearly related

composition and dissolution of black smoker particulates from active vents on the juan de fuca ridge during two atlantis ii alvin cruises to the juan de fuca ridge in 1984 active high temperature 140 284 c vents were sampled for black smoker particulates using the grassle pump individual mineral phases were identified using standard x ray diffraction and petrographic procedures in addition elemental compositions and particle morphologies were determined by x ray energy spectrometry and scanning electron microscope x ray energy spectrometry techniques the vent particulates from the southern juan de fuca ridge vent sites were highly enriched in s si fe zn and cu and were primarily composed of sphalerite wurtzite pyrite pyrrhotite barite chalcopyrite cubanite hydrous iron oxides and elemental sulfur two additional unidentified phases which were prevalent in the samples included an fe si phase and a ca si phase the grain sizes of the individual particle phases ranged from 100 m for the fe si particles grain size and current meter data were used in a deposition model of individual phase dispersal for many of the larger sulfide and sulfate particles the model predicts dispersal to occur over length scales of only several hundreds of meters the high temperature black smokers from the more northerly endeavour segment vents were highly enriched in fe s ca cu and zn and were primarily composed of anhydrite chalcopyrite sphalerite barite sulfur pyrite and other less abundant metal sulfide minerals the grain sizes of the individual particles ranged from 10 m to slightly larger than 500 m the composition and size distributions of the mineral phases are highly suggestive of high temperature mixing between vent fluids and seawater a series of field and laboratory studies were conducted to determine the rates of dissolution of several sulfate and sulfide minerals the dissolution rates ranged over more than 3 orders of magnitude from 3 2 10 8 cm s 1 for anhydrite to 1 2 10 12 cm s 1 for chalcopyrite the results indicate that for some minerals particularly anhydrite and marcasite total dissolution occurs within a few hours to a few weeks of their formation for other more stable minerals including pyrite sphalerite and chalcopyrite the time required for total dissolution is much longer and consequently individual crystals may be expected to persist in the sediments for considerable periods of time after deposition

a model of an atlantic and pacific hydrothermal plume a turbulent entrainment model of a hydrothermal plume was modified to include the effects of vertical structure in the background temperature and salinity fields owing to the stably stratified salinity field in the pacific relatively warm salty water spreads at an equilibrium level where the plume density equals the ambient density in contrast the unstable salinity profile in the atlantic gives the plume added buoyancy and it continues to rise to an equilibrium level where it is relatively cool and fresh these attributes result from the mixing between the plume and the surrounding seawater at each level below the spreading layer the equilibrium temperature anomaly is not directly proportional to the source strength but depends on the mixing and background salinity gradient the net entrainment of surrounding seawater is greater in the atlantic plume model than in the pacific plume model for the same buoyancy flux recent data from the tag hydrothermal field at 26 n on the mid atlantic ridge and data from a vent field on the endeavor segment of the juan de fuca ridge are compared to model results the model predicts the penetration height and s characteristics of the atlantic plume and it explains some of the differences between the observed atlantic and pacific hydrothermal plumes

constrained circulation at endeavour ridge facilitates colonization by vent larvae understanding how larvae from extant hydrothermal vent fields colonize neighbouring regions of the mid ocean ridge system remains a major challenge in oceanic research1 2 among the factors considered important in the recruitment of deep sea larvae are metabolic lifespan the connectivity of the seafloor topography and the characteristics of the currents3 here we use current velocity measurements from endeavour ridge to examine the role of topographically constrained circulation on larval transport along ridge we show that the dominant tidal and wind generated currents in the region are strongly attenuated within the rift valley that splits the ridge crest and that hydrothermal plumes rising from vent fields in the valley drive a steady near bottom inflow within the valley extrapolation of these findings suggests that the suppression of oscillatory currents within rift valleys of mid ocean ridges shields larvae from cross axis dispersal into the inhospitable deep ocean this effect augmented by plume driven circulation within rift valleys having active hydrothermal venting helps retain larvae near their source larvae are then exported preferentially down ridge during regional flow events that intermittently over ride the currents within the valley

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

geochemistry and morphology of metalliferous sediments and oxyhydroxides from the endeavour segment juan de fuca ridge abstract we present first data on the geochemistry mineralogy and morphology of near vent sediments 35 and 200 m from active vent and ridge flank sediments approximately 3 km from the vent field as well as oxyhydroxide deposits from the endeavour segment juan de fuca ridge the purpose of the study was to understand better the origin and characteristic features of metalliferous sediments associated with base and precious metal massive sulfides in volcanic terrains hydrothermal components in sediments are fe si s rich and mn fe si rich phases sulfides and barite which were exclusively derived from plume fallout sulfides are only a minor constituent of near vent sediments 2 4 wt and were not detected in ridge flank sediments the study suggests that the distribution of hydrothermal phases and associated elements in near vent and ridge flank sediments is affected mainly by processes of agglomeration dissolution absorption and settling that take place within a plume and to a lesser extent post depositional processes rapid deposition of sulfides in the vicinity of the vents is reflected in a sharp drop of the cu concentrations in sediments with increasing distance from the vents besides sulfides important carriers of pb cu zn and co in near vent sediments are fe mn oxyhydroxides that occur together with silica as aggregates of gel like material and flaky particles and as coatings on filaments away from the vents trace metals are mostly in fe mn oxyhydroxides and authigenic fe rich montmorillonite oxyhydroxides at the main endeavour field are interpreted to have originated from oxidation of mound sulfides accompanied by precipitation of primary fe oxyhydroxide silica from low temperature fluids at the mothra field seafloor deposits and chimney crusts composed of fe oxyhydroxide mn silica are considered to be direct precipitates from hydrothermal fluids that have been less diluted with seawater oxyhydroxide deposits exhibit unique microtextures that resemble mineralized microorganisms and may indicate existence of diverse microbial communities

evidence for a weakly stratified europan ocean sustained by seafloor heat flux we interpret chaos type features on the surface of europa as melt through structures formed by rotationally confined steady and or episodic oceanic plumes that rise to the base of the ice shell from magmatically heated regions of the seafloor smaller lenticular features in the vicinity of chaos type regions might be formed by baroclinically unstable vortices that spin off the main convective plume or by persistent heating from localized hydrothermal venting sites the ocean is assumed to be weakly stratified because of turbulent convection generated by heating from below and cooling from above seafloor heating maintained by tidal dissipation in the rocky interior generates an estimated global heat flux of 8 7 1012 w and limits the mean ice thickness to 2 5 km for seafloor heat sources with radii r that are less than the ocean s deformation radius rd nd n is the brunt vaisala frequency d is the water depth and is the coriolis parameter the diameters of chaos type regions are expected to diminish from o 100 km within equatorial regions to o 10 km at high latitudes assuming spatially uniform water depth and density structure where r rd the scale of the source region determines the scale of the melt through features provided there is sufficient time before refreezing ice rafts in large melt through regions are imbedded in episodes of preferentially anticyclonic circulation corresponding to clockwise counterclockwise motions in the northern southern hemisphere we calculate that 1021 j were required to melt the ice in the 100 km diameter conamara chaos region and that for a steady localized heat flux f 1011 w 1 of the global heat flux it took 1000 years for the initial melt through to occur assuming that ice raft displacements in conamara chaos occurred during a major melt through event maximum current speeds in the region were o 10 cm s 1 and refreezing occurred within 20 hours a lack of well defined ice drift in other major melt through regions suggests that these regions formed through episodes of melting and refreezing that modified the existing structure but left little time for the establishment of organized advective motion

strong sediments at the deformation front and weak sediments at the rear of the nankai accretionary prism revealed by triaxial deformation experiments nineteen whole round core samples from the nankai accretionary prism iodp expeditions 315 316 and 333 from a depth range of 28 128 m below sea floor were experimentally deformed in a triaxial cell under consolidated and undrained conditions at confining pressures of 400 1000 kpa room temperature axial displacement rates of 0 01 9 0 mm min and up to axially compressive strains of 64 despite great similarities in composition and grain size distribution of the silty clay samples two distinct rheological groups are distinguished the first group shows deviatoric peak stress after only a few percent of compressional strain 10 or does not weaken at all this is characteristic of structurally strong material the strong samples tend to be overconsolidated and are all from the drillsites at the accretionary prism toe while the weak and normally consolidated samples come from the immediate hanging wall of a megasplay fault further upslope sediments from the incoming plate are also structurally weak the observed differences in mechanical behavior may hold a key for understanding strain localization and brittle faulting within the uniform silty and clayey sedimentary sequence of the nankai accretionary prism

plume dispersed hydrothermal particles a time series record of settling flux from the endeavour ridge using moored sensors abstract settling particles collected with moored sediment traps near an area of active hydrothermal venting in the northeast pacific ocean provide information about the deep sea distribution of hydrothermal material results of an 8 day near field single trap experiment and a 340 day multi instrument 5 trap deployment two kilometers from the endeavour ridge vent field show variability in the quantity and composition of the plume to sediment particle flux over time with depth and with distance from the vent field hydrothermal elemental fluxes vary by a factor of two among the four sampling intervals of the longer experiment a factor of three variation in fe s among these periods reflects temporal changes in phase composition changes in flux with depth for example a 200 fold increase in cu and zn between above and below plume traps are evidence of hydrothermal input comparison of fluxes determined in the near and far field experiments reveals the spatial influence of vent effluent near field particle flux exceeds that measured two kilometers away by a factor of 10 to 20 and is relatively enriched in fe cu and organic carbon but depleted in mn calculations of the residence time of the plume with respect to settling indicate that more than 90 percent of the particulate material is transported farther than two kilometers from the vent field interpretation of elemental mineralogical and grain size analyses suggest that chemical precipitation kinetics other physical and biological particle forming processes changes in vent output and lateral transport of hydrothermal material by abyssal currents are tenable explanations for our observations our findings suggest that these rate interdependent processes be included in realistic models of hydrothermal particle flux

growth of large sulfide structures on the endeavour segment of the juan de fuca ridge abstract mapping and sampling with dsrv alvin has established that sulfide blocks 0 5 m across dredged from the axial valley of the endeavour segment at 47 57 n are samples of unusually large sulfide structures the steep sided structures up to 30 m in length 20 m in height and 10 15 m across are localized by venting along normal faults at the base of the western axial valley wall and are distributed for about 200 m along strike paralleling the 020 trend of the ridge crest high temperature fluids 350 to more than 400 c pass through the massive sulfide structures and enter seawater through small concentric nozzle like features projecting from the top or the sides of the larger vent structures diffuse low temperature flow is pervasive in the vicinity of the active sulfide structures exiting from basalt and sulfide surfaces alike evidence of recent volcanic activity is sparse the two largest samples taken with the dredge would not have been recoverable using the submersible these samples represent massive complex portions of the sulfide structures which were not closely associated with rapid high temperature fluid flow at the time of sampling they contain textural evidence of sealed hydrothermal fluid exit channels mineralogy is dominated by fe sulfides nnd amorphous silica pyrite marcasite wurtzite chalcopyrite and iss are the most common sulfide phases pyrrhotite galena and sphalerite are present in trace amounts barite amorphous silica and chalcedony are the only non sulfide phases anhydrite is not observed in any of the dredge samples although it is common in the chimney like samples recovered by alvin specific mineralogical textural zones within the dredge samples are anaoogous to individual layers in east pacific rise at 21 n and southern juan de fuca ridge samples with two exceptions a coarse grained highly porous fe sulfide rich interior containing sulfidized tubeworm casts and a 2 5 cm thick zone near the outer margin of the samples dominated by late stage amorphous silica the porous interior may have formed by dendritic crystal growth from a slowly circulating fluid within a large enclosed chamber the amorphous silica deposited from a seawater hydrothermal fluid mixture percolating slowly through the walls of the enclosed chamber conductive cooling of the fluid as it traversed the walls allowed amorphous silica to precipitate these silica rich zones are the densest most durable portions of the structures and may be responsible for the lasting stability of the large sulfide features observations in these samples are consistent with two distinct phases of development phase 1 is analogous to chimney growth and construction at 21 n and ends when flow channels become sealed to rapid flow of through going fluid the flow is evidently redirected within the structure phase 2 includes dissolution of anhydrite and precipitation of amorphous silica during conductive cooling of sluggishly circulating hydrothermal fluid or seawater hydrothermal fluid mixtures evolution of vent structures through phase 2 allows lateral and vertical growth of unusually large structures

velocity porosity relationships for slope apron and accreted sediments in the nankai trough seismogenic zone experiment integrated ocean drilling program expedition 315 site c0001 in this study we focused on the porosity and compressional wave velocity of marine sediments to examine the physical properties of the slope apron and the accreted sediments this approach allows us to identify characteristic variations between sediments being deposited onto the active prism and those deposited on the oceanic plate and then carried into the prism during subduction for this purpose we conducted ultrasonic compressional wave velocity measurements on the obtained core samples with pore pressure control site c0001 in the nankai trough seismogenic zone experiment transect of the integrated ocean drilling program is located in the hanging wall of the midslope megasplay thrust fault in the nankai subduction zone offshore of the kii peninsula sw japan penetrating an unconformity at 200 m depth between slope apron sediments and the underlying accreted sediments we used samples from site c0001 compressional wave velocity from laboratory measurements ranges from 1 6 to 2 0 km s at hydrostatic pore pressure conditions estimated from sample depth the compressional wave velocity porosity relationship for the slope apron sediments shows a slope almost parallel to the slope for global empirical relationships in contrast the velocity porosity relationship for the accreted sediments shows a slightly steeper slope than that of the slope apron sediments at 0 55 of porosity this higher slope in the velocity porosity relationship is found to be characteristic of the accreted sediments textural analysis was also conducted to examine the relationship between microstructural texture and acoustic properties images from micro x ray ct indicated a homogeneous and well sorted distribution of small pores both in shallow and in deeper sections other mechanisms such as lithology clay fraction and abnormal fluid pressure were found to be insufficient to explain the higher velocity for accreted sediments the higher slope in velocity porosity relationship for accreted sediments can be explained by weak cementation critical porosity or differences in loading history

methane in aging hydrothermal plumes methane concentration and the stable carbon isotopic composition 13 c ch4 were measured in aging hydrothermal plume waters originating at the endeavour segment juan de fuca ridge ch4 concen trations decreased systematically from 600 nm at on axis stations to 11 nm at a distal station 15 km off axis background ch4 concentrations are 2 nm ch4 behaves nonconservatively in plume waters and does not correlate with conservative parameters such as temperature anomaly but is highly correlated with nh4 and total dissolvable mn 13 c ch4 values for plume depth samples varies inversely and significantly r 2 0 89 with methane concentrations normalized to some 13 c ch4 values 1 8 and 10 9 measured at the 15 km off axis station are among the heaviest yet reported from a natural marine environment less than 5 of original hydrothermal methane remains in the plume at this station the data are consistent with extensive microbial methane oxidation a narrow range of fractionation factors rc 1 0072 to 1 0077 was calculated for the endeavour samples these fractionation factors are less than those reported by coleman et al 1981 but fall near the trend line of their rc versus temperature data when extrapolated to plume water temperature 2 c copyright 2002 elsevier science ltd

diamond garnet geobarometry the role of garnet compressibility and expansivity abstract we report p v and t v equations of state eos for synthetic single crystal pyrope py mg3al2si3o12 and the p v eos for synthetic single crystals of almandine alm fe3al2si3o12 as well as an intermediate composition py60alm40 as measured by in situ high pressure and high temperature x ray diffraction experiments the unit cell volumes of the three samples were measured at room temperature and different pressures and up to about 8 gpa in a diamond anvil cell the high temperature experiment was carried out using a micro furnace the pressure volume data were fitted to a third order birch murnaghan eos giving the following coefficients v0 1506 15 16 a3 kt0 163 7 1 7 gpa and k 6 4 4 for pyrope v0 1533 52 10 a3 kt0 172 6 1 5 gpa and k 5 8 5 for almandine and v0 1516 32 13 a3 kt0 167 2 1 7 gpa and k 5 6 5 for the intermediate py60alm40 composition the unit cell volume along the pyrope almandine join changes linearly within the error of measurement indicating ideality in the volume of mixing behaviour for the solid solution the first pressure derivative k for all three garnets is similar within experimental uncertainty with an average value of k 6 0 4 the thermal expansion parameters for end member pyrope as described using to the kroll type eos with the einstein temperature e fixed to 320 k are 303 k 1 bar 2 543 5 10 5 k 1 and v0 1504 64 4 a3 the determined thermoelastic parameters were combined in a thermal pressure type eos thermal p type eos to calculate the entrapment pressures pe for natural garnet inclusions in diamonds at mantle temperatures a calculated pressure of 5 8 gpa at t 1500 k using our newly determined thermoelastic parameters appears to be more consistent with diamond forming conditions compared to the higher pressures that range up to 6 8 gpa at t 1500 k which are obtained using thermoelastic data in the literature

elastic anisotropy modeling of kimmeridge shale 1 anisotropy of elastic properties in clay rich sedimentary rocks has been of long standing interest these rocks are cap rocks of oil and gas reservoirs as well as seals for carbon sequestration elasticity of shales has been approached by direct velocity measurements and by models based on microstructures here we are revisiting the classical kimmeridge shale studied by hornby 1998 by first quantifying microstructural features such as phase volume fractions grain shapes and grain orientations and pore distributions with advanced analytical methods and then using this information in different models to explain bulk elastic properties it is shown that by application of a self consistent algorithm based on eshelby s 1957 model of inclusions in a homogeneous medium it is possible to explain most experimental elastic constants though some discrepancies remain which may be due to the interpretation of experimental data using a differential effective medium approach an almost perfect agreement with experimental stiffness coefficients can be obtained though the physical basis of this method may be questionable the influence of single crystal elastic properties grain shapes preferred orientation and volume and shapes of pores on elastic properties of shale is explored

hydrothermal venting and geothermal heating in cascadia basin observations in cascadia basin on the eastern flank of the northern juan de fuca ridge reveal significant bottom water modification as a result of regional conductive heating and local hydrothermal venting seafloor conductive heating occurs throughout the sedimented basin while hydrothermal fluid discharge is confined to small 1 km2 isolated igneous basement outcrops in the northern sector of the plateaulike basin the vertical fluxes of heat and mass associated with these seafloor processes lead to the formation of a 250 to 350 m thick geothermal boundary layer characterized by anomalously high temperature reduced vertical stability and high dissolved silicate concentration using a basinwide average lithospheric heat flux of 0 3 w m 2 and the observed thermal anomaly structure of the water column we obtain a mean residence time of 1 to 2 years for the deep water over cascadia basin detailed water property data collected in 1992 and 1993 within the immediate vicinity of three isolated igneous basement outcrops in the north central sector of the basin indicate that local bottom water heating arises from low temperature venting through the summit and flanks of the outcrops near the smallest edifice especially well defined layers of anomalously warm particle laden water were found within 20 m of the outcrop summit depth of 2610 m maximum anomalies of temperature light attenuation coefficient and dissolved silicate concentration in the layers were 0 040 c 0 015 m 1 and 5 mol l 1 respectively we estimate the local heat flux fo from the smallest outcrop to be 2 4 0 8 u 109 w where u m s 1 is the mean horizontal current at the venting depth for reasonable mean currents in the range 10 3 to 10 2 m s 1 we find fo 0 2 to 2 4 107 w assuming that the depressed conductive heat flow of 0 05 w m 2 observed through the sedimented seafloor surrounding the smallest outcrop reflects the advective loss of heat through the outcrop the radial distance over which crustal fluids must collect heat and converge on the outcrop is about 10 km

mid ocean ridge sulfide deposits evidence for heat extraction from magma chambers or cracking fronts numerous seafloor observations show that the sizes of high temperature hydrothermal sulfide edifices vary dramatically with spreading rate on fast spreading ridges venting occurs from spindly chimneys which reach heights of about 10 m while on slow spreading ridges vents are frequently located near the tops of large sulfide mounds whose volumes may reach 105 106 m3 we argue that such variations are the result of a profound difference in the nature of hydrothermal heat extraction between fast spreading ridges where spreading occurs predominantly by magmatism and slow spreading ridges where there is a significant component of tectonic extension along fast spreading ridges a steady state axial magma chamber can be insulated by a relatively thick conductive boundary layer because heat extraction is limited by low permeability in such systems episodes of vigorous venting are linked to diking events not only because these introduce a significant heat source into the upper crust but also because the intrusion of a dike is the primary source of increased permeability near the ridge axis over a time scale of the order of a decade mineral deposition tends to clog the reaction and upflow zones and there is a high probability that young sulfide structures will be buried by subsequent eruptions on slow spreading ridges tectonic extension maintains the fluid pathways necessary to support vigorous convection in the waning stages of magmatism hydrothermal circulation driven by a downward migrating cracking front can cool the entire crust leading to the formation of very large sulfide deposits as the depth of circulation increases overburden pressures reduce the permeability and the hydrothermal heat fluxes decrease progressively once hydrothermal fluids penetrate the moho serpentinization clogs the fluid pathways and high temperature venting ceases until it is reactivated by a fresh magmatic intrusion

physical characteristics of the endeavour ridge hydrothermal plume during july 1988 abstract we conducted ctd transmissometer tows from 8 to 26 july 1988 within 15 km of the central hydrothermal vent site 47 57 n 129 06 w on the endeavour segment of juan de fuca ridge anomalies of temperature salinity and light attenuation reveal possible new vent sites 4 and 8 km northeast and 6 km south of the central vent site as a result of widespread plume dispersion background values of potential temperature salinity and light attenuation below the 1900 m depth exceeded those for pristine ambient waters by 0 05 c 0 05 psu and 0 03 m 1 respectively maximum plume anomalies relative to the background waters were of the order of 0 10 c 0 010 psu and 0 10 m 1 at core depths of 2000 2050 m heat and salt anomalies were detectable more than 5 km from the central vent site whereas light attenuation particle anomalies were confined to within 2 5 km of the vent site based on the background water property anomalies and moored current meter records the mean time averaged heat fluxes for the survey region were 2 3 1 5 10 8 w in the along ridge direction 20 t and 7 7 4 7 10 8 w in the cross ridge direction 110 t mean along and cross ridge salt fluxes were 7 5 and 25 15 kg s 1 mean particle fluxes were 0 09 0 06 and 0 29 0 18 kg s 1 estimates of the instantaneous fluxes derived from coincident current and plume measurements indicate that heat fluxes from the central vent field may have been as high as1 2 0 6 10 10 w and corresponding particulate fluxes as high as6 3 kg s 1

relationships between hydrothermal activity and axial magma chamber distribution depth and melt content 1 hydrothermal activity especially high temperature discharge is commonly thought to require the presence of an axial magma chamber amc but this association has not been examined systematically the availability of six lengthy 170 560 km ridge sections with continuous surveys of both amcs and hydrothermal plumes now makes it possible to quantitatively compare the distribution and intensity of hydrothermal activity to amc extent depth and at a few locations melt content these six ridge sections span spreading rates from 55 to 145 mm a and total 20 second order segments at the section multisegment scale the linear incidence of hydrothermal plumes increases with amc incidence amci r2 0 64 excluding the hot spot affected galapagos spreading center for all six sections plume incidence increases as amc depth below the seafloor decreases amcz r2 0 66 at the second order segment scale plume incidence is poorly correlated with both amci r2 0 12 and amcz r2 0 25 finally at the subsegment or local scale 0 75 km long bins plume intensity increases as amcz shallows r2 0 85 of bins where plumes are most intense and thus closest to their seafloor sources 68 13 lie directly over an amc as do at least 37 of the 40 known high temperature vent fields the data also allow tests of other hypotheses linking amc properties and hydrothermal activity existing data though still sparse do not support the hypothesis that lenses of melt rich magma preferentially support vigorous long lasting venting also the suggestion that increased hydrothermal cooling within a segment locally depresses amcz finds no support within any ridge section evidence for magma bodies is much scarcer on slow spreading ridges but the data are nevertheless consistent with those from faster ridges observations from all spreading rates thus demonstrate that high temperature vent fields are almost universally associated with the presence or inference of magma hot rock or other nonmagmatic heat sources are insufficient

the relationship of strain and preferred orientation of phyllosilicate grains in rocks a review abstract the calculation of strain from the preferred orientation of platy phyllosilicate grains taking strain to be homogeneous is surprisingly often successful from a pragmatic point of view although numerous theoretical objections to this procedure can be and have been raised for the calculation to succeed the original distribution of mica or clay mineral grains must be known or assumed the usual assumption that the starting distribution was random and therefore nearly uniform implies that the strain history of pelitic sediments is taken to start when the watery sediment was last stirred by waves or bioturbation or if it was laid down in still anoxic waters from the moment of first deposition it therefore includes compaction as part of the strain history which is usually neglected by structural geologists originally random distribution of clay flake orientations has been observed in recently deposited sediments in many instances and has been theoretically explained in various ways exceptions probably exist but their frequency has not been estimated biologically disturbed sediments may be brought nearer to a state of random orientation of clay flakes than they were at deposition with the exception of whole beds or concretions in which pores are early filled with cement mudrocks are almost always compacted by the weight of later deposits the strain so induced is axially symmetric about the vertical and thus about the pole of the bedding planes compaction acts on soft unconsolidated rocks but may continue with increasing overburden on rocks strengthened by cementation and by increasing interlocking of grains tectonic deformation may start early before the rock is fully compacted or it may begin only much later and the mechanisms of deformation increase in complexity with the increasing temperature and hydrostatic stress of deeper burial up to a certain level of mild metamorphism however phyllosilicate grains seem to be rather permanent if not as mineral species then at least by preservation of their constituent sheets of silica tetrahedra given this permanence the mechanisms of rock deformation whether intergranular slip cataclasis or dissolution and reprecipitation and various intragranular processes in the non phyllosilicate matrix hardly matter as far as the recording of strain is concerned even a certain amount of stress solution at especially heavily loaded points of the phyllosilicate grains themselves does not destroy their value as strain markers as long as the dissolved material is disposed of mainly by crystallographically continuous overgrowth on existing grains the same holds for local dissolution of phyllosilicates in portions of old grains that have become thermodynamically unstable by bending widespread irregular or random nucleation and growth of newly nucleated phyllosilicate grains at intermediate or high grades of metamorphism can reduce the reliability of the strain record and so can domainal structure of the deformed rock if both the volume fraction of phyllosilicate grains and the intensity of total strain differ sharply from one domain to another if distinct domains are not equant and have themselves a strong preferred orientation of shape nevertheless rocks containing an anastomosing network of zones rich in phyllosilicate grains as slates and other deformed mudrocks commonly do have in many cases been found to have average preferred orientations of mica or chlorite which if used to estimate the strain give results similar to those obtained from independent strain markers in the same rocks thus the most significant verification of the usefulness of strain estimates by measurement of preferred orientation is the mutual confirmation with independently determined strain the limit to such verification is the rarity of strain markers so that the coexistence of two independent ones is exceptionally rare for this reason however it is important to make full use of the orientation of phyllosilicate grains where they are available the alternative to have no strain measurements at all causes structural geologists to miss potential information

texture development in naturally compacted and experimentally deformed silty clay sediments from the nankai trench and forearc japan the petrophysical properties of fine grained marine sediments to a large extent depend on the microstructure and crystallographic preferred orientations cpos in this contribution we show that rietveld based synchrotron texture analysis is a new and valuable tool to quantify textures of water saturated fine grained phyllosilicate rich sediments and assess the effects of compaction and tectonic deformation we studied the cpo of compositionally almost homogeneous silty clay drillcore samples from the nankai accretionary prism slope and the incoming philippine sea plate offshore sw japan basal planes of phyllosilicates show bedding parallel alignment increasing with drillhole depth thus reflecting progressive burial and compaction in some samples calcite and albite display a cpo due to crystallographically controlled non isometric grain shapes or nannofossil tests consolidated undrained experimental deformation of a suite of thirteen samples from the prism slope shows that the cpos of phyllosilicate and calcite basal planes develop normal to the experimental shortening axis there is at least a qualitative relation between cpo intensity and strain magnitude scanning electron micrographs show concurrent evolution of preferred orientations of micropores and detrital illite flakes normal to axial shortening this indicates that the microfabrics are sensitive strain gauges and contribute to anisotropic physical properties along with the cpo

geochemical and physical structure of the hydrothermal plume at the ultramafic hosted logatchev hydrothermal field at 14 45 n on the mid atlantic ridge abstract the hydrothermal plume generated in deep waters above the logatchev hydrothermal field lhf about 15 n on the mid atlantic ridge was investigated and mapped for its 3d distributions using a combination of in situ optical light scattering data temperature and salinity data as well as concentrations of hydrogen methane total dissolvable fe and total dissolvable mn based on the results obtained for these meaningful parameters we report the geochemical and physical characteristics of the fluids expelled from the ultramafic lhf and the chemical structure of its hydrothermal plume in the water column the hydrothermal plume is sourced by at least seven distinct vent sites and possibly additional diffusive fluid and gas discharge it comprises a water body characterized by strong nephelometric anomalies expressed as ntu nephelometric turbidity units and high concentrations of fe and mn 5 times seawater concentration and the gas plume with several times the h 2 and ch 4 concentrations of normal seawater up to three plume levels with a total vertical extension of about 350 m from the seafloor were classified in the hydrothermal plume the ntu plume could be followed to approximately 2 5 km to the north and to the south from the vent site while the gas plume spread several km farther from the hydrothermal source high concentrations of h 2 up to 1598 nmol l 1 and ch 4 up to 323 nmol l 1 accompanied by relatively low dissolvable fe concentrations up to 270 nmol l 1 as well as low concentrations of dissolvable mn 112 nmol l 1 compared to basaltic hydrothermal systems are the characteristics of the plume the low metal gas ratios showed a decrease with increasing distance from the vent site our data demonstrate that ultramafic systems such as the lhf serve both as sources and sinks for elements with respect to metal and gas inputs into the oceanic water column the relevance of such systems is underlined by the discovery and postulated frequency of further ultramafic hosted hydrothermal systems on slow spreading ridges

numerical simulation of hydrothermal vent induced circulation at endeavour ridge hydrothermal venting at endeavour ridge is focused at five major vent fields spaced at 2 km intervals along a narrow 1 km wide north south trending rift valley a princeton ocean model with an axially symmetric rift valley centered within a rotating full depth 2400 m stratified ocean is used to simulate the circulation and associated temperature and salinity fields generated by vertical heat fluxes from the five vent fields steady seafloor heat fluxes are introduced instantaneously to a quiescent ocean using the model provision for boundary fluxes the model achieves a quasi steady state in about 80 days results are consistent with observation in particular buoyant plumes emanating from the larger vent fields rise to trapping depths as shallow as 1850 m 350 m above bottom and spread laterally with the vent induced circulation plume anomalies remain confined to within several kilometers of the ridge except to the west where anomalies extend more than 10 km off axis as with observations modeled currents within the valley consist of a strong 10 cm s 1 poleward inflow within roughly 75 m of the bottom and an equally strong equatorward counterflow above that depth weak 1 cm s 1 equatorward flow develops over the flank of the west ridge and weak poleward flow over the flank of the east ridge cyclonic relative vorticity dominates the model circulation within the valley while anticyclonic vorticity prevails above the depth of neutral plume buoyancy findings support the notion that turbulent entrainment by buoyant plumes forces a mean poleward flow within the confines of the endeavour ridge axial valley

chemical reaction rates and entrainment within the endeavour ridge hydrothermal plume the aging of the hydrothermal plume over the endeavour segment of the juan de fuca ridge was estimated by measuring the222rn3he ratio in the plume as it dispersed despite uncertainties in the source function of hydrothermal input it wa determined that the relative sequence of removal from the plume ish2 c 222rn ch4 mn where c is a measure of particle concentration and the mean life of222rn is 5 5 days h2 is removed from the plume within hours of input while mn is not removed within the two week timescale of the radon helium clock entrainment of bottom water within the buoyant plume may introduce additional chemical signatures into the spreading effluent layer over that which would be introduced by hydrothermal discharge alone this is particularly significant for those chemical species which are not greatly enriched in the vent fluids relative to bottom water concentration and which display a nutrient like profile in the deep ocean thus we found that significant fractions of the si and226ra anomalies in the plume were not of hydrothermal origin but were derived from entrained bottom water which has a higher concentration of these elements than ambient water at plume height

hydrothermal plume dynamics on europa implications for chaos formation 1 hydrothermal plumes may be responsible for transmitting radiogenic or tidally generated heat from europa s rocky interior through a liquid ocean to the base of its ice shell this process has been implicated in the formation of chaos regions and lenticulae by melting or exciting convection in the ice layer in contrast to earlier work we argue that europa s ocean should be treated as an unstratified fluid we have adapted and expanded upon existing work describing buoyant plumes in a rotating unstratified environment we discuss the scaling laws governing the flow and geometry of plumes on europa and perform a laboratory experiment to obtain scaling constants and to visualize plume behavior in a europa like parameter regime we predict that hydrothermal plumes on europa are of a lateral scale at least 25 50 km comparable to large chaos regions they are too broad to be responsible for the formation of individual lenticulae plume heat fluxes 0 1 10 w m 2 are too weak to allow complete melt through of the ice layer current speeds in the plume 3 8 mm s are much slower than indicated by previous studies the observed movement of ice blocks in the conamara chaos region is unlikely to be driven by such weak flow index terms 6218 planetology solar system objects jovian satellites 5418 planetology solid surface planets heat flow 5430 planetology solid surface planets interiors 8147 4540 oceanography physical ice mechanics and air sea ice exchange processes 4568 oceanography physical turbulence diffusion and mixing processes keywords chaos europa hydrothermal plumes

hydrothermal plumes over the carlsberg ridge indian ocean 1 indian ocean ridges north of the rodriguez triple junction remain poorly explored for seafloor hydrothermal activity with only two active sites confirmed north of 25 s we conducted water column surveys and sampling in 2007 and 2009 to search for hydrothermal plumes over a segment of the carlsberg ridge here we report evidence for two separate vent fields one near 3 42 n 63 40 e and another near 3 41 5 n 63 50 e on a segment that is apparently sparsely magmatic both sites appear to be located on off axis highs at the top of the southern axial valley wall at depths of 3600 m or shallower 1000 m above the valley floor at the 63 40 e site plume sampling found local maxima in light scattering temperature anomaly oxidation reduction potential orp dissolved mn and3he no water samples are available from the 63 50 e site but it showed robust light scattering and orp anomalies at multiple depths implying multiple sources orp anomalies are very short lived so the strong signals at both sites suggest that fluid sources lie within a few kilometers or less from the plume sampling locations although ultramafic rocks have been recovered near these sites the light scattering and dissolved mn anomalies imply that the plumes do not arise from a system driven solely by exothermic serpentinization e g lost city instead the source fluids may be a product of both ultramafic and basaltic gabbroic fluid rock interaction similar to the rainbow and logatchev fields on the mid atlantic ridge

extensive distribution of hydrothermal plumes along the superfast spreading east pacific rise 13 30 18 40 s a general model relating relative hydrothermal activity to the rate of plate creation requires data from the full spectrum of mid ocean ridge spreading rates to obtain data from a superfast spreading environment the japanese u s ridge flux project used continuous hydrographic optical tow yos in 1993 to map the distribution of hydrothermal plumes along the southern east pacific rise from 13 30 to 18 40 s plume incidence the fraction of the spreading axis length overlain by a significant plume was 0 6 including a virtually continuous vent field stretching 150 km from 17 20 to 18 40 s hydrothermal venting was most concentrated along portions of the ridge crest with an inflated cross sectional area and an observable axial magma chamber reflector this pattern agrees with previous results from the northern east pacific rise such consistency implies that hydrothermal circulation on fast spreading ridges is vigorous where the relative volume of partial melt is high and meager where melt volume is low or undetectable hot rock alone is generally insufficient to drive significant hydrothermal circulation using a mean temperature anomaly of 0 014 0 010 c we estimated the hydrothermal heat flux from the study area as 1 5 1 1 107 mw ux where ux is the cross axis flow at the ridge crest no direct measurements of ux are yet available combining the plume distribution found here with prior data from slow intermediate and fast spreading ridges yields a significant linear correlation between plume incidence and spreading rate that extends across the full range of plate motion this correlation which mirrors that for near axis heat flux calculated by recent models implies that magma supply rate is the principal control on the large scale distribution of axial hydrothermal venting

influence of particle shape on the frictional strength of sediments a numerical case study abstract numerous direct and ring shear tests supply that composition and texture of sediments are crucial for shear and frictional strength we simulate particle interactions on a micro scale level during shearing with a numerical shear box using the discrete element method dem based on the granular model approach detailed information about slip localisation and rates forces at particle contacts as well as particle rotation can be quantified as function of particle shape stress conditions and shear rate our numerical experiments show that the deformation behaviour of clay is largely controlled by their particle shape two key factors sphericity and roughness could be identified as relevant for frictional strength shear zone development and particle rotation with sphericity dominating over roughness decreasing sphericity leads to complex initial microfabrics whose breakdown with increasing strain is caused by particle rotation preferred particle orientation favours low friction decreasing volume strain and the evolution of particle domains of similar orientation decreasing roughness results in particle interlocking of different degrees which obstructs slip and rotation to preferred orientations so that friction is high and volume strain positive spatial and temporal shear zone development depends on the roughness of particles increasing roughness precludes shear zone development and particle domain evolution shear zone localisation depends on particle sphericity increasing sphericity leads to more localised but vertically dilated deformation

artefacts of isotropic inversion applied to magnetotelluric data from an anisotropic earth summary 2 d isotropic approaches are standard for magnetotelluric mt data modelling and inversion unfortunately the real subsurface structure is not isotropic everywhere and one should be aware of the possible consequences of applying an isotropic inversion to data from an anisotropic earth the work presented herein was motivated by discovering an unusual mid to lower crust conductor that appeared to be bent downward into the lithospheric mantle compared to the neighbouring terranes when applying routine 2 d isotropic inversion one major difference between the terranes is the presence of the okavango giant mafic dyke swarm northeastern botswana beneath which the conductor is imaged to be in the lithospheric mantle rather than the lower crust the limited width of the dykes makes them an anisotropic feature rather than a normal 2 d structure at mt scale to examine the possible effects of the dykes synthetic data were generated from an 1 d model accounting for the dyke swarm by using an anisotropic block and then inverted isotropically the synthetic tests showed that the normal 2 d decomposition and strike analysis techniques are not removing these large scale anisotropic effects and that an isotropic inversion result obtained in the presence of an anisotropic structure has to be treated with caution the comparison of the synthetic data with the presented case history strongly suggests that the lithospheric depths of the conductor is an artefact and it is most likely located in the lower crust as everywhere else in the area focused anisotropic inversion of the dyke swarm area supports the assumption of a crustal anisotropic structure related to the okavango dyke swarm that affects the isotropic inversion results

the rainbow hydrothermal plume 36 15 n mar the rainbow hydrothermal plume was discovered during a recent geophysical survey along 200 km of the mid atlantic ridge mar sw of the azores triple junction in which at least seven new sites of hydrothermal activity were identified here we present the first hydrographic study of the rainbow plume 36 15 n the strongest of the features located during that survey the plume is detectable from real time in situ nephelometer anomalies and extends 10 15km w e in a non transform discontinuity ntd between two adjacent ridge segments maximum anomalies in the rainbow plume indicate particle enrichments at least as large as those seen directly above the trans atlantic geotraverse tag vent field 26 n mar analysis of hydrographic data indicates a vent source at rainbow with a thermal output of up to 98 mw representing 140 of the thermal output previously attributed to tag both lines of evidence indicate the rainbow plume to be the strongest such feature yet found on the mar

estimation of heat and chemical fluxes from a seafloor hydrothermal vent field using radon measurements the circulation of seawater through newly formed ocean crust at mid ocean ridge spreading centres is important in the oceanic heat and chemical budgets the heat transfer in submarine hydrothermal systems accounts for 25 of the total global heat loss1 the transfer of mass resulting from basaltic alteration affects the geochemistry of seawater2 and is responsible for the formation of ore deposits on and beneath the seafloor3 various geophysical techniques have been employed in efforts to determine the heat output from hydrothermal vent fields4 9 however the magnitudes of the heat and chemical fluxes through these systems remain uncertain here we introduce a geochemical approach for estimating the flux from a vent field based on radon 222rn measurements in the overlying effluent plume this method was applied successfully in september 1986 during a 23 day expedition to an active vent field on the 170 km endeavour segment of the juan de fuca ridge fig la we estimate the heat flux from this site to be l 5 l09 w

is the south pacific helium 3 plume dynamically active it is suggested that the hydrothermal vents of the south pacific rise produce a beta governed circulation at mid depth and that perhaps the associated plume of excess3he lupton and craig 1 points westward because of the dynamics of this circulation rather than as a passive tracer

excess 222rn above the cleft segment of the juan de fuca ridge the distributions of excess 222rn above the north cleft segment of the juan de fuca ridge were determined from 1988 to 1991 in order to obtain a better understanding of the temporal variability of heat flux from this ridge segment based on the inventories of excess 222rn the hydrothermal heat flux was estimated to be 2 7 1 3 gw in 1990 the large uncertainty results from errors in calculating the standing crop of 222rn the difficulty in determining the 222rn to heat ratio and the problem of entrainment of low temperature waters into the rising plume the high resolution sampling program carried out in 1990 permitted the mapping of apparent plume ages for different sections of the neutrally buoyant plume about 3 km to the east of the ridge axis there is a sharp gradient in the apparent age of the plume while to the west the plume appears to be younger this plume age distribution could indicate a current flow to the northwest during the time this transect was sampled the mean 222rn heat ratios for the high and low temperature vent fluids were 0 0017 disintegrations per minute dpm g 1 c 1 and 0 0044 dpm g 1 c 1 respectively the extrapolated end member concentrations of excess 222rn in low temperature vent fluids are a factor of 6 greater than the high temperature fluids based on the regressions of excess 222rn activity and mg concentration in north cleft segment vent fluid samples

modeling the large scale influence of geothermal sources on abyssal flow an approximate temperature perturbation equation for a continuously stratified deep ocean layer on an equatorial plane is presented a vertically integrated flow is allowed which in the present study is forced by uniform upwelling solutions with truncated vertical structure are presented which are forced by a representation of localized geothermal heat sources solutions exhibit a range of behavior from passive advection to plume depending on the ratio of barotropic flow speed to long rossby wave phase speed owing to the different phase speeds of the vertical modes the forced vertical structure is dispersed which can result in positive and negative temperature anomalies both upstream and downstream of the source furthermore it is clear that the ability of localized sources to influence upstream conditions is enhanced in equatorial regions compared with higher latitudes where the background flow is better able to advect the temperature perturbations downstream depending on the strength of the imposed temperature anomaly at the source there may or may not be a large scale reversal of the total flow field thus the correlations between temperature geostrophic shear and a passive tracer such as 3he can be positive negative or zero away from the geothermal source

characteristics of hydrothermal plumes from two vent fields on the juan de fuca ridge northeast pacific ocean deep ctd transmissometer tows and water bottle sampling were used during 1985 to map the regional distribution of the neutrally buoyant plumes emanating from each of two major vent fields on the southern symmetrical segment sss and endeavour segment es of the juan de fuca ridge at both vent fields emissions from point and diffuse hydrothermal sources coalesced into a single 200 m thick plume elongated in the direction of current flow and with characteristic temperature anomalies of 0 02 0 05 c and light attenuation anomalies of 0 01 0 08 m 1 10 80 g l above background temperature anomalies in the core of each plume were uniform as far downcurrent as the plumes were mapped 10 15 km downcurrent light attenuation trends were non uniform and differed between plumes apparently because different vent fluid chemistries at each field cause significant differences in the settling characteristics of the hydrothermal precipitates vent fluids from the sss are metal dominated and mostly precipitate very fine grained hydrous fe oxides that remain suspended in the plume vent fluids from the es are sulfur dominated and precipitate a high proportion of coarser grained fe sulfides that rapidly settle from the plume the integrated flux of each vent field was estimated from measurements of the advective transport of each plume heat flux was 1700 1100 mw from the es and 580 351 mw from the sss particle flux varied from 546 312 g s to 204 116 g s at the es depending on distance from the vent field and was 92 48 g s from the sss

measurements of plume flow from a hydrothermal vent field simultaneous velocity temperature and conductivity data were taken in the convective flow emanating from a hydrothermal vent field located at 10 56 n 103 41 w on the east pacific rise the horizontal profiles obtained indicate that the flow field approaches an ideal plume in the temperature and velocity distribution such parameters as total heat flow and maximum plume height can be estimated using either the velocity or the temperature information the results of these independent calculations are in close agreement yielding a total heat flow from this vent site of 3 7 0 8 mw and a maximum height of 150 10 m the nonlinear effects of large temperature variations on heat capacity and volume changes slightly alter the calculations applied to obtain these values

a major helium 3 source at 15 s on the east pacific rise an extensive plume of water enriched with helium 3 has been discovered in the deep pacific ocean at latitude 15 s on the east pacific rise in the core of the plume at a depth of 2500 meters over the ridge crest the helium 3 helium 4 ratio is 50 percent higher than the ratio in atmospheric helium indicating a strong injection of mantle or primordial helium at the spreading center axis through local hydrothermal systems the helium 3 plume is completely absent east of the rise but it can be traced over 2000 kilometers to the west above a newly observed physical feature a density discontinuity here caled the ridge crest front the injected plume provides a unique deep sea tracer with an asymmetric distribution which shows that the deep circulation across the rise is from east to west the striking intensity and lateral extent of this helium 3 anomaly compared to observations at known oceanic hydrohrmal sites suggest that the largest hydrothermal fields in the ocean are yet to be discovered and that they will be found near 15 s on the east pacific rise

locating hydrothermal vents by detecting buoyant advected plumes an improved method of detecting buoyant hydrothermal plumes and locating their source vents is introduced plumes are detected by computing fluid stability from conductivity temperature and depth measurements acquired during navigated towed vertically oscillating casts over the endeavour segment of the juan de fuca ridge for each instability detected the maximum range to its hydrothermal source is estimated by multiplying a theoretical plume equilibration time by a measured current velocity using an estimate of current direction the method reliably locates plume sources where they are known to exist in all of the focused vent fields mapped by submersible and in several isolated diffuse flow sites the method generates a distribution of hydrothermal sources that is more consistent with variations in surface permeability than with circulation cells spaced evenly along a uniformly permeable axis axial instabilities are nearly continuous along the heavily fissured and fractured western wall of the axial valley beyond the axial valley instabilities evidence that hydrothermal upflow penetrates the outer slopes of the ridge crest probably along a boundary between sheet and pillow flows