the role of kelp in sediment transport observations from southeast new zealand abstract large marine algae such as kelp can when dislodged remove part of the substrate to which they are attached and subsequently transport it quantification of this process and the causes behind it have rarely been investigated this study examines sediment transport by the kelps durvillaea antarctica and durvillaea willana in southeast new zealand five beaches near dunedin were surveyed weekly for 30 weeks and all beach cast kelp was counted and identified the weight and nature of any attached sediment was also recorded additionally 60 sections of the southeast new zealand coastline totaling 130 km were surveyed once and all beach cast kelp examined rocks were found to be attached to 27 of examined holdfasts usually occurring as a thin veneer the weight per holdfast ranged from 0 1 g to 83 kg with an unusual outlier of 365 kg with a mean weight of 77 1 g se 15 3 n 1560 most holdfasts transported less than 100 g carbonate material occurred on 84 of holdfasts with weights per holdfast ranging from 0 02 to 116 56 g with a mean of 7 8 g se 0 3 n 1695 the character of local rock strongly influences the kelp mediated flux south coast beaches with schist rocks nearby received a significantly greater mean flux 7 49 g m2 year 1 than basalt surrounded peninsula beaches mean flux 0 50 g m2 year 1 kelp transported material provides a small but significant source of sediment to beaches in southeast new zealand and therefore should be considered when constructing sediment budgets in coastal areas this study has shown that it is often necessary for geologists to consider biological processes

dropstones their origin and significance abstract dropstones are clasts which pose a hydrodynamic paradox or which have an exotic or extra basinal lithology with uncertain provenance recently dropstones have been consistently interpreted as the product of ice rafting and have been used to substantiate the presence of cool climatic phases in the geological record this review illustrates that not all dropstones are reliable as palaeoclimatic indicators dropstones are the product of two distinct entrainment and transport mechanisms by a rafting agent or as a projectile four main processes of transport and formation are identified biological rafting ice rafting floatation and projectiles biological rafting agents include vertebrate gastroliths and other stomach contents vegetational rafting and anthropomorphic activity ice rafting can be the product of ice bergs or seasonal sea river or lake ice floatation of clasts in calm waters has been recorded for stones up to 25 mm in length projectiles are most commonly of volcanic origin given the range of transport and depositional mechanisms reviewed we argue that great care is required in the interpretation of the origin of dropstones

long submarine lava flows observations and results from numerical modeling long 100 km lava flows are relatively common on mars and venus and have been identified on the moon but they are rarely documented on earth however although 75 of the earth s surface is covered by water only a small percentage of the ocean floor has been investigated at a resolution sufficient to unequivocally identify the boundaries of long submarine lava flows even so basaltic lava flows as long as 110 km have been identified on the deep 1500 m seafloor near hawaii and the east pacific rise ambient conditions on the deep ocean floor may favor the development of long lava flows for the following reasons first high pressures 15 mpa keep volatiles dissolved in basaltic lavas preventing viscosity increases associated with exsolution and vesiculation second seawater rapidly quenches the surface of submarine basalt flows so that an insulating glass layer 1 5 cm thick encases submarine flows within seconds after their emplacement this glass rind effectively insulates the molten flow interior from additional heat loss making submarine basalt flows behave as well insulated subaerial tube fed flows thus for identical basalt flows emplaced on the deep seafloor and subaerially a submarine flow could advance farther before stopping results of numerical modeling indicate that thin 1 m submarine basalt flows behave similarly to identical subaerial flows but thicker submarine flows may advance significantly farther than their subaerial counterparts

voyages of seaweeds the role of macroalgae in sediment transport abstract at least 26 genera of macroalgae have been observed attached to and carrying sedimentary materials rock veneer or carbonate to beaches and the sea floor most studies have been descriptive with few quantifying flux rates this study reviews and quantifies the mainly biological literature on macroalgae mediated sediment transport in coastal and marine environments dislodgement of macroalgae from a hard substrate can result in erosion up to 0 01 mm y 1 the proportion of buoyant macroalgae carrying rock material usually a thin veneer but sometimes boulders up to 365 kg varies by location but can reach as high as 35 macroalgae carrying attached carbonate material e g crustose coralline algae shells are more common up to 85 of holdfasts on a beach may contain up to 116 g caco 3 each sand and carbonate trapped in haptera along with encrusting carbonate producing organisms have a mass of up to tens of grammes while most dislodged macroalgae land close to where they came from others may be stranded many thousands of km from their place of origin sediment deposited by macroalgal transport depends largely on the characteristics of local source rocks while hard rocks may only provide a few g m 2 y 1 highly fractured basalt has been shown to provide up to 78 g m 2 y 1 rock transport by macroalgae is less than carbonate flux which may reach over 8000 g m 2 y 1 based on the projected abundance of macroalgal rafts some 350 tonnes of sediment both carbonate and clastic may be afloat in the southern ocean at any one time macroalgal transport may explain the presence of exotic rocks and shells found far from their source environment while cold climate rafting of sediment is dominated by glacial processes in warm periods rafting may be limited to biological transport overall the transport of rock and carbonate by macroalgae has the potential in some environments of being a significant source of sediment

seabeam depth changes associated with recent lava flows coaxial segment juan de fuca ridge evidence for multiple eruptions between 1981 1993 after a swarm of earthquakes was detected on the coaxial segment of the juan de fuca ridge in june july 1993 the area was resurveyed with seabeam multibeam sonar to search for depth changes associated with a submarine volcanic eruption quantitative comparison of the 1993 seabeam survey with surveys in 1981 82 and 1991 shows one area of seafloor depth change up to 29 m between 1991 93 exactly where a pristine lava flow was discovered in addition two other depth anomalies up to 37 m and 20 m are identified between 1981 91 evidence that other recent eruptions have occurred along this spreading ridge segment

water column hydrothermal plumes on the juan de fuca ridge hydrographic surveys on the juan de fuca ridge jdfr carried out from 1980 to 1987 show a complex pattern of sup 3 he and mn rich water column plumes produced by venting from several submarine hot spring areas in the vicinity of axial volcano at latitude 46 degree n distinct plumes were detected in 1980 1982 and 1983 with sup 3 he signatures up to delta sup 3 he 64 at approximately 1 500 m depth at distances of approximately 10 km from the seamount summit however the same plumes had no detectable thermal signature a paradox which is attributed to the high sup 3 he heat ratios and low salinities of the fluids venting within the caldera of axial volcano profiles directly over the seamount show hydrothermal sup 3 he in the water column up to 300 m above the caldera floor with the sup 3 he signal increasing with depth to very high and uniform ratios of delta sup 3 he 108 150 below the approximately 1 500 m caldera sill depth another apparent locus of hydrothermal input is helium basin a depression on the northeast flank of axial volcano which had delta sup 3 he 51 when first sampled in 1980 however subsequent hydrocasts into helium basin in 1982 and 1983 yielded lowermore helium enrichments suggesting either a decrease in hydrothermal input or flushing of the basin via a mixing event to the south of axial volcano high delta sup 3 he values of approximately 40 observed over the ridge axis at 45 degree 18 prime n and 45 degree 39 prime n indicate venting on this previously unexplored section of the ridge the water column plumes over the us geological survey vent site at approximately 44 degree 40 prime n on the southern jdfr have very high mn sup 3 he ratios of 4 600 mol cm sup 3 an apparently unique characteristic which can be used to distinguish these plumes from those originating at other jdfr vent fields less

dynamics of intertidal gravel dunes 1 this paper examines the dynamics of intertidal gravel dunes subjected to high energy unsteady and reversing tidal currents in the severn estuary uk the dunes were composed of shale particles with median grain size d50 around 4 mm and had mean heights and wavelengths of 60 cm and 7 m respectively acoustic instruments were deployed above a dune to measure the streamwise profile and flow turbulence radar was used to map a section of the dune field on two occasions measurements during the peak spring ebb tides showed that when the measured bed shear stress exceeded the threshold for entrainment of the coarse surface armor sediments were eroded from the upper surface of the dune the dune prograded in the direction of the ebb flow a distance o 1 m and some eroded sediments were deposited on the stoss slope downstream the dune was then reworked during the following flood tide to reestablish approximately the same pre ebb profile shape and location radar data showed also that the exposed dunes at low water occupied approximately the same location over a period of 4 days during peak spring tides the relationship between the depth mean current and the net volume of dune sediments moved was found to conform to a power law the predicted critical bed shear stress crit for d50 4 mm was approximately 1 n m2 however owing to a coarse armor layer on the surface of the dune a measured crit value o 4 n m2 was required to initiate dune erosion although the van rijn formula was found to overestimate both the height and wavelength of dunes at this site the dunes appear to be equilibrium bed forms controlled by local hydraulics and sediment supply sediment transport predicted by bed load formula were within a factor of two of the observed mass transport of dune sediments for ebb tides

hydrothermal exploration of the fonualei rift and spreading center and the northeast lau spreading center we report evidence for active hydrothermal venting along two back arc spreading centers of the ne lau basin the fonualei rift and spreading center frsc and the northeast lau spreading center nelsc the ridge segments investigated here are of particular interest as the potential source of a mid water hydrothermal plume 1500 2000 m depth which extends more than 2000 km across the sw pacific ocean dispersing away from an apparent origin close to the most northeastern limits of the lau basin our results indicate the presence of at least four new hydrothermal plume sources three along the frsc and one on the nelsc the latter situated within 150 km of the maximum for the previously identified sw pacific regional scale plume however tdfe and tdmn concentrations in the southernmost frsc plume that we have identified only reach values of 19 and 13 nmol l and dissolved 3he anomalies in the same plume are also small both in relation to the sw pacific plume and to local background which shows evidence for extensive 3he enrichment throughout the entire lau basin water column our results reveal no evidence for a single major point hydrothermal source anywhere in the ne lau basin instead we conclude that the regional scale sw pacific hydrothermal plume most probably results from the cumulative hydrothermal output of the entire topographically restricted lau basin discharging via its ne most corner

geochemistry of some gases in hydrothermal fluids from the southern juan de fuca ridge five samples of hydrothermal fluids from two vent areas on the southern juan de fuca ridge were analyzed for dissolved gases concentrations in the end member hydrothermal fluid of h2 270 527 mol kg ch4 82 118 mol kg and co2 3920 4460 mol kg are well above values in ambient seawater and are similar to concentrations reported for other ridge crest hydrothermal systems the carbon isotopic ratios of the ch4 13c 17 8 to 20 8 and co2 13c 3 6 to 4 7 suggest that at least some of the ch4 and co2 in the fluids is basalt derived the range of 13c values for the basalt derived co2 is 6 8 to 9 7 calculated by assuming conservation of recharge co2 during hydrothermal circulation apparent temperatures of equilibration between the ch4 and the basalt derived co2 range from 640 c to 750 c small amounts of ethane c2h6 ch4 0 9 10 3 2 2 10 3 propane and butane detected in the samples may also have formed in the basalt one sample of almost pure 95 5 hydrothermal fluid contained a significant fraction up to 63 and 74 respectively of the recharge ar and n2 this suggests that the fluid has not undergone extensive vapor liquid phase separation

comment on helium heat and the generation of hydrothermal event plumes at mid ocean ridges by j e lupton e t baker and g j massoth

manganese and iron in hydrothermal plumes resulting from the 1996 gorda ridge event abstract we sampled hydrothermal plumes over the n gorda ridge four times between march and august 1996 to document mn and fe discharge resulting from a magmatic intrusion seafloor eruption two separate event plumes ep96a and b and chronic hydrothermal emissions lasting 6 months were characterized shipboard time series measurements of an event plume sample were used to calibrate an fe phase clock useful for estimating sample age up to 6 days after fluid discharge samples collected from ep96a and b had mn heat 2 mol mol 1 ratios similar to historical event plume observations we suggest these signature ratio values are generally characteristic of event plumes and hypothesize that mn and fe may be supplied to event plumes by different processes mn by entrainment of fluids from an extant shallow subseafloor reservoir and fe by short lived high temperature water rock reaction coincident with dike emplacement calculations based on the fe phase clock indicate that the two event plumes were released more than a month apart the largest event plume ep96a 2 3 106 m mn and 13 106 m fe formed 7 march soon after seismic activity began the smaller ep96b 0 49 106 m mn and 3 5 106 m fe was not discharged until 11 april 3 weeks after the cessation of seismic activity detectable by sosus t phase monitoring we hypothesize that the subseafloor disturbance that triggered ep96b also resulted in the episodic flushing of a reservoir of chronic plume like fluids total event plume inventories of mn and fe at n gorda ridge are much smaller than those associated with the 1986 event at n cleft segment of the juan de fuca ridge but comparable to event plume inventories at n cleft segment in 1987 and coaxial segment in 1993 mn heat values for chronic plumes over the eruption site underlying ep96a evolved from moderate 0 25 oonmol j 1 reflecting probable admixture with event plume formation fluids to high 0 7 nmol j 1 typical of chronic plumes to low 0 1 nmol j 1 similar to diffuse vent fluid values marking a complete episode of intrusion eruption induced hydrothermal discharge

temporal and spatial variability of hydrothermal manganese and iron at cleft segment juan de fuca ridge a unique data set for hydrothermal mn and fe was collected at cleft segment on the juan de fuca ridge between 1983 and 1991 the data set includes observations of focused and diffuse venting fluids and neutrally buoyant plumes formed by chronic and episodic venting manganese heat and iron heat ratios for plumes from the north end of the cleft segment were combined with independently determined estimates of plume heat flux to yield annually averaged chronic venting fluxes for mn of 0 36 0 17 mol s 1 and for fe of 0 61 0 34 mol s 1 over 6 years of plume measurements at north cleft segment observed episodic hydrothermal discharge accounted for 15 of the total vented mn and 35 of vented fe the chronic fluxes for mn and fe at a second venting center located at the south end of the cleft segment were estimated to be approximately equal to the fluxes at north cleft segment chronic plumes at north cleft segment are mixtures of focused and diffuse discharge that contribute heat mn and fe in variable proportions similar examination of south cleft segment data strongly suggests the presence of an as yet unobserved venting source relatively depleted in mn and fe but contributing substantially to the overall heat temporal and spatial variations in the concentrations of mn and fe and in mn heat and fe heat ratios for focused seafloor vents were difficult to resolve within complex chronic plumes manganese heat and iron heat ratios of megaplumes suggest they may have derived from reservoirs of diffuse fluids while smaller event plumes may have formed by different processes and have properties similar to chronic plumes the accurate assessment of segment scale hydrothermal fluxes of mn and fe requires coordinated measurements of representative seafloor sources and the neutrally buoyant plume that integrates all seafloor discharge

sulfur speciation oxidation state and sulfur concentration in backarc magmas we have determined the fe3 fe2 ratio s content and the so2 4s2 ratio by sk x ray peak shift for a suite of twelve mid ocean ridge and backarc basin glasses these samples range in oxidation state from two orders of magnitude below to 1 5 orders of magnitude above the ni nio oxygen buffer sulfur speciation in the glasses is strongly dependent on magmatic oxidation state we propose that the solubility mechanism for s in basaltic melts changes over the o2 range which characterizes convergent margin magmas in low o2 sulfide saturated basaltic melts such as most morbs s is present largely as s2 and its concentration is controlled by sulfide melt silicate melt equilibrium and varies positively with o2 at higher o2 s where sulfide melt becomes unstable s concentration is controlled by melt vapor equilibrium and decreases with increasing o2 for the range of o2 s in the back arc basin basalts in this study nno 1 to nno 1 5 s is dissolved in the silicate in both oxidized and reduced forms for the lower o2 end of this range s2 dominates and equilibria involving reduced s species control s melt concentration at higher o2 s the proportion of oxidized s increases and melt vapor equilibria involving these species become more important because s partitions strongly into the vapor phase over the o2 range investigated magmatic degassing may be effective at stripping s from the melt the low s concentrations in island arc and backarc lavas do not necessarily reflect a low s source but rather result from higher o2 in the convergent margin environment

helium radon and manganese at the tag and snakepit hydrothermal vent fields 26 and 23 n mid atlantic ridge abstract 3 he 4 he and 222 rn have been measured in samples of vent fluids from the tag and snakepit hydrothermal vent fields and in samples from the neutrally buoyant plume at tag the hydrothermal end member 3 he 4 he at tag normalised to the ratio in air is r r a 7 5 0 9 222 rn 187 dpm kg 1 and 226 ra 13 5 0 4 dpm kg 1 at snakepit r r a 7 86 1 85 222 rn 82 dpm kg 1 and 226 ra 10 3 0 2 dpm kg 1 helium isotope tracer systematics have been described which allow hydrothermal end member properties to be estimated from plume data or non conservative behaviour in the buoyant plume to be identified comparison of plume mn concentrations and he isotope data shows that mn behaves conservatively in the plume system over the time scale considered in contrast 222 rn is highly non conservative and there is a large 222 rn excess in the neutrally buoyant plume the hydrothermal end member accounts for less than 2 5 of the excess 222 rn enrichment at the top of the buoyant plume attributed to entrainment of 222 rn enriched fluids during buoyant plume rise 222 rn 3 he ages of the tag plume give values of around 1 2 days close to the vent field and up to 10 days in adjacent waters 222 rn mn ages are similar except for a region of anomalous water at the level of neutral buoyancy which is enriched in 222 rn and si but not 3 he and mn and which may be derived from the walls of the axial valley at the depth of the neutrally buoyant plume

volcanic and hydrothermal processes in submarine calderas the kulo lasi example sw pacific abstract the study area is located at the transition between the northern end of the tonga trench and the north fiji fracture zone where tectonic movements are reputed to be the fastest in the world to the southeast of futuna island a broad area of volcanism occurs within a region characterized by a change in the tectonic fabric between a ne sw oriented volcanic graben and the n s oriented alofi ridge in 2010 the active volcano kulo lasi which represents the most recent volcanic episode in the futuna area was discovered in the center of this extensive volcanic zone kulo lasi is a 20 km diameter shield volcano that rises 400 m above the seafloor it is composed of basaltic to trachy andesitic lava with no obvious geochemical affinity with the tonga subduction that occurs 500 km to the east the central caldera is 5 km in diameter and 300 m deep and is located at a water depth of 1500 m diving operations with the submersible nautile and high resolution auv mapping have revealed the presence of numerous active and inactive hydrothermal fields on the floor and the walls of the caldera four tectono volcanic stages can be distinguished at kulo lasi caldera in stage 1 the shield volcano is built annular reverse faults develop at the summit and control circulation of water rock dominated hydrothermal fluids and high temperature alteration of rocks along the nascent normal faults mixing of hydrothermal fluids with seawater is favored along normal superficial faults leading to the formation of low temperature fe mn mineralization at the summit of the volcano during stage 2 the caldera collapse gradually revealing outcrops of the altered and mineralized zones formed during stage 1 as the magma chamber cools and collapses less heat is available as a result medium to low temperature

the effect of arc proximity on hydrothermal activity along spreading centers new evidence from the mariana back arc 12 7 n 18 3 n back arc spreading centers bascs form a distinct class of ocean spreading ridges distinguished by steep along axis gradients in spreading rate and by additional magma supplied through subduction these characteristics can affect the population and distribution of hydrothermal activity on bascs compared to mid ocean ridges mors to investigate this hypothesis we comprehensively explored 600 km of the southern half of the mariana basc we used water column mapping and seafloor imaging to identify 19 active vent sites an increase of 13 over the current listing in the interridge database irdb on the bathymetric highs of 7 of the 11 segments we identified both high and low i e characterized by a weak or negligible particle plume temperature discharge occurring on segment types spanning dominantly magmatic to dominantly tectonic active sites are concentrated on the two southernmost segments where distance to the adjacent arc is shortest 48 mm yr and tectonic extension is pervasive re examination of hydrothermal data from other bascs supports the generalization that hydrothermal site density increases on segments 90 km from an adjacent arc although exploration quality varies greatly among bascs present data suggest that for a given spreading rate the mean spatial density of hydrothermal activity varies little between mors and bascs the present global database however may be misleading on both bascs and mors the spatial density of hydrothermal sites mapped by high quality water column surveys is 2 7 times greater than predicted by the existing irdb trend of site density versus spreading rate

noble gases in vent water from the juan de fuca ridge abstract hydrothermal vent fluids collected with the dsrv alvin from the southern limb of the juan de fuca ridge are chemically unique having chloride concentrations 2 times ambient seawater the same fluids contain noble gases in relative abundances like 2 c air saturated seawater the expected recharge composition however the absolute noble gas abundances are depleted by 30 relative to seawater the combination of very high chloride and moderately depleted noble gases appears to require the formation of a cl rich gas free brine by phase separation this brine is mixed with recharge seawater at temperatures in excess of 340 c and therefore deep in the hydrothermal system

hydroacoustic detection of volcanic activity on the gorda ridge february march 1996 abstract beginning at 0700 gmt on 28 february 1996 intense seismicity was detected in the northeast pacific ocean using the t phase monitoring system developed by noaa pmel to access the u s navy s sound surveillance system sosus in the north pacific the event was preliminarily located on the northernmost segment of the gorda ridge near 42 67 n and 126 8 w in the vicinity of the ridge segment high narrowgate the nature of the seismicity was similar to that observed in june 1993 at the coaxial segment of the juan de fuca ridge which was later documented to be a lateral magma injection with subsequent eruption due to several gaps in the data the detection information was not as comprehensive as during the coaxial event but an initial migration of epicenters from the narrowgate area down rift is inferred based on arrival bearings from a single array there is evidence for an additional diking event on the second and third day of activity there is also indication of a concentration of epicenters located near 42 6 n as occurred during the coaxial episode at what was later determined to be an eruption site examination of t wave rise times generally supports this interpretation based on the nature and duration of the activity a response effort was initiated which later confirmed hot water plumes and fresh lava flows at the site based on both hydroacoustic information and field observations it is proposed that the episode began with a lateral dike injection possibly with eruptive activity in the summit region followed by multiple magma pulses and eventual focusing of the seismic activity and extrusion near 42 6 n

the movement of gravel by the alga fucus vesiculosus l on an arctic intertidal flat abstract measurements of the weights of gravel clasts with attached fucoid algae fucus vesiculosus from an arctic intertidal flat at pangnirtung in northern canada indicate that stones are floated when the algae is at least three times heavier than the clast when the algae is not large enough to float the clast to which it is attached the form drag and lift which it provides allow moderate tidal currents to move very much larger stones than would be possible otherwise these mechanisms offer a means of transporting coarse sediments to deep water where they may augment or be confused with ice rafted materials

the formation of deep sea limu o pele deep sea sheet hyaloclastite consists mostly of sand sized blocky and splinter shaped shards but also contains subordinate mm to cm sized thin curved and wrinkled plates and sheets of sideromelane this latter type of shard termed limu o pele has been observed forming subaerially on kilauea by entrapment of water in flowing lava followed by expansion of steam to form large bubbles which burst into thin fragments deep marine limu has been inferred from comparative morphological studies and assessment of physical bubble forming conditions to form in a similar way but with the increased ambient pressure and the higher viscosity of water reducing bubble expansion differing mechanisms of heat transfer and rates of magma chilling also modify the limu forming process in the deep sea this paper evaluates a variety of deep sea limu forming processes and develops a new and quantitatively supported model based on observed limu forming processes and criteria derived from dive samples and observations at seamount six cocos plate it is inferred that water and or water saturated sediment was trapped in extremely thin fluid and rapidly advancing lava flows by various processes bubble formation might also occur during small scale magma fountaining driven by magmatic volatile exsolution and extreme vent constriction or during collapse of pillows and rapid drainage of the magma but we found no deposits clearly resulting from these processes

lava seawater interactions at shallow water submarine lava flows hydrothermal plumes associated with nearshore lava flows from kilauea volcano hawaii were studied on five occasions during 1989 1990 to address the current lack of data on direct lava seawater interactions the following enrichments were found in the sea surface hydrothermal plumes above the active underwater lava flows h2 15 000x ambient seawater concentrations mn 250x and si 20x water temperatures reached 46 c lower concentrations and temperatures were observed in the plumes with increasing distance from shore with h2 si and mn concentrations linearly related to seawater temperature unlike deep sea spreading center hydrothermal plumes no ch4 enrichment was observed the elevated h2 is likely to be from water rock reactions rather than from the release of magmatic gas the plume mass heat ratios presented here suggest that submarine flood basalts although aerially large should be relatively small immediate contributors to oceanic geochemical cycles compared to hydrothermal circulation through the crust

enriched h2 ch4 and 3he concentrations in hydrothermal plumes associated with the 1996 gorda ridge eruptive event abstract the rapid field response in march and april 1996 to diking eruptive events on the northern gorda ridge allowed the first detailed co measurement of 3 he h 2 and ch 4 in plumes associated with a submarine eruption and examination of the chemical transformation of these volatile species during aging of the plumes the diking eruptive event generated extensive event plumes with a low 3 he heat ratio of 0 4 10 12 cm 3 stp gm 1 c 1 while chronic plumes had 3 he heat ratios of 2 1 6 2 10 12 cm 3 stp gm 1 c 1 hydrogen concentrations in the initial event plume ep96a reached a maximum value of 47 nm 235 times that of ocean background concentration 0 2 nm in contrast roughly one month later h 2 concentrations although still elevated had drastically dropped to 40 times background values methane concentrations in both event and chronic plume fluids were also elevated and similar to stable hydrothermal systems in unsedimented ridge environments reaching a maximum value of 7 nm the intense rapid interaction at very high temperatures of hydrothermal fluids with the host rock during dike intrusion or with seawater during extrusion of the basalt likely generated high localized concentrations of h 2 the data indicate however that maximum rates of h 2 production during event plume formation could have lasted a few days at most

solidification and morphology of submarine lavas a dependence on extrusion rate the results of recent laboratory experiments with wax extruded beneath relatively cold water may be extrapolated to predict the surface morphology of submarine lavas as a function of the extrusion rate and melt viscosity the experiments with solidifying wax indicated that the surface morphology was controlled by a single parameter the ratio of the time taken for the surface to solidify and a time scale for lateral flow for submarine basalts a solution of the cooling problem which is dominated by conduction in the lava but convective heat transfer in the water and estimates of lava viscosities place this parameter within the empirically determined pillowing regime over a wide range of extrusion rates this result is consistent with the observation that pillow basalts are the most common products of submarine eruptions smoother surfaces corresponding to the various types of submarine sheet flows are predicted for sufficiently rapid extrusions of basaltic magma still higher eruption rates in regions of low topographic relief may produce submarine lava lakes minimum emplacement times can be calculated for submarine volcanic constructs of a single lava flow type

the water column chemical signature after the 1998 eruption of axial volcano the eruption of axial volcano in january 1998 produced extensive plumes in the overlying water column with large anomalies in fe mn ph light attenuation and temperature a strong correlation between total iron and light attenuation dc suggests a low abundance of particulate sulfur ps in the plumes because total carbon dioxide co2 samples were not collected high precision ph measurements were used to estimate maximum co2 anomalies co2 which when compared to the other physical and chemical data suggest that the fluids being vented 3 weeks after eruption were formed by the interaction between an intruded dike and a mixture of interstitial seawater and mature hydrothermal fluids

lava flows from a mid 1980s submarine eruption on the cleft segment juan de fuca ridge a series of lava flows with a total volume of 0 05 km3 were erupted in the mid 1980s along 17 km of the northern cleft segment of the juan de fuca ridge observations from camera tows and submersible dives show that the new flows are all similar in appearance and consist entirely of pillow lava with a mixture of smooth and striated surface textures suggesting a relatively uniform eruption rate approaching 1 m3 s at point source vents the flows vary in size from small patches to large steep sided ridges and were probably erupted from a dike intruded along the ridge axis because they are aligned along a linear fissure graben system observations at north cleft show that the physical appearance of new flows changes more rapidly than previously realized and that earlier qualitative dating of young lavas based on sediment cover and glassy surface texture were probably overestimates by an order of magnitude sediment accumulation on the lavas is quite variable and locally surprisingly substantial mainly due to hydrothermal deposits that formed while the lava flows were cooling biological vent communities photographed on the new flows in 1989 show that vent animals can colonize new vent sites rapidly but that warm water was still venting only in a few places nonvent animals are much slower to colonize the new flows and rates of colonization observed at north cleft may be useful for making improved age estimates of young 10 years lava flows elsewhere the north cleft eruption represents about 2 of the estimated average annual volcanic output along the global mid ocean ridge implying that many other submarine eruptions are occurring undetected

hydrothermal activity in the northwest lau backarc basin evidence from water column measurements 1 the northwest lau backarc basin consisting of the northwest lau spreading center nwlsc and the rochambeau rifts rr is unique in having elevated 3he 4he ratios up to 28 ra in the erupted lavas clearly indicating a hot spot or ocean island basalt oib type signature this oib type helium signature does not appear in any other part of the lau basin water column plume surveys conducted in 2008 and 2010 identified several sites of active hydrothermal discharge along the nwlsc rr and showed that the incidence of hydrothermal activity is high consistent with the high spreading rate of 100 mm year hydrocasts into the central caldera and southern caldera of the nwlsc detected elevated3he 4he 3he 55 and 100 respectively trace metals tmn tfe and suspended particles indicating localized hydrothermal venting at these two sites hydrocasts along the northern rift zone of the nwlsc also had excess 3he tmn and suspended particles suggesting additional sites of hydrothermal activity the rr are dominated by lobster caldera a large volcano with four radiating rift zones hydrocasts into lobster caldera in 2008 detected high 3he up to 239 and suspended particle and tmn signals indicating active venting within the caldera a repeat survey of lobster in 2010 confirmed the site was still active two years later plumes at lobster caldera and central caldera have end member3he 4he ratios of 19 ra and 11 ra respectively confirming that hot spot type helium is also present in the hydrothermal fluids

chemical and physical diversity of hydrothermal plumes along the east pacific rise 8 45 n to 11 50 n we conducted a survey of water column hydrothermal plumes along a 350 km long section of the east pacific rise epr axis stretching from 8 45 n to 11 50 n including the region at 9 45 54 n where 1991 alvin dives found evidence for recent seafloor volcanic eruptions our survey included measurements of temperature anomaly light attenuation suspended particles methane hydrogen iron manganese helium and aluminum we detected strong light attenuation plumes in two main regions south of the clipperton transform fault ctf from 9 27 n to 9 57 n and north of the ctf from 11 05 n to 11 35 n however the plumes at these two regions had very different physical and chemical characteristics south of the ctf where the ridge is thought to be magmatically robust the plumes had very high ratios of he heat methane mn and s fe plumes north of the ctf had much lower ratios of he heat methane mn and s fe these striking differences in volatile heat and volatile metal ratios suggest that the plumes in the vicinity of 9 50 n were derived from a young evolving hydrothermal system while the activity at 11 10 n is from an older stable system this survey demonstrated that water column plume measurements can be used not only to accurately locate regions of hydrothermal activity but also to determine the first order chemical and physical characteristics of the hydrothermal fluids exiting from the seafloor

the remarkable longevity of submarine plumes implications for the hydrothermal input of iron to the deep ocean the longevity of submarine plumes generated at sea floor hydrothermal systems constrains the hydrothermal input of chemical species into the deep ocean decades of observations of episodic event plumes suggest that a key process governing the dynamics of an hydrothermal cloud spreading out laterally from a buoyant rising plume is the production of internal layering here we use analog experiments on turbulent hot particle laden plumes and clouds to show that this layering occurs where particle diffusive convection driven by the differential diffusion of heat and small mineral precipitates gives rise to a large scale double diffusive instability where hydrothermal clouds are enriched in fine minerals this particle diffusive convection can extend the longevity of an event plume to 2 yr after its emplacement the very long residence time imposed by diffusive convective effects enables complete dissolution of fine sulfide and sulfate minerals we develop a new theoretical model that includes both sedimentation and dissolution processes to quantify the potential amount of iron produced by the dissolution of iron sulfide minerals settling through the cloud by diffusive convection a key prediction is that the concentration of dissolved iron in hydrothermal clouds can reach up to 19 3 nm19 3 nm which represents about 5 of the global hydrothermal discharge if these results are representative of all hydrothermal vent fields hydrothermal systems could provide 75 of the global budget of dissolved iron in the deep ocean regionally this flux is expected to scale in magnitude with mid ocean ridge heat flow consistent with observations and global ocean models

geochronology and geochemistry of lavas from the 1996 north gorda ridge eruption abstract radiometric dating of three north gorda ridge lavas by the 210 po 210 pb method confirms that an eruption occurred during a period of increased seismic activity along the ridge during late february early march 1996 these lavas were collected following detection of enhanced t phase seismicity and subsequent ocean bottom photographs documented the existence of a large pillow mound of fresh appearing lavas 210 po 210 pb dating of these lavas indicates that an eruption coinciding with this seismicity did occur within analytical error and that followup efforts to sample the recent lava flows were successful compositions of the three confirmed young lavas and eleven other samples of this contiguous new flow sequence are distinct from older lavas from this area but are variable at a level outside analytical uncertainty these intraflow variations can not easily be related to a single common parent magma compositional variability within the new flow is compared to that of other recently documented individual flow sequences and this comparison reveals a strong positive correlation of compositional variance with flow volumes spanning a range of 2 orders of magnitude the geochemical heterogeneity in the north gorda new flow probably reflects incomplete mixing of magmas generated from a heterogeneous mantle source or from slightly different melting conditions of a single source the compositional variability range in sample ages up to 6 weeks and range in active seismicity 4 weeks imply that this relatively large flow was erupted over an interval of several weeks

extensive young silicic volcanism produces large deep submarine lava flows in the ne lau basin new field observations reveal that extensive up to 402 km2 aphyric glassy dacite lavas were erupted at multiple sites in the recent past in the ne lau basin located about 200 km southwest of samoa this discovery of volumetrically significant and widespread submarine dacite lava flows extends the domain for siliceous effusive volcanism into the deep seafloor although several lava flow fields were discovered on the flank of a large silicic seamount niuatahi two of the largest lava fields and several smaller ones northern lava flow fields were found well north of the seamount the most distal portion of the northernmost of these fields is 60 km north of the center of niuatahi caldera we estimate that lava flow lengths from probable eruptive vents to the distal ends of flows range from a few km to more than 10 km camera tows on the shallower near vent areas show complex lava morphology that includes anastomosing tube like pillow flows and ropey surfaces endogenous domes and or ridges some with crease like extrusion ridges and inflated lobes with extrusion structures a 2 1 5 km 30 m deep depression could be an eruption center for one of the lava flow fields the lau lava flow fields appear to have erupted at presumptive high effusion rates and possibly reduced viscosity induced by presumptive high magmatic water content and or a high eruption temperature consistent with both erupted composition 66 sio2 and glassy low crystallinity groundmass textures the large areal extent 236 km2 and relatively small range of compositional variation 0 60 for wt si02 within the northern lava flow fields imply the existence of large eruptible batches of differentiated melt in the upper mantle or lower crust of the ne lau basin at this site the volcanism could be controlled by deep crustal fractures caused by the long term extension in this rear arc region submarine dacite flows exhibiting similar morphology have been described in ancient sequences from the archaean through the miocene and in small batches on present day seafloor spreading centers this study shows that extensive siliceous lavas can erupt on the modern seafloor under the right conditions

rapid dike emplacement leads to eruptions and hydrothermal plume release during seafloor spreading events the creation of ocean crust by rapid injection of magma at mid ocean ridges can lead to eruptions of lava onto the seafloor and release of event plumes which are huge volumes of anomalously warm water enriched in reduced chemicals that rise up to 1 km above the seafloor here we use seismic data to show that seafloor eruptions and the release of hydrothermal event plumes correspond to diking episodes with high injection velocities and rapid onset of magma emplacement within the rift zone these attributes result from high excess magma pressure at the dike source likely due to a new influx of melt from the mantle these dynamic magmatic conditions can be detected remotely and may predict the likelihood of event plume release during future seafloor spreading events

first hydrothermal discoveries on the australian antarctic ridge discharge sites plume chemistry and vent organisms the australian antarctic ridge aar is one of the largest unexplored regions of the global mid ocean ridge system here we report a multiyear effort to locate and characterize hydrothermal activity on two first order segments of the aar kr1 and kr2 to locate vent sites on each segment we used profiles collected by miniature autonomous plume recorders on rock corers during r v araon cruises in march and december of 2011 optical and oxidation reduction potential anomalies indicate multiple active sites on both segments seven profiles on kr2 found 3 sites each separated by 25 km forty profiles on kr1 identified 17 sites some within a few kilometer of each other the spatial density of hydrothermal activity along kr1 and kr2 plume incidence of 0 34 is consistent with the global trend for a spreading rate of 70 mm yr the densest area of hydrothermal activity named mujin occurred along the 20 km long inflated section near the segment center of kr1 continuous plume surveys conducted in january february of 2013 on r v araon found ch4 3he 1 15 106 and ch4 mn 0 01 0 5 ratios in the plume samples consistent with a basaltic hosted system and typical of ridges with intermediate spreading rates additionally some of the plume samples exhibited slightly higher ratios of h2 3he and fe mn than others suggesting that those plumes are supported by a younger hydrothermal system that may have experienced a recent eruption the mujin field was populated by kiwa crabs and seven armed paulasterias starfish previously recorded on the east scotia ridge raising the possibility of circum antarctic biogeographic connections of vent fauna

on the calculation of total heat salt and tracer fluxes from ocean hydrothermal events abstract total heat salt and other tracer masses released during a hydrothermal event are shown to be proportional to but not necessarily equal to volume integrals of resulting water column anomalies proportionality coefficients depend on anomaly definition on background hydrographic and tracer profiles on expansion coefficients of the equation of state at an appropriate pressure and on tracer to heat anomaly ratios at the venting source for gorda ridge event plumes which are described in other papers of this issue volume integrals of conventionally defined heat anomalies underestimate actual released heat by a factor of 2 4 if the discharge is not anomalously saline under certain combinations of hydrographic and source anomaly conditions not unlike those found on the east pacific rise at 10 n the apparent total heat released during an event can be deceivingly zero this analysis also establishes a linear relationship between the ratio of tracer anomaly to heat anomaly at any point in the plume to the same ratio at the source one consequence is that the ratio of anomalous 3 he to heat in gorda ridge event plumes is approximately 2 4 larger in the water column than it is at the source results are independent of the entrainment process involved in event plume formation and they are shown to hold true even for background hydrographic profiles that do not vary linearly with depth

the role of attached kelp seaweed in lowering threshold of coarse gravel entrainment in tidal flows lag deposits of cobble sized clasts found within the troughs of fine gravel dunes on a rock bed intertidal zone are explained by the process of enhanced fluid transport due to attached seaweed field determinations of the competence of the tidal currents demonstrated that the currents were not able to transport weed free clasts on the tidal flat controlled flume experiments were used to demonstrate that the presence of the fucoid alga fucus vesiculosus l attached to clasts reduces the critical velocities and shear stresses to levels that allow the tidal currents to entrain the clasts and move them over the backs of dunes to the lee sides subsequent burial of the weedy clasts in the dune troughs by dune progression kills the weed leaving the latterly exposed clasts as an accumulating weed free lag the weight of attached weed typically equals the submerged weight of the clast and the critical velocity and critical shear stress for initial motion are both roughly halved by the presence of the weed the drag induced by weed is four times that experienced by weed free clasts and mobile clasts exhibit near equal mobility

gases and helium isotopes in high temperature solutions sampled before and after odp leg 158 drilling at tag hydrothermal field 26 n mar hydrothermal fluids were sampled for dissolved gases at tag 26 n mar during two alvin dive series in april may 1993 and march 1995 respectively 17 months before and 4 months after ocean drilling program leg 158 total gas volumes extracted from the 1993 and 1995 samples are of the same order of magnitude even if some increase in h2s c02 and ch4 was noticeable in the 1995 samples no significant difference was observed in helium concentration and helium isotopic ratio 3 he 4 he 8 2 0 1 ra the ch4 3 he ratio found in black smokers sampled in 1993 is around 9 x 10 6 close to ratio found in mar basalts in 1995 this ratio is around 4 times higher due to the ch4 increase the 13 c in c02 measured on two black smokers sampled in 1993 is uniform at 8 4 to 8 8 versus pdb while a 13 c value of 13 is measured at a new site sampled in 1995 located 40 meters east of the main black smokers complex bsc ch4 3 he and 13 c values both point to a mainly magmatic abiogenic origin of the carbon species in the system overall comparison of the 1993 and 1995 data suggests that the hydrothermal circulation is continuing through the basaltic layer carrying helium c02 and ch4 of magmatic origin even if some thermogenic ch4 contribution seems likely in the 1995 samples collected in the new active area we can speculate that the 1994 drilling may have modified the previously steady state hydrothermal circulation in the tag mound and temporarily enhanced the hydrothermal circulation by opening new pathways favouring transient input of ch4 rich fluids before coming back to a new steady state

seaweed assisted benthic gravel transport by tidal currents abstract traction transport of pebbles and cobbles occurs subtidally at current velocities below 0 5 m s 1 when seaweed attached to clasts provides additional lift and drag to the clast in the juan de fuca strait british columbia canada the seaweed cymanthere triplicata commonly attaches to pebbles and provides sufficient additional surface area for tidal currents to drag the clast along the seafloor using in situ measurements of current velocities at 13 m water depth the threshold for initiation of motion of a 30 mm pebble with attached seaweed is 0 3 m s 1 this is approximately one order of magnitude less than the activation velocity for a 30 mm pebble without attached seaweed in addition to kelp rafted floated gravel seaweed assisted benthic gravel transport is possible in marine settings where unidirectional currents e g tidal currents storm induced bottom currents are sufficient to transport pebbles alongshore and into and across the offshore below fairweather wave base if preserved in the rock record deposits of algal enhanced gravel deposited via unidirectional subtidal currents will likely appear as isolated gravel clasts encased in sandstone reflecting the similar current velocities required to transport these two clast groups

anomalous helium and heat signatures associated with the 1998 axial volcano event juan de fuca ridge water column plumes sampled before and after the 1998 eruptive event on axial volcano juan de fuca ridge showed a complex pattern in he and heat very different from that observed during previous magma injection events plumes detected in 1997 before the event had relatively high he heat consistent with the volatile rich character of the known hydrothermal vents within the summit caldera a response cruise in february 1998 immediately after the event failed to detect any event plumes but instead found an intense steady state type plume with high temperature signals moderate he enrichments and low he heat ratios this plume was present over the caldera and extended a considerable distance off axis in contrast all of the plumes detected during follow up cruises in july september 1998 had moderate temperature signals high he concentrations and elevated he heat ratios comparison with previous events suggests that the low he heat plume sampled in february was either the remnant of an event plume which had already migrated off axis or an event plume like discharge which was smeared out by strong currents it is likely that the high he heat plumes observed in july september were derived from the injected dike or from the new lava flow

1998 eruption of axial volcano multibeam anomalies and sea floor observations lava flows erupted during the january february 1998 seismic swarm at axial volcano on the juan de fuca ridge have been identified by differencing of pre and post event multibeam bathymetric surveys and by seafloor observations a sheet flow more than 3 km in length and 500 800 m wide erupted from the uppermost south rift is the site of a robust hydrothermal system 1998 lavas occur over about 9 km of the upper south rift zone or about 20 of the along axis length of the seismicity event 50 km the estimated volume of lava erupted is 18 76 106 m for the extrusion and 100 106 m for the intrusion the total volume is consistent with the volume from modeling of seafloor strain measurements recorded during the event

venting of acid sulfate fluids in a high sulfidation setting at nw rota 1 submarine volcano on the mariana arc a comprehensive survey of hydrothermal plumes and their geochemistry revealed 16 hydrothermally active volcanoes along 1 200 km of the mariana arc from 13 5 n to 22 5 n of these 16 one volcano nw rota 1 is discharging sulfuric acid rich fluids producing some of the largest chemical anomalies ever observed in non buoyant hydrothermal plumes two types of venting were observed one of focused flow rich in al s si co2 fe mn and 3he and a second of diffuse flow rich in fe mn co2 and 3he but without al s and si the plume waters from the acid rich focused flow showed decreases in ph up to 0 73 ph units compared to ambient seawater and an estimated 80 percent of this change was brought about by volcanic and or hydrothermal so2 and possibly hcl with the remaining 20 percent by co2 conservative estimates suggest that the ph of the venting fluids was 1 0 the chemistry of this plume was also greatly different from that observed in any other hydrothermal setting an acid sulfate or high sulfidation fluid source was indicated from plume water samples rich in sulfuric acid native s al sulfate phosphate minerals and weathered silicate and amorphous silica phases the alunite group minerals and acidic fluids indicate that the most important hydrothermal reactions taking place within the hydrothermal system were resulting in advanced argillic alteration of the host volcanic rocks gas chemistry of the fluids revealed an he isotope signature with r ra 8 35 and 3he co2 3 25 109 these values are similar to those found along the global midocean ridge mor suggesting that the source of the 3he and co2 found here is predominantly from the upper mantle consistent with the position of nw rota 1 closest to the back arc in a chain of cross arc volcanoes the chemical signatures of the plumes arising from the diffuse flow remain greatly enriched in fe mn 3he and co2 but compared to the focused flow are low in mineral acidity h al and s the source of this diffuse flow is likely a fluid similar to that producing the acid rich plume but with a lower water to rock ratio and a longer reaction time resulting in the consumption of the mineral acidity this results in an increase in ph and the deposition of the alunite phosphate sulfate and other minerals close to the seafloor by comparison it is likely that the low ph focused flow ejects the vast majority of its mineral load into the water column which is not conducive to the formation of a mineral deposit at the sea floor and potentially would produce a barren prospect

seasonality of modern pollen and sediment deposition in an estuarine context the severn estuary levels southwest england recent sedimentological and palynological research on subfossil holocene banded sediments from the severn estuary levels suggested seasonality of deposition registered by variations in mineral grain size and pollen assemblages between different parts of the bands here we provide data that strengthen this interpretation from sampling of modern sediments and pollen deposition on an active mudflat and saltmarsh on the margin of the severn estuary and comparison with a vegetation survey and contemporary records of climate river and tidal regimes the results of grain size analysis indicate deposition of comparatively coarse grained silts during the relatively cool and windy conditions of winter and comparatively fine grained sediments during relatively warm and calm summer months pollen analysis demonstrates the significance of long term storage of pollen grains and fern spores in the estuarine waterbody superimposed on which seasonal variations in pollen inputs from local and regional vegetation remain detectable copyright c 2007 john wiley sons ltd

current tectonics of the tonga new hebrides region abstract this paper proposes a present day tectonic map of the tonga new hebrides region the result of combining data from bathymetry satellite derived gravity seismicity magnetic measurements and gps observations focal mechanisms and a large amount of bathymetric data collected during the last decade in the lau and north fiji back arc basins indicate a deformation distributed over numerous spreading ridges rather than diffuse and shear dominated as was previously thought the region is also characterised by a large variation of both consumption rate along the arcs and opening rate along the back arc basins spreading centres geometry and rate of spreading in the lau and north fiji back arc basins and segmentation of the tonga and new hebrides arcs are closely related to and significantly influenced by the subduction of aseismic ridges namely the louisville d entrecasteaux and loyalty ridges where subducting aseismic ridges enter the trenches we observe slower convergence arc transverse strike slip faulting thrusting at the rear of arcs and slow or absent back arc spreading rapid subduction correlates with fast back arc opening sometimes parallel spreading ridges in the back arc domain are required to accommodate the fast motion of convergence

bulk properties of hot smoker plumes the hot buoyant fluid from a submarine vent rises as a plume entraining fluid from the stratified ocean until a height is reached where the driving buoyant force vanishes and shortly thereafter the diluted plume fluid intrudes into the ocean interior at its equilibrium depth while a large positive heat flux issues from the vent the potential temperature anomaly of the horizontally spreading fluid at the equilibrium height depends on the oceanic stratification of both salinity and potential temperature and on the salinity of the source fluid at the vent for example in the atlantic ocean the oceanic stratification causes the spreading plume fluid to be cooler and fresher than the surrounding seawater of the same density while in the pacific the spreading fluid is relatively warm here the well known solution for a turbulent plume rising in a linearly stratified environment is used to obtain simple formulae for 1 the potential temperature anomaly of the horizontally spreading fluid 2 the heat flux associated with this injection of diluted vent water at this density level and 3 the nondimensional solution for the evolution of tracer properties e g salinity and potential temperature in the rising plume

diversity of rock varnish bacterial communities from black canyon new mexico 1 scientists vigorously debate the degree to which rock varnish is formed through the actions of microorganisms to investigate this enigma we utilized a three pronged approach that combined 1 culture independent molecular methods to characterize bacterial communities associated with varnish that coats the rhyolitic volcanic rocks of black canyon new mexico and rocks with no visible varnish 2 culturing of varnish in media supplemented with reduced forms of manganese and or iron and no or low amounts of carbon to isolate bacteria capable of precipitating iron and or manganese oxides and 3 scanning electron microscopy sem of varnish and nearby rock that lacks macroscopically visible varnish our culture independent studies revealed significant differences between varnish and nonvarnish communities chloroflexi and ktedobacteria dominated one varnish site while the other varnish site was dominated by cyanobacteria the nonvarnish sites were dominated by actinobacteria and to a lesser extent cyanobacteria and were the only samples to contain deinococcus thermus sequences approximately 65 of varnish cultures produced visible manganese precipitates most culture isolates were not closely related to known manganese oxidizers with the exception of bacillus spp sem revealed microbial morphologies and two types of varnish morphologies 1 relatively smooth layers and 2 patches of botryoidal pinnacles which were often associated with increased manganese concentrations bare rock showed evidence of incipient varnish these results have important implications for the detection of life on extraterrestrial planets such as mars where putative varnish coatings have been observed and represent some of the first culture independent characterizations of varnish communities

evidence for volcanic eruption on the southern juan de fuca ridge between 1981 and 1987 the formation of new ocean crust at mid ocean ridges is known to be a discontinuous process in both space and time but little is known about the frequency and duration of eruptions along an active ridge segment here we present evidence from sea beam surveys and underwater photography for the eruption of lavas along a segment of the juan de fuca ridge between 1981 and 1987 although previous studies have inferred volcanic activity on ridges in areas where recent seismicity or young lava flows have been observed1 4 none has yet had direct evidence to date such a recent submarine eruption the temporal coincidence between this eruptive episode and the megaplumes huge sudden emissions of hot mineral laden water observed over this part of the ridge5 6 in 1986 and 1987 supports previous suggestions5 10 that megaplumes are caused by sea floor spreading events

understanding a submarine eruption through time series hydrothermal plume sampling of dissolved and particulate constituents west mata 2008 2012 four cruises between 2008 and 2012 monitored the continuing eruption of west mata volcano in the ne lau basin as it produced plumes of chemically altered water above its summit although large enrichments in 3he co2 fe and mn were observed in the plumes the most notable enrichment was that of h2 which reached concentrations as high as 14 843 nm strongly enriched h2 concentrations in the water column result from reactions between seawater or magmatic water and extremely hot rocks in 2008 the observation of elevated h2 concentrations in the water column above west mata pointed to vigorous ongoing eruptions at the volcano s summit the eruption was confirmed by visual observations made by the rov jason 2 in 2009 and demonstrated that h2 measurements are a vital instrument to detect ongoing volcanic eruptions at the seafloor elevated h2 in 2010 showed that the eruption was ongoing although at a reduced level given a maximum h2 concentration of 4410 nm in 2012 h2 levels in the water column declined significantly to a maximum of only 7 nm consistent with visual observations from the quest 4000 rov that found no evidence of an ongoing volcanic eruption methane behaved independently of other measured gases and its concentrations in the hydrothermal plume were very low we attribute its minimal enrichments to a mixture of mantle carbon reduced to ch4 and biological ch4 from diffuse flow sites this study demonstrates that ongoing submarine volcanic eruptions are characterized by high dissolved h2 concentrations present in the overlying water column

direct observation of a submarine volcanic eruption from a sea floor instrument caught in a lava flow our understanding of submarine volcanic eruptions has improved substantially in the past decade owing to the recent ability to remotely detect such events and to then respond rapidly with synoptic surveys and sampling at the eruption site but these data are necessarily limited to observations after the event in contrast the 1998 eruption of axial volcano on the juan de fuca ridge was monitored by in situ sea floor instruments one of these instruments which measured bottom pressure as a proxy for vertical deformation of the sea floor was overrun and entrapped by the 1998 lava flow the instrument survived being insulated from the molten lava by the solidified crust and was later recovered the data serendipitously recorded by this instrument reveal the duration character and effusion rate of a sheet flow eruption on a mid ocean ridge and document over three metres of lava flow inflation and subsequent drain back after the brief two hour eruption the instrument also measured gradual subsidence of 1 4 metres over the next several days reflecting deflation of the entire volcano summit as magma moved into the adjacent rift zone these findings are consistent with our understanding of submarine lava effusion as previously inferred from seafloor observations terrestrial analogues and laboratory simulations

observations of manganese and iron at the coaxial seafloor eruption site juan de fuca ridge magmatic injection during june july 1993 on the coaxial segment of the juan de fuca ridge resulted in an axial lava eruption an overlying chronic style hydrothermal plume and at least three megaplume like event plumes a comprehensive view of mn and fe liberated as hydrothermal discharge during this volcano hydrothermal event was obtained as part of a rapid field response that included continuous in situ measurements of mn and fe distribution maps for mn fe and temperature anomaly within the chronic and event plumes reveal significant thermochemical diversity that we ascribe to variable extraction efficiencies for heat and metals by source fluids and to plume age differences the mn heat data suggest that diffuse fluids could be the primary source of the chronic plume at northern coaxial segment but make only a minor contribution to event plumes measurements of ephemeral fe ii were used to estimate that the event plumes were 2 4 and 6 8 days old when first sampled corresponding to formation times during peak seismic activity the inventories of mn and fe within the three coaxial event plumes were similar to values previously reported for megaplumes

low temperature volatile production at the lost city hydrothermal field evidence from a hydrogen stable isotope geothermometer although commonly utilized in continental geothermal work the water hydrogen and methane hydrogen isotope geothermometers have been neglected in hydrothermal studies here we report d ch4 and d h2 values from high temperature black smoker type hydrothermal vents and low temperature carbonate hosted samples from the recently discovered lost city hydrothermal field methane deuterium content is uniform across the dataset at 120 12 hydrogen d values vary from 420 to 330 at high temperature vents to 700 to 600 at lost city the application of several geothermometer equations to a suite of hydrothermal vent volatile samples reveals that predicted temperatures are similar to measured vent temperatures at high temperature vents and 20 60 c higher than those measured at the lost city vents we conclude that the overestimation of temperature at lost city reflects 1 that methane and hydrogen are produced by serpentinization at n110 c and 2 that isotopic equilibrium at temperatures b70 c is mediated by microbial sulfate reduction the successful application of hydrogen isotope geothermometers to low temperature lost city hydrothermal samples encourages its employment with lowtemperature diffuse hydrothermal fluids 2005 elsevier b v all rights reserved

unique event plumes from a 2008 eruption on the northeast lau spreading center the creation of ocean crust by lava eruptions is a fundamental earth process involving immediate and immense transfers of heat and chemicals from crust to ocean this transfer creates event plumes megaplumes massive ellipsoidal eddies with distinctive and consistent chemical signatures here we report the discovery of unique event plumes associated with a 2008 eruption on the northeast lau spreading center instead of a large plume hundreds of meters thick we detected at least eight individual plumes each 50 m thick and apparently only 1 3 km in diameter yet still rising 200 1000 m above the eruption site low and uniform 3he heat 0 041 10 17 mol j and dissolved mn heat 0 04 nmol j ratios in water samples were diagnostic of event plumes high h2 concentrations up to 9123 nm and basalt shards confirmed extensive interactions between molten lava and event plume source fluids remote vehicle observations in 2009 mapped a new small 1 5 5 8 106 m3 lava flow our results suggest that event plumes are more variable and thus perhaps more common than previously recognized small event plumes may be preferentially associated with small or sheet flow eruptions and massive event plumes with slowly extruding pillow mounds 25 75 m thick despite this correlation and high h2 concentrations existing theory and seafloor observations argue that cooling lava cannot transfer heat fast enough to create the buoyancy flux required for event plumes the creation of event plumes under a broad range of eruption conditions provides new constraints for any theory of their formation

a three plate kinematic model for lau basin opening 1 we present revised compilations of bathymetry magnetization acoustic imagery and seismicity data for the lau basin and surrounds we interpret these data to more precisely locate the plate boundaries in the region and to derive a three plate kinematic model for the opening of the lau basin during the brunhes chron our tectonic model includes the niuafo ou microplate which is separated from the australian plate along the peggy ridge lau extensional transform zone central lau spreading center and from the tongan plate along the newly discovered fonualei rift and spreading center our model shows that australia tonga plate motion between 15 5 s and 19 s is partitioned across the microplate and it resolves the former apparent conflict between geodetic versus seafloor spreading velocities regionally this implies that the angular rate of opening of the lau basin has been faster than that of the havre trough to the south and that the rapid current rates of pacific subduction at the tonga trench 240 mm yr at 16 s have been sustained for at least 0 78 m y

quantitative constraints on the growth of submarine lava pillars from a monitoring instrument that was caught in a lava flow 1 lava pillars are hollow vertical chimneys of solid basaltic lava that are common features within the collapsed interiors of submarine sheet flows on intermediate and fast spreading mid ocean ridges they are morphologically similar to lava trees that form on land when lava overruns forested areas but the sides of lava pillars are covered with distinctive evenly spaced thin horizontal lava crusts referred to hereafter as lava shelves lava stalactites up to 5 cm long on the undersides of these shelves are evidence that cavities filled with a hot vapor phase existed temporarily beneath each crust during the submarine eruption of axial volcano in 1998 on the juan de fuca ridge a monitoring instrument called vsm2 became embedded in the upper crust of a lava flow that produced 3 to 5 m high lava pillars a pressure sensor in the instrument showed that the 1998 lobate sheet flow inflated 3 5 m and then drained out again in only 2 5 hours these data provide the first quantitative constraints on the timescale of lava pillar formation and the rates of submarine lava flow inflation and drainback they also allow comparisons to lava flow inflation rates observed on land to theoretical models of crust formation on submarine lava and to previous models of pillar formation a new model is presented for the rhythmic formation of alternating lava crusts and vapor cavities to explain how stacks of lava shelves are formed on the sides of lava pillars during continuous lava drainback each vapor cavity is created between a stranded crust and the subsiding lava surface a hot vapor phase forms within each cavity as seawater is syringed through tiny cracks in the stranded crust above eventually the subsiding lava causes the crust above to fail quenching the hot cavity and forming the next lava crust during the 1998 eruption at axial volcano this process repeated itself about every 2 min during the 81 min long drainback phase of the eruption based on the thickness and spacing of the lava shelves the vsm2 data show that lava pillars are formed during short lived eruptions in which inflation and drainback follow each other in rapid succession and that pillars record physical evidence that can be used to interpret the dynamics of seafloor eruptions

quantification of submarine lava flow morphology through analog experiments we developed a technique for determining paleoeffusion rates and emplacement times for submarine lava flows using observations of flow morphology and estimates of eruption volume eruption temperature lava viscosity and preflow topography using laboratory simulations and correlating these results with sea floor observations four submarine lava flow morphologies are considered to be diagnostic of specific effusion rates jumbled folded and lineated sheets and striated pillows we applied this approach to the coaxial flow emplaced in less than 14 days on the juan de fuca ridge in early summer 1993 and we calculated an effusion rate of 100 m3 s giving an emplacement time of 10 days

hydrothermal vent waters at 13 n on the east pacific rise isotopic composition and gas concentration abstract gas concentrations and isotopic compositions of water have been measured in hydrothermal waters from 13 n on the east pacific rise in the most mg depleted samples 5 10 3 moles kg the gas concentrations are 3 4 5 10 5 cm 3 stp kg helium 0 62 1 24 cm 3 stp kg ch 4 10 80 16 71 10 3 moles kg co 2 the samples contain large quantities 95 126 cm 3 kg of h 2 and some carbon monoxide 0 26 0 36 cm 3 kg which result from reaction with the titanium sampling bottles 13 c in methane and co 2 16 6 to 19 5 and 4 1 to 5 5 respectively indicate temperatures between 475 and 550 c whereas 13 c co is compatible with formation by reduction of co 2 on ti at 350 c close to the sampling temperature 3 he 4 he are very homogeneous at 7 5 0 1 r a 3 he 4 he 1 0 10 5 and very similar to already published data as well as ch 4 3 he ratios between 1 4 and 2 1 10 6 18 o and d in water show enrichments from 0 39 to 0 69 and from 0 62 to 1 49 respectively these values correspond to w r ratios of 0 4 7 the distinct 18 o enrichments indicate that the isotopic composition of the oceans is not completely buffered by the hydrothermal circulations the 3 he enthalpy relationship is discussed in terms of both hydrothermal heat flux and 3 he mantle flux

a systematic analysis of eight decades of incipient motion studies with special reference to gravel bedded rivers data compiled from eight decades of incipient motion studies were used to calculate dimensionless critical shear stress values of the median grain size t c 50 calculated t c 50 values were stratified by initial motion definition median grain size type surface subsurface or laboratory mixture relative roughness and flow regime a traditional shields plot constructed from data that represent initial motion of the bed surface material reveals systematic methodological biases of incipient motion definition t c 50 values determined from reference bed load transport rates and from visual observation of grain motion define subparallel shields curves with the latter generally underlying the former values derived from competence functions define a separate but poorly developed field while theoretical values predict a wide range of generally higher stresses that likely represent instantaneous rather than time averaged critical shear stresses the available data indicate that for high critical boundary reynolds numbers and low relative roughnesses typical of gravel bedded rivers reference based and visually based studies have t c50 ranges of 0 052 0 086 and 0 030 0 073 respectively the apparent lack of a universal t 50 for gravel bedded rivers warrants great care in choosing defendable t c50 values for particular applications

detection of an unusually large hydrothermal event plume above the slow spreading carlsberg ridge nw indian ocean about 90 of earth s volcanism occurs along the global mid ocean ridge system here sporadic volcanic and tectonic activity is thought to cause cataclysmic release of hydrothermal fluids forming event plumes each plume often contains as much hydrothermal effluent and heat as chronic hydrothermal venting from a typical vent site discharges during a year to date only a few event plumes have been detected and only above intermediate rate spreading ridges in the pacific here we report the first evidence for an unusually large event plume that originated from the slow spreading 3 cm yr full rate carlsberg ridge in the nw indian ocean at 70 km long up to 4540 km3 in volume and with up to 24 1016 j of excess heat this event plume was substantially larger than previous ones and demonstrates that dispersion of hydrothermal heat and biological products from slow spreading ridges may be more significant and effective than hitherto imagined

helium heat and the generation of hydrothermal event plumes at mid ocean ridges hydrothermal event plumes are unique water column features observed over mid ocean ridges presumably generated by the sudden release of large volumes of hot buoyant fluid although the specifics of event plume generation are unknown event plumes have been attributed to the rapid emptying of a hydrothermal reservoir or to rapid heat extraction from a recently emplaced dike or seafloor lava flows the chemical and thermal signatures of event plumes as compared to the underlying steady state plumes offer important clues to the generation of event plumes event plumes have low 3he heat ratios of 0 4 10 17 mol j 1 similar to vent fluids from mature hydrothermal systems in contrast the steady state plumes found beneath the event plumes have elevated and variable 3he heat ratios of 2 to 5 10 17 mol j 1 fluids collected directly over fresh lava flows have even higher 3he heat ratios of 2 to 8 10 17 mol j 1 up to 30 times the event plume values these disparate 3he heat ratios place strong constraints on models of event plume generation especially models which rely on heat extraction from seafloor eruptions published by elsevier science b v

deep submarine pyroclastic eruptions theory and predicted landforms and deposits submarine pyroclastic eruptions at depths greater than a few hundred meters are generally considered to be rare or absent because the pressure of the overlying water column is sufficient to suppress juvenile gas exsolution so that magmatic disruption and pyroclastic activity do not occur consideration of detailed models of the ascent and eruption of magma in a range of sea floor environments shows however that significant pyroclastic activity can occur even at depths in excess of 3000 m in order to document and illustrate the full range of submarine eruption styles we model several possible scenarios for the ascent and eruption of magma feeding submarine eruptions 1 no gas exsolution 2 gas exsolution but no magma disruption 3 gas exsolution magma disruption and hawaiian style fountaining 4 volatile content builds up in the magma reservoir leading to hawaiian eruptions resulting from foam collapse 5 magma volatile content insufficient to cause fragmentation normally but low rise speed results in strombolian activity and 6 volatile content builds up in the top of a dike leading to vulcanian eruptions we also examine the role of bulk interaction steam explosivity and contact surface steam explosivity as processes contributing to volcaniclastic formation in these environments we concur with most earlier workers that for magma compositions typical of spreading centers and their vicinities the most likely circumstance is the quiet effusion of magma with minor gas exsolution and the production of somewhat vesicular pillow lavas or sheet flows depending on effusion rate the amounts by which magma would overshoot the vent in these types of eruptions would be insufficient to cause any magma disruption the most likely mechanism of production of pyroclastic deposits in this environment is strombolian activity due to the localized concentration of volatiles in magma that has a low rise rate magmatic gas collects by bubble coalescence and ascends in large isolated bubbles which disrupt the magma surface in the vent producing localized blocks bombs and pyroclastic deposits another possible mode of occurrence of pyroclastic deposits results from vulcanian eruptions these deposits being characterized by the dominance of angular blocks of country rocks deposited in the vicinity of a crater should be easily distinguishable from strombolian and hawaiian eruptions however we stress that a special case of the hawaiian eruption style is likely to occur in the submarine environment if magmatic gas buildup occurs in a magma reservoir by the upward drift of gas bubbles in this case a layer of foam will build up at the top of the reservoir in a sufficient concentration to exceed the volatile content necessary for disruption and hawaiian style activity the deposits and landforms are predicted to be somewhat different from those of a typical primary magmatic volatile induced hawaiian eruption specifically typical pyroclast sizes might be smaller fountain heights may exceed those expected for the purely magmatic hawaiian case cooling of descending pyroclasts would be more efficient leading to different types of proximal deposits and runout distances for density flows would be greater potentially leading to submarine pyroclastic deposits surrounding vents out to distances of tens of meters to a kilometer in addition flows emerging after the evacuation of the foam layer would tend to be very depleted in volatiles and thus extremely poor in vesicles relative to typical flows associated with hawaiian style eruptions in the primary magmatic gas case we examine several cases of reported submarine volcaniclastic deposits found at depths as great as 3000 m and conclude that submarine hawaiian and strombolian eruptions are much more common than previously suspected at mid ocean ridges furthermore the latter stages of development of volcanic edifices seamounts formed in submarine environments are excellent candidates for a wide range of submarine pyroclastic activity due not just to the effects of decreasing water depth but also to 1 the presence of a summit magma reservoir which favors the buildup of magmatic foams enhancing hawaiian style activity and episodic dike emplacement which favors strombolian style eruptions and 2 the common occurrence of alkalic basalts the co2 contents of which favor submarine explosive eruptions at depths greater than tholeiitic basalts these models and predictions can be tested with future sampling and analysis programs and we provide a checklist of key observations to help distinguish among the eruption styles

geochemistry of high h2 and ch4 vent fluids issuing from ultramafic rocks at the rainbow hydrothermal field 36 14 n mar the flores diving cruise was part of the mast iii amores 1995 1998 program funded by the european union one of the major achievements of the flores cruise was the discovery of the rainbow hydrothermal field hosted in ultramafic rocks south of the amar segment on the mid atlantic ridge mar the rainbow hydrothermal fluids exhibit temperatures of 365 c ph of 2 8 high chlorinity 750 mmol kg and low silica 6 9 mmol kg the uniformity in endmember major minor trace element concentrations and gas contents suggests that all rainbow fluids originate from the same deep source although h2s content is relatively low 1 20 mmol kg all vent fluids show extraordinary high h2 16 mmol kg ch4 2 5 mmol kg and co 5 mol kg endmember concentrations compared to fluids collected from other vent sites along the mar hydrogen represents more than 40 of the total gas volume extracted from the fluids at rainbow h2 production is likely associated with alteration of olivine and orthopyroxene minerals during serpentinization given that exposures of ultramafic rock may be common particularly along slow spreading ridges the production of h2 may have important implications for microbial activity at and beneath the seafloor

discovery of abundant hydrothermal venting on the ultraslow spreading gakkel ridge in the arctic ocean submarine hydrothermal venting along mid ocean ridges is an important contributor to ridge thermal structure and the global distribution of such vents has implications for heat and mass fluxes from the earth s crust and mantle and for the biogeography of vent endemic organisms previous studies have predicted that the incidence of hydrothermal venting would be extremely low on ultraslow spreading ridges ridges with full spreading rates 2 cm yr 1 which make up 25 per cent of the global ridge length and that such vent systems would be hosted in ultramafic in addition to volcanic rocks here we present evidence for active hydrothermal venting on the gakkel ridge which is the slowest spreading 0 6 1 3 cm yr 1 and least explored mid ocean ridge on the basis of water column profiles of light scattering temperature and manganese concentration along 1 100 km of the rift valley we identify hydrothermal plumes dispersing from at least nine to twelve discrete vent sites our discovery of such abundant venting and its apparent localization near volcanic centres requires a reassessment of the geologic conditions that control hydrothermal circulation on ultraslow spreading ridges

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

conductive heat loss in recent eruptions at mid ocean ridges a new technique for measuring conductive heat flow from unsedimented volcanic rocks on the sea floor has been tested on two new eruption sites in the ne pacific this technique consists of isolating the surficial rocks from sea water using water saturated urethane foam as an insulating thermal blanket the thermal gradient transferred from the outcrop to the thermal blanket is a quantitative measurement of the conductive heat flow that takes place in unsedimented volcanic areas we deployed two thermal blankets at 13 sites on the 1993 and 1996 juan de fuca gorda ridge flows and found 1 a factor of 10 decrease in heat flow over a period of 12 months on the 1993 coaxial flow 2 a value of 6950 mw m 2 on the 8 month old gorda flow and 3 measurements of heat flow versus age since eruption indicate that newly extruded volcanic units are quite permeable to fluid circulation and cool rapidly by convection in only a few years these new heat flux data confirm that the extrusive volcanic layer is not the primary heat source for long lived high temperature hydrothermal systems which must instead rely on a more isolated thermal reservoir within the lower crustal rocks

mantle 3he in hydrothermal vents and plume of the lucky strike site mar 37 17 n and associated geothermal heat flux abstract the helium isotopes composition of hydrothermal fluids at the lucky strike vent field has been determined by mass spectrometry 4he end member concentration is in the 1 2 10 5 2 6 10 5 cm3 stp g range and the 3he 4he ratio r is 8 13 0 05 ra the helium 3 budget over the active vent field was estimated from the 3he mapping of the plume advected along the rift valley using a 1 d advection box model constrained with current velocity measurements this leads to a 3he flux of 2 3 10 8 mol s the scaling of the heat flux from the lucky strike site to that of 3he using the 3he heat ratio measured in the hydrothermal fluids 5 2 10 18 7 7 10 18 mol j allows an initial evaluation of the total geothermal heat output from the site in the 3800 1200 mw range this flux of the same order as that of the endeavour segment juan de fuca ridge indicates the present high level of the hydrothermal activity in this section of the mid atlantic ridge

the morphodynamics and internal structure of intertidal fine gravel dunes hills flats severn estuary uk in the macrotidal severn estuary uk the dynamics of intertidal fine gravel dunes were investigated these dunes are migrating across a bedrock platform systematic observations were made of hydraulic climate geometry migration rates and internal sedimentary structures of the dunes during spring tides the ebb flow is dominant dunes grow in height and have ebb orientated geometry with bedrock floors in the troughs during neap tides a weak flood flow may dominate dunes then are flood orientated or symmetrical neap dune heights decrease and the eroded sediment is stored in the dune troughs where the bedrock becomes blanketed by muddy gravel during spring tides instantaneous bed shear stresses reach 8 n m 2 sufficient to disrupt a 9 mm gravel armour layer however a sustained bed shear stress of 4 n m 2 is required to initiate dune migration at which time the critical depth mean velocity is 1 m s 1 ebb and flood inequalities in the bed shear stress explain the changes in dune asymmetry and internal structures during flood tides the crests of the dunes reverse such that very mobile sedimentary caps overlie a more stable dune core because ebb tides dominate internal structures of the caps often are characterised by ebb orientated steep open work foresets developed by strong tidal currents and some lower angle crossbeds deposited as weaker currents degrade foresets the foresets forming the caps may be grouped into cosets tidal bundles and are separated from mud infused cores of crossbeds that lie below by reactivation and erosion surfaces blanketed by discontinuous mud drapes the cores often exhibit distinctive muddy toe sets that define the spacing of tidal cosets

hydrothermal cooling along the eastern lau spreading center no evidence for discharge beyond the neovolcanic zone heat transported from the mantle beneath spreading centers creates an astonishingly narrow ribbon of convective heat discharge at plate boundaries as apparently demonstrated by exhaustive exploration for hydrothermal discharge sites over the last three decades recent observations and models are now challenging this assumption of exclusively axis centric high temperature venting one example is the proposal that intense cooling along the vertical boundaries of a broad low velocity volume lvv of hot crust could generate high temperature fluids several kilometers off axis to test the hypothesis that substantial hydrothermal discharge might occur beyond the lvv we conducted a dense survey grid of the ridge and surrounding seafloor up to 5 km along 175 km of the eastern lau spreading center and valu fa ridge 1800 km of track line our sampling array extended from 50 to 400 m above bottom and included light scattering oxidation reduction potential and hydrographic sensors attached to the tow line and beneath the imi120 sonar mapping system the surveys successfully mapped plumes from several vent fields in the neovolcanic zone 1 5 km about the axis but did not detect evidence of significant discharge anywhere farther off axis at a few locations on the valu fa ridge however we did record oxidation reduction potential anomalies with hydrographic density signatures that imply low temperature hydrothermal sources on the axial flank although these sites are hundreds of meters deeper than the adjacent crest they are above not beyond the previously mapped lvv our results thus do not support a simple picture of high temperature fluids ascending undiluted through the crust to the seafloor several kilometers off axis however we cannot exclude the possibilities that the largely unmapped lvv is narrower here than seen on other ridges that hydrothermal fluids formed beyond the lvv are channeled to the axis or that discharge beyond the neovolcanic zone occurs only as dispersed very low temperature fluids our observations do demonstrate that high temperature discharge predominantly exits the seafloor within a narrow 1 5 km axial ribbon regardless of the presence or absence of an axial magma chamber

abiotic hydrogen h2 sources and sinks near the mid ocean ridge mor with implications for the subseafloor biosphere free hydrogen h2 is a basal energy source underlying chemosynthetic activity within igneous ocean crust in an attempt to systematically account for all h2 within young oceanic lithosphere

reply to comment by m r palmer and g g j ernst on helium heat and the generation of hydrothermal event plumes at mid ocean ridges by j e lupton e t baker and g j massoth

particle over passing on depth limited gravel bars an experimental channel is used to examine the transport of mixed sand and gravel bedload over the crestal platform of hump back bars and along the top of planar gravel sheets hydraulic processes result in the simultaneous transport of cobbles and pebbles over a static closely packed bed consisting of like sized and finer particles for prescribed conditions flat upper stage plane sand beds develop over the crestal location with pebbles rolling easily over the sandy bed at the brinkpoint flow separation ensures effective segregation of the gravel from the sand over the slip face the deposition rate of the sand is insufficient to fill fully the interstices within the gravel foresets before rapid deposition of gravel further advances the bed form consequently distinctive vertical assemblages of open work and closed contact framework gravels could be generated as another bar migrates over and preserves the initial structure in respect to the observed mechanisms of sorting over the bars a mathematical expression is developed to explain the critical conditions allowing coarse particle mobility over planar sand or gravel beds under upper stage plane bed conditions on the crestal platform the model then is used to ascertain whether the depositional environment ascribed to certain facies in the bunter pebble beds described in a recent publication is appropriate given the distinctive facies assemblages generated in this experiment and the known hydrodynamic control of the particle segregation process

entrainment of gravel from naturally sorted riverbed material the critical dimensionless shear stress ci required to entrain a given size particle di was computed from extensive bedload transport measurements in three self formed rivers that have naturally sorted gravel and cobble bed material the value of ci was determined to be significantly affected by the size distribution of the riverbed material for bed particles between 0 3 to 4 2 times the median diameter of the subsurface bed material d50 the average critical dimensionless shear stress ci is equal to ci 0 0834 di d50 0 872 with a coefficient of determination of 0 980 for bed particles larger than 4 2 times d50 ci appears to approach a constant value of approximately 0 020 in a noneroding channel thus ci varies from 0 25 to 0 020 for a given particle depending on the ratio of di to d50 previous investigations of the hydraulic conditions required to entrain coarse riverbed material have reported critical shear stress values ranging from 0 25 to 0 020 for a given particle size this analysis indicates that virtually all of the variation is due to differences in the subsurface bed material size distribution this analysis shows that ci varies almost inversely proportional to the particle diameter for a nonuniform bed material therefore bed particles between 0 3 to 4 2 times d50 are entrained at nearly the same discharge an investigation of the hydraulic characteristics and bed material mobility of 24 self formed gravel bed rivers in colorado determined that particles as large as the median diameter of the bed surface were entrained by discharges equal to the bankfull stage or less particles as large as the 90th percentile fraction were entrained by the bankfull discharge in nine of the rivers studied consequently in these rivers a significant fraction of the riverbed was entrained rather frequently over a period of years although the transport rate was very small