clumped isotope geochemistry of carbonates a new tool for the reconstruction of temperature and oxygen isotope composition of seawater abstract clumped isotope geochemistry deals with the state of ordering of rare isotopes in molecules in particular with their tendency to form bonds with other rare isotopes rather than with the most abundant ones among its possible applications carbonate clumped isotope thermometry is the one that has gained most attention because of the wide potential of applications in many disciplines of the earth sciences in particular it allows reconstructing the temperature of formation of carbonate minerals without knowledge of the isotopic composition of the water from which they were formed in addition the o isotope composition of the waters from which they were formed can be calculated using the 18 o of the same carbonate sample this feature offers new approaches in paleoclimatology for reconstructing past global geochemical cycles in this contribution two applications of this method are presented first the potential of a new analytical method of measurement of clumped isotopes on small samples of foraminifera for high resolution sst and seawater 18 o reconstructions from marine sediments is shown furthermore the potential of clumped isotope analysis of belemnites for reconstructing seawater 18 o and temperatures in the cretaceous is shown

volatiles in alkaline magmatism abstract gases extracted during fusion of volcanic glasses substantiate other sources of evidence that alkaline magmatism is distinguished from calc alkaline by higher co2 so2 contents with relatively smaller proportions of h2o in addition to higher c and s activity alkaline magmas are characterised by high cl and f which are released with alkalis during fusion experiments in a graphic reminder that alkalis should be regarded as mobile elements in magmatism the association of c s and halogens in alkaline magmatism is well shown in some high temperature residua characterized by assemblages such as calcite vishnevite calcium is an important cation in many aspects of alkaline magmatism and the links with carbonatite further emphasize the key position of c s cl and f throughout and the subordinate role of h2o

barremian cenomanian palaeotemperatures for australian seas based on new oxygen isotope data from belemnite rostra oxygen isotope ratio 18o data are presented for 55 belemnite rostra specimens of late aptian albian age from the great artesian basin of eastern australia and of late barremian early cenomanian age from the carnarvon basin of western australia the belemnites were carefully screened for diagenesis by means of catholuminescence and trace element geochemistry the 18o record for the carnarvon basin palaeolatitude 40 s is interpreted to show a long term warming trend from 10 c in the aptian to a maximum of 15 c in the cenomanian shorter term fluctuations in the mid albian are also apparent data from the great artesian basin palaeolatitude 40 55 s show a wider 18o range which is attributed to the epeiric sea context of this sample set the larger range in 18o is interpreted to reflect differing water mass characteristics due possibly to a combination of variable freshwater dilution seasonal temperature fluctuations and isotopic exchange between seawater and pyroclastic sediment within the epeiric sea nevertheless the aptian albian 18o trends are still comparable to those obtained for the carnarvon basin the inferred palaeotemperature trends for the australian sedimentary basins are consistent with records from other regions and demonstrate that broadly synchronous palaeotemperature variations can be recognised on a global scale during the cretaceous although our data do indeed show cool temperatures within the aptian and the albian reaching a low of 10 c these estimates are somewhat warmer than those based on glendonite genesis however the exact position of the glendonite bearing and dropstone rich parts of the bulldog shale is poorly constrained with respect to our isotope data and the warmest temperatures were of late albian and early cenomanian age however to conform to greenhouse warmth and equability and to achieve parity with other palaeotemperature records the 18oseawater estimate of 1 0 that is generally used for cretaceous oceans in an ice free world needs to be adjusted to a slightly higher value

rivera ocean seismic experiment rose overview the rivera ocean seismic experiment rose was designed as a combined sea and land seismic program to utilize both explosive and earthquakes to study a number of features of the structure and evolution of a mid ocean ridge a major oceanic fracture zone and the transition region between ocean and continent the primary region selected for the experiment included the rivera fracture zone the crest and eastern flank of the east pacific north of the rivera and adjacent areas of baja california and mainland mexico the experiment included 1 study of the east pacific rise south of the orozco fracture zone primarily using ocean bottom recording and explosive sources 2 a seismicity program at the orozco and 3 a land based program of recording natural events along the coastal region of mexico a considerable amount of useful data was obtained in each of the three subprograms in the marine parts of the experiment we were able to address a variety of problems including structure and evolution of young oceanic crust and mantle structure and dynamics of the east pacific rise seismicity of the orozco fracture zone and partitioning of energy transmission between the ocean volume and the crust lithosphere on land the fortuitousmore occurrence of the petatlan m7 6 earthquake of march 14 1979 permitted the acquisition of an excellent data set of foreshocks and aftershocks of this large event which provide new insight into the filling of a major seismic gap in the region this overview describes the scientific rationale and the design of the experiments along with some general results less

northern east pacific rise evolution from 25 m y b p to the present the northeast pacific topography and magnetic lineations 25 m y b p to the present record the traces of three major spreading reorganizations only one spreading center is observed today but there is evidence for several ephemeral episodes of twin spreading accompanying the evolution from an extensive pacific guadalupe plate boundary to a much shortened pacific cocos and pacific rivera plate boundary the 25 m y b p plate reorganization culminated with the formation of the guadalupe plate bound by the murray fracture zone to the north and the cocos nazca spreading ridge to the south between 25 and 12 5 m y b p spreading continued while the plate retained the same general outline the 12 5 11 m y b p reorganization resulted in the creation of a much shortened pacific cocos plate boundary located in its early stages over the mathematician seamounts and a much reduced cocos plate the last reorganizaton 6 5 3 5 m y b p resulted in the abandonment of the mathematician spreading ridge as a pacific cocos plate boundary in favor of the east pacific rise

the concentration of co_ 2 in submarine tholeiitic basalts the concentrations of co 2 and h 2 o in glass separates from submarine tholeiitic basalts have been determined the presence of vesicles in samples precludes accurate determination of the co 2 concentration in glass unless steps are taken to analyze only uncontaminated vesicle free glass special care has been exercised during sample preparation to avoid these problems a new analytical technique for micro determination of h 2 o and co 2 has been applied to five mid ocean ridge and two kilauea submarine east rift zone basalts quenched at pressures as great as 0 5 kb the uncertainty of quenching pressure due to flow of lava subsequent to surface quenching global sea level variations and local deformation of the sea floor are discussed non tectonic uncertainties of quenching pressure have been estimated the concentration of co 2 in glass separates from submarine tholeiitic basalts increases linearly with quenching pressure the apparent solubility of co 2 in tholeiitic basalts

time variability of the earth s gravity field hydrological and oceanic effects and their possible detection using grace the grace satellite mission scheduled for launch in 2001 is designed to map out the earth s gravity field to high accuracy every 2 4 weeks over a nominal lifetime of 5 years changes in the gravity field are caused by the redistribution of mass within the earth and on or above its surface grace will thus be able to constrain processes that involve mass redistribution in this paper we use output from hydrological oceanographic and atmospheric models to estimate the variability in the gravity field i e in the geoid due to those sources we develop a method for constructing surface mass estimates from the grace gravity coefficients we show the results of simulations where we use synthetic grace gravity data constructed by combining estimated geophysical signals and simulated grace measurement errors to attempt to recover hydrological and oceanographic signals we show that grace may be able to recover changes in continental water storage and in seafloor pressure at scales of a few hundred kilometers and larger and at timescales of a few weeks and longer with accuracies approaching 2 mm in water thickness over land and 0 1 mbar or better in seafloor pressure

dynamic modelling of the subduction zone of central mexico summary in central mexico some significant normal faulting events have occurred within the subducted oceanic cocos plate just below or near the down dip edge of the strongly coupled interface these normal faulting shocks followed large shallow thrust earthquakes in other subduction zones such events generally precede the up dip thrust events a vertical 2 d finite element modelling has been used to simulate the subduction of the cocos plate beneath the north american plate when the slab is driven by an active convergence velocity or slab pull we find that the latter mechanism plays only a minor role due to shallow subduction the modelling results show that the stress pattern is very sensitive to the geometry of the plates in particular normal faulting earthquakes that follow large thrust events can be explained on the basis of the flexural response of the overriding and subducting plates to the peculiar geometry of this subduction zone where the subducting slab becomes horizontal at about 100 km from the trench this horizontal part of the subducting plate down dip with respect to the main thrust zone is under an extensional stress field this provides an alternative explanation to the slab pull for the occurrence of normal faulting intraplate earthquakes in order for normal faulting earthquakes to occur in the early part of the seismic cycle it is necessary that the large up dip thrust events have a partial stress drop we find that for small fractional stress drop a wide region of extension remains below the down dip edge of the main fault plane following a large thrust earthquake thus the main thrust earthquakes do not invert the polarity of the active stress field which is compressional and extensional up dip and down dip respectively with respect to the main thrust fault larger fractional stress drops result in larger delays in the occurrence of normal faulting events after the main thrust events

ocean bottom pressure variability can it be reliably modeled ocean bottom pressure obp variability serves as a proxy of ocean mass variability the knowledge of which is needed in geophysical applications the question of how well it can be modeled by the present general ocean circulation models on time scales in excess of one day is addressed here by comparing the simulated obp variability with the observed one to this end a new multi year data set is used obtained with an array of bottom pressure gauges deployed deeply along a transect across the southern ocean we present a brief description of obp data and show large scale correlations over several thousand kilometres at all time scales red using daily and monthly averaged data annual and semi annual cycles are weak close to the agulhas retroflection signals of up to 30 cm equivalent water height ewh are detected further south signals are mostly intermittent and noisy it is shown that the models simulate consistent patterns of bottom pressure variability on monthly and longer scales except for areas with high mesoscale eddy activity where high resolution is needed to capture the variability due to eddies furthermore despite good agreement in the amplitude of variability the in situ and simulated obp show almost no correlation

application of redox sensitive proxies and carbonate clumped isotopes to mesozoic and palaeozoic radiaxial fibrous calcite cements abstract ancient marine radiaxial calcite cements are commonly exploited as archives of marine porewater properties based on the argument that they lack metabolic effects often assigned to biogenic carbonates here we critically test the significance of conventional versus with respect to these fabrics less conventional proxy data from pennsylvanian triassic and cretaceous case examples conventional proxies include cathodoluminescence carbon and oxygen isotope ratios main and trace elemental concentrations less conventionally applied proxies are clumped isotope 47 redox sensitive and rare earth elements sampled across a succession of triassic radiaxial fibrous calcites radiaxial calcites are subdivided in three groups based on their luminescence characteristics non luminescent patchy luminescent and bright luminescent luminescence patterns are in fair agreement with isotope ratios in particular with those of oxygen the data fall into or are close to the range of reconstructed marine seawater values and often plot to the positive end member of the isotopic range these results disagree with the commonly held view that isotope data from luminescent cements reflect a priori non marine values further evidence for this comes from ree concentration patterns and cerium anomalies suggesting normal marine porewater values for all except the very last generation of radiaxial calcites this implies that luminescent radiaxial calcites must not necessarily represent significant diagenetic resetting kinetic effects during precipitation and different activator elements must be considered marine and earliest burial porewater temperatures of 12 26 c are suggested by conventional calcite 18 o thermometry conversely the application of the clumped isotope thermometer to the same radiaxial calcites suggests temperatures of 180 200 c reflecting solid state resetting of fully cemented limestones under a low water rock ratio redox sensitive elements particularly zn cd u and cu are affected by kinetic processes overriding fluid eh manganese concentrations and ce anomaly data point to gradually decreasing marine porewater oxygen levels from outer to inner cement fringes judging from ree patterns and ce anomalies the cement layers in the central portions of the pore filling cement succession witnessed the end of marine precipitation and the onset of shallow marine diagenesis consequently radiaxial calcite precipitation is suggested to continue in the early shallow marine burial domain this study underscores the potential of radiaxial calcite successions as archives of marine porewater to shallow burial diagenetic pathways the combination of conventional and less conventional proxies is a clear strength of this study and documents that abiogenic carbonate archives are often underexplored

new field and laboratory evidence for the origin of hyaloclastite flows on seamount summits new field observations with the submersible alvin and photographic evidence from a study of the summits of seamounts near the east pacific rise show that hyaloclastite deposits occur commonly hyaloclastite outcrops were found on six volcanoes at depths from 1240 to 2500 m these new observations plus laboratory study of new hyaloclastite specimens extend the results of previous studies most of the hyaloclastite samples are of hydrovolcanic eruptive origin but a few show evidence of a predominantly sedimentary origin primarily from morphology we identify several vent areas from which hyaloclastite presumably erupted the surface appearance of the hyaloclastite deposits varies with distance to these vents leading us to propose a facies model for deep sea hyaloclastites on seamount summits hyaloclastites of hydromagmatic origin exhibit weak normal grading and bedding parallel alignment of platy shards they consist of blocky sliver and fluidal basalt glass shards and lithics in a matrix that contains pelagic sediment the shards themselves are remarkably free of even the tiniest crystals and are usually chemically homogeneous we propose that the shards form mainly by cooling contraction granulation but cannot rule out the possibility of limited steam explosivity hyaloclastites are closely associated with submarine pahoehoe and we propose that a very rapid eruption rate promoting clastic dominated versus flow dominated eruptive behavior is the dominant control on hyaloclastite formation we propose that shard formation occurs during submarine lava fountaining gravitational instability of the resulting slurry of shards sea water and possibly steam causes gravity flow that carries the shards outward from the vent further field and modelling studies are needed to test these ideas and more quantitatively constrain the ascent mechanism eruption dynamics and deposition of deep sea hyaloclastites

highly vesicular pumice generated by buoyant detachment of magma in subaqueous volcanism many submarine caldera volcanoes are blanketed with deposits of highly vesicular pumice typically attributed to vigorous explosive activity1 2 3 4 however it is challenging to relate volcanic products to specific eruptive styles in submarine volcanism5 6 here we document vesicularity and textural characteristics of pumice clasts dredged from the submarine macauley volcano in the kermadec arc southwest pacific ocean we find that clasts show a bimodal distribution with corresponding differences in vesicle abundances and shapes specifically we find a sharp mode at 91 vesicularity and a broad mode at 65 80 subordinate clasts show gradients in vesicularity we attribute the bimodality to a previously undocumented eruptive style that is neither effusive nor explosive the eruption rate is insufficient to cause magma to fragment explosively yet too high to passively feed a lava dome instead the magma foam buoyantly detaches at the vent and rises as discrete magma parcels or blebs while continuing to vesiculate internally the blebs are widely distributed by ocean currents before they disintegrate or become waterlogged this disintegration creates individual clasts from interior and rim fragments yielding the bimodal vesicularity characteristics we conclude that the generation and widespread dispersal of highly vesicular pumice in the marine environment does not require highly explosive activity

lava balloons peculiar products of basaltic submarine eruptions between december 1998 and april 2001 a submarine basaltic eruption occurred west of terceira island azores portugal in water depths between 300 and 1 000 m physical evidence for the eruption was provided by the periodic occurrence of hot lava balloons floating on the sea surface the balloons consisted of a large gas filled cavity surrounded by a thin shell a few centimetres thick the shells of the collected balloons are composed of two layers termed the outer layer and the inner layer defined by different bubble number density bubble sizes and crystal content the inner layer is further divided into three sublayers defined by more subtle differences in vesicularity the outer layer is glassy golden coloured and highly porous it shows signs of fluidal deformation and late stage extension cracks interstitial glass contains 0 29 wt h2o and co2 is below detection melt inclusions contain up to 1 18 wt h2o and 1 500 ppm co2 from different inclusions cooling rates of the outermost glass of the outer layer are found to be as high as 1 259 k s during ascent of low viscosity magma to the ocean floor volatiles dominated by co2 exsolved from the magma melt crystals the buoyancy of the vapour phase that accumulated below a thin crust on lava ponded at the vent caused bulging and ultimately cracking of the crust this allowed large bubbles central cavity surrounded by a film of vesicular magma balloon shell to leak into the water column on contact with the seawater the outermost part of the outer layer of the shell hyperquenched if an entirely closed shell was produced during detachment the trapped gas inside allowed buoyant rise only balloons with the right balance of physical properties e g size and bulk density rose all the way to the sea surface

clumped isotope geochemistry the study of naturally occurring multiply substituted isotopologues clumped isotope geochemistry is concerned with the state of ordering of rare isotopes in natural materials that is it examines the extent to which rare isotopes d 13 c 15 n 18 o etc bond with or near each other rather than with the sea of light isotopes in which they swim abundances of isotopic clumps in natural materials are influenced by a wide variety of factors in most cases their concentrations approach within ca 1 relative the amount expected for a random distribution of isotopes deviations from this stochastic distribution result from enhanced thermodynamic stability of heavy isotope clumps slower kinetics of reactions requiring the breakage of bonds between heavy isotopes the mass dependence of diffusive and thermo gravitational fractionations mixing between components that differ from one another in bulk isotopic composition biochemical and photochemical fractionations that may reflect combinations of these simpler physical mechanisms and in some cases other processes we do not yet understand although clumped isotope geochemistry is a young field several seemingly promising applications have already emerged most importantly it appears that proportions of 13 c 18 o bonds in carbonate minerals are sensitive to their growth temperatures independent of bulk isotopic composition thus clumped isotope analysis of ancient carbonates can be used as a quantitative paleothermometer that requires no assumptions about the 18 o of waters from which carbonates grew this approach has been used to reconstruct marine temperatures across the phanerozoic reaching back to the silurian terrestrial ground temperatures across the cenozoic thermal histories of aqueously altered meteorites among other applications clumped isotope geochemistry is also placing new constraints on the atmospheric budget and stratospheric photochemistry of co 2 and should be capable of placing analogous new constraints on the budgets of other atmospheric gases finally this field could be extended to encompass sulfates volatile hydrocarbons organic moieties and other materials

isotope records c o sr of late pliensbachian early toarcian environmental perturbations in the westernmost tethys majorca island spain the late pliensbachian early toarcian early jurassic was a time of major environmental changes that culminated with the early toarcian oceanic anoxic event t oae ca 183 ma this period is marked by significant disturbances in the carbon cycle and rapid climatic changes to improve the understanding of the expression of these events in westernmost tethyan domains this study provides new belemnite and bulk carbonate c and o stable isotope records of a 5 myr upper pliensbachian to middle toarcian marine succession of the balearic basin es cosconar section majorca time resolution has been improved by combination of biostratigraphic ammonoids and brachiopods and geochronologic 87sr 86sr methods seawater paleotemperatures derived from 18o belemnite records reveal cooler paleotemperatures in the upper part of the spinatum zone the uppermost spinatum zone is characterized by the onset of a warming event that crosses the pliensbachian toarcian boundary culminating with the warmer temperatures up to 10 c of warming for the serpentinum zone of the lower toarcian this warming event has been detected contemporaneously in many other european and tethyan basins and is interpreted to represent generalized raised seawater temperatures linked to the t oae four significant 13c events have been recorded in belemnite and bulk carbonate records the first is a negative carbon isotope excursion cie around the pliensbachian toarcian boundary which is best represented in the belemnite record soon after the bulk carbonate record shows a positive shift in the lower tenuicostatum zone concomitant with a return to background values in the belemnite record suggesting strong water stratification or decoupling probably related with export of neritic carbonate to the basin the third is a negative cie represented in bulk carbonate across the tenuicostatum serpentinum zonal transition which could be correlated with the negative excursion characterizing the onset of the t oae in other sections the position of this excursion corresponds with a gap in the belemnite record finally in the lower toarcian both the bulk carbonate and belemnite carbon isotope records show pronounced positive cies in the lower middle part of the serpentinum zone these cies testify the impact of the t oae in the balearic basin

volcanic eruption of the mid ocean ridge along the east pacific rise crest at 9 45 52 n direct submersible observations of seafloor phenomena associated with an eruption event in april 1991 in april 1991 we witnessed from the submersible alvin a suite of previously undocumented seafloor phenomena accompanying an in progress eruption of the mid ocean ridge on the east pacific rise crest at 9 45 n 52 n the volume of the eruption could not be precisely determined although comparison of pre and post eruption seabeam bathymetry indicate that any changes in ridge crest morphology resulting from the eruption were 10 m high effects of the eruption included 1 increased abundance and redistribution of hydrothermal vents disappearance of numerous vent communities and changes in characteristics of vent fauna and mineral deposits within the eruption area since december 1989 2 murkiness of bottom waters up to tens of meters above the seafloor due to high densities of suspended mineral and biogenic particulates 3 destruction of a vent community by lava flows mass wasting and possible hydrovolcanic explosion at a site known as tubeworm barbecue in the axial summit caldera asc at 9 50 6 n 4 near critical temperatures of hydrothermal vent fluids ranging up to 403 c 5 temporal variations over a 2 week interval in both temperatures and chemical isotopic compositions of hydrothermal fluids 6 unusual compositions of end member vent fluids with ph values ranging to a record low of 2 5 salinities ranging as low as 0 3 wt nacl one twelfth that of seawater and dissolved gases reaching high concentrations 65 mmol l for both co2 and h2s 7 venting at temperatures above 380 c of visually detectable white vapor that transformed to plumes of gray smoke a few centimeters above vent orifices 8 disorganized venting of both high temperature fluids black and gray smoke and large volumes of cooler diffuse hydrothermal fluids directly from the basaltic seafloor rather than from hydrothermal mineral constructions 9 rapid and extensive growth of flocculent white bacterial mats species unknown on and under the seafloor in areas experiencing widespread venting of diffuse hydrothermal fluid and 10 subseafloor downslope migration of magma normal to the ridge axis in a network of small scale 1 5 m diameter lava tubes and channels to distances at least 100 200 m outside the asc we suggest that in april 1991 intrusion of dikes in the eruption area to 200 m beneath the asc floor resulted in phase separation of fluids near the tops of the dikes and a large flux of vapor rich hydrothermal fluids through the overlying rubbly cavernous lavas low salinities and gas rich compositions of hydrothermal fluids sampled in the eruption area are appropriate for a vapor phase in a seawater system undergoing subcritical liquid vapor phase separation boiling and phase segregation hydrothermal fluids streamed directly from fissures and pits that may have been loci of lava drainback and or hydrovolcanic explosions these fissures and pits were lined with white mats of a unique fast growing bacteria that was the only life associated with the brand new vents the prolific bacteria which covered thousands of square meters on the ridge crest and were also abundant in subseafloor voids may thrive on high levels of gases in the vapor rich hydrothermal fluids initially escaping the hydrothermal system white bacterial particulates swept from the seafloor by hydrothermal vents swirled in an unprecedented biogenic blizzard up to 50 m above the bottom the bacterial proliferation of april 1991 is likely to be a transient bloom that will be checked quickly either by decline of dissolved gas concentrations in the fluids as rapid heat loss brings about cessation of boiling and or by grazing as other organisms are re established in the biologically devastated area

reconstruction of vertical temperature gradients in past oceans proxy data from the hauterivian early barremian early cretaceous of the boreal realm abstract a total of 25 mudstone samples from two sections in northwest germany and northeast england palaeolatitude 40 n respectively of hauterivian and early barremian age were analysed using tex86 palaeothermometry in addition the stable isotope ratio 18o 13c and trace element content mg sr fe and mn of 138 belemnite guards from the same two outcrops were determined to reconstruct past sea water temperatures the tex86 based sea surface temperatures were constantly warm 24 26 c throughout the entire hauterivian for both sections and even warmer for the early barremian 27 30 c stable water temperatures prevailed over a period of 6 4 ma in the southernmost part of the boreal realm these findings clearly support the view of warm equable conditions for the hauterivian at least for mid latitudinal settings a constant off set between higher tex86 derived temperatures and lower 18obel derived temperatures was observed this may be explained by either increased sea water salinities a deep dwelling habitat of belemnites or a combination of both a by 3 higher salinity of the sea water below the thermocline which would at least partly explain the by 2 too positive 18obel values cannot be ruled out in these epicontinental settings a habitat of the belemnites below the thermocline in perhaps 50 100 m water depth may have contributed to the positive 18obel data as well the temperature difference between the tex86 and the 18obel findings might therefore reflect a vertical temperature gradient of the water column of 4 5 c and a salinity of the subsurface waters of 38

origin of volcanic seamounts at the continental margin of california related to changes in plate margins volcanic samples collected with the monterey bay aquarium research institute s rov tiburon from eight seamounts at the continental margin offshore central to southern california comprise a diverse suite of mainly alkalic basalt to trachyte but also include rare tholeiitic basalt and basanite all samples experienced complex crystal fractionation probably near the crust mantle boundary based on the presence in some of mantle xenoliths incompatible trace elements poorly correlated with isotopic compositions suggest variable degrees of partial melting of compositionally heterogeneous mantle sources ranging from morb like to relatively enriched oib high precision 40ar 39ar ages indicate episodes of volcanic activity mainly from 16 to 7 ma but document one eruption as recent as 2 8 ma at san juan seamount synchronous episodes of volcanism occurred at geographically widely separated locations offshore and within the continental borderland collectively the samples from these seamounts have age ranges and chemical compositions similar to those from davidson seamount identified as being located atop an abandoned spreading center these seamounts appear to have a common origin ultimately related to abandonment and partial subduction of spreading center segments when the plate boundary changed from subduction dominated to a transform margin they differ in composition age and origin from other more widespread near ridge seamounts which commonly have circular plans with nested calderas and from age progressive volcanoes in linear arrays such as the fieberling guadalupe chain that occur in the same region each volcanic episode represents decompression melting of discrete enriched material in the suboceanic mantle with melts rising along zones of weakness in the oceanic crust fabric the process may be aided by transtensional tectonics related to continued faulting along the continental margin

source characteristics of hypothetical subduction earthquakes in the northwestern united states historic earthquake sequences on subduction zones that are similar to the cascadia subduction zone are used to hypothesize the nature of shallow subduction earthquakes that might occur in the northwestern united states based on systematic comparisons of several physical characteristics including physiography and seismicity subduction zones that are deemed most similar to the cascadia subduction zone are those in southern chile southwestern japan and colombia these zones have all experienced very large earthquake sequences and if the cascadia subduction zone is also capable of storing elastic strain energy along its greater than 1000 km length then earthquakes of very large size m w 8 1 2 must be considered circumstantial evidence is presented that suggests but does not prove that large subduction earthquakes along the cascadia subduction zone may have an average repeat time of 400 to 500 yr

the mechanism of surtseyan volcanism during eruption the vent of a surtseyan volcano is occupied by a highly mobile slurry of tephra hyaloclastite and water as magma ascends rapidly through the slurry mixing occurs due to velocity shear acceleration of the fluid fluid contacts and mass incorporation consequent expansion of the admixed water causes the jetting and continuous up rush activity that characterizes surtseyan volcanism there is no evidence of the widely accepted fuel coolant interaction mechanism

warming in the agulhas current system since the 1980 s 1 since the 1980 s the sea surface temperature of the agulhas current system has increased significantly the warming is due to an augmentation of its transport in response to an increase in wind stress curl in the south indian ocean at relevant latitudes this causes an increase in the fluxes of salt and heat into the atlantic ocean and in the transfer of energy from the ocean to the atmosphere therefore the changes we are witnessing in the region could have far reaching consequences on top of the regional impacts on ecosystem and climate the increase in wind stress curl is consistent with a poleward shift of westerly wind in the southern hemisphere reported by others

lava fountain heights at pu u o o kilauea hawaii indicators of amount and variations of exsolved magma volatiles we review the factors determining lava fountain heights in hawaiian style basaltic eruptions such as those occurring during the recent series of eruptive episodes at the pu u o o vent east rift zone of kilauea volcano hawaii numerical solutions to the equations describing the fluid dynamics of such eruptions predict that lava fountain heights which are indicators of the velocity of magmas in vents should be controlled much more strongly by amounts of exsolved volatiles than by any other factors the next most important factor is the width of the conduit system which determines frictional losses and can be characterized by the volume flux of magma the diameter of the surface vent required to accommodate a given discharge i e instantaneous volume rate of eruption is also a function of exsolved magma volatile content but is less dependent on this factor than is the fountain height we simulate fountain heights for typical discharges at pu u o o and find implied exsolved gas contents very close to those determined by other methods the corresponding predicted vent diameters are comparable with and somewhat smaller than the observed diameter of the pu u o o vent the discrepancy is probably a consequence of processes occurring in the lava pond which is present over the vent during eruptions the variation of fountain height with time during eruptive episodes can be used as a measure of changing exsolved magma volatile content which can be related to processes occurring in the subsurface dike system

temporal evolution of overlapping spreading centers at 16 20 n on the east pacific rise abstract a compilation of sea beam data on the right stepping overlapping spreading center osc at 16 20 n on the east pacific rise epr and in the surrounding region is presented sea beam data from the rise flanks at 16 20 n reveal three deep basins at least one of which is flanked by a curved ridge whose morphology is similar to that at the offset the widths of these offaxis basins range from 8 to 13 km and allow an estimate to be made of the sizes of the offsets at which they were formed it is proposed that they represent traces on the rise flank which provide a record of the development of the offset and a basis for prediction of its future evolution the repeated pattern of abandoned overlap basins and cuspate ridges supports the notion that oscs are episodic on a time scale of approximately 200 000 years and are cyclic in nature it also reveals an alternation of dominance between the southern and northern segments which have shifted the location of the discontinuity along the axis so as to create zig zag traces of basins on the ridge flanks

gas rich submarine exhalations during the 1989 eruption of macdonald seamount in january 1989 we observed submarine eruptions on the summit of macdonald volcano during a french german diving programme with the ifremer submersible cyana gas streaming of large amounts of ch4 co2 and so2 from summit vents inferred from water column anomalies and observed by submersible was accompanied on the sea surface by steam bursts turbulence red glowing gases and black bubbles comprising volcanic ash sulphur and sulphides chloride depletion of water sampled on the floor of an actively degassing summit crater suggests either boiling and phase separation or additions of magmatic water vapour submersible observations in situ sampling and shipboard geophysical and hydrographic measurements show that the hydrothermal system of this hotspot volcano is distinguished by the influence of magmatic gases released from its shallow summit

core top calibration of δ18o and δ13c of g ruber white and u mediterranea along the southern adriatic coast of italy we carried out an extensive survey of the oxygen and carbon isotope composition of the planktic species g ruber white and of the benthic species u mediterranea along the southern italian coast of the adriatic sea and in the gulf of taranto comparison with seasonal satellite based sea surface temperature maps and water column profiles as well as with several sets of water samples allows estimation of the effect of salinity and different nutrient supply on the 18o and 13c of foraminifera the results indicate that g ruber white while being highly affected by different salinity and nutrient distributions related to circulation patterns of major water masses dominantly record summer temperature conditions u mediterranea reflect the recent environmental conditions such as nutrient supply and bottom water temperature characteristics along the southern italian coast and clearly show the transition from a near coastal eutrophic system to an offshore oligotrophic system

an approach to estimate lower jurassic seawater oxygen isotope composition using δ18o and mg ca ratios of belemnite calcites early pliensbachian northern spain palaeotemperature estimates from the oxygen isotope compositions of belemnites have been hampered by not knowing ancient seawater isotope compositions well enough we have tackled this problem using mg ca as a proxy for temperature and here we present a 2 ma record of paired mg ca and d 18 o measurements of jurassic early pliensbachian belemnites from the asturian basin as a palaeo proxy of seawater oxygen isotope composition from the combined use of the two approaches we suggest a d 18 ow composition of about 0 1 for the jamesoni ibex zones this value may have been increased by about 0 6 during the davoei zone due to the effect of waters with a different d 18 ow composition these findings illustrate the inaccuracy of using a globally homogeneous ice free value of d 18 ow 1 for d 18 ocarbbased palaeotemperature reconstructions our data suggest that previous palaeotemperatures calculated in the region from d 18 o values of belemnites may have been underestimated as the seawater oxygen isotopic composition could have been higher

miocene spreading centre south of isla guadalupe the juan de fuca rivera cocos and nazca plates in the eastern pacific are small fragments remaining from the fragmentation and partial subduction of the long narrow farallon plate which began as early as 55 myr ago1 this fragmentation apparently resulted from interaction of the very irregular pacific farallon ridge with the uneven coastlines of the american plate1 4 and involved the formation of ridge trench transforms5 creation of new spreading centres6 ridge jumps1 7 8 deactivation of subduction zones2 and other phenomena1 we present here magnetic evidence that a fifth small fragment of the farallon plate is preserved in the eastern pacific off the coast of baja california its preservation is the result of an eastward jump of the pacific farallon ridge 12 5 myr ago which was preceded by asymmetric spreading and perhaps pivotal subduction1 menard1 first suggested this idea and showed that anomaly 5b is symmetric and parallel to a linear trough trending southwards from guadalupe island he named the small fragment of the farallon plate preserved between this trough and the margin of baja california to the east the guadalupe plate1 we show that both anomalies 5b and 5c symmetrically flank the guadalupe trough our results and conclusions are not based on new magnetic data but rather on reinterpretation of aeromagnetic profiles published by taylor et al 9

anthropogenic impact on agulhas leakage recent work suggests that changes of the southern hemisphere sh winds led to an increase in agulhas leakage and a corresponding salinification of the atlantic climate model projections for the 21st century predict a progressive southward migration and intensification of the sh westerlies the potential effects on the ocean circulation of such an anthropogenic trend in wind stress are studied here with a high resolution ocean model forced by a step function change in sh wind stress that involves a 7 increase in westerlies strength and a 2 shift in the zero wind stress curl the model simulation suggests a rapid dynamic adjustment of agulhas leakage by 4 5 sv about a third of its original value after a few years the change in leakage is reflected in a concomitant change in the transport of the south atlantic subtropical gyre but leads only to a small increase in the atlantic meridional overturning circulation amoc of o 1 sv after three decades a main effect of the increasing inflow of indian ocean waters with potential long term ramifications for the amoc is the salinification and densification of upper thermocline waters in the south atlantic which extends into the north atlantic within the first three decades

kinematic evolution of the northern cocos plate as the east pacific rise approached and impinged upon the western margin of north america the farallon plate began to break into multiple smaller plates whose relative movements changed strongly from those before breakup movements of the northern cocos plate inferred from magnetic lineations show rapidly changing rates and pole positions which may be characteristic of other small remnants of the farallon plate finite difference poles and rates for cocos pacific relative motion during the time intervals 2 5 5 7 7 10 and 10 14 m y have been determined from the geometry of magnetic isochrons prior to about 2 m y the cocos pacific pole was 10 to 20 south of its present location and angular rates were 2 4 times the present rate from 10 7 m y we deduce a large degree of oblique convergence between the cocos and north american plates which may be an important constraint on the evolution of the structure of the continental margin of southwestern mexico the tehuantepec ridge originated as the ridge ridge transform which generated the clipperton fracture zone spreading rate estimates for the southern part of the cocos plate suggest the possibility that the tehuantepec ridge was reactivated as a ridge trench transform between the two cocos plate fragments prior to about 7 m y

comparison of in situ bottom pressure data with grace gravimetry in the crozet kerguelen region two time series of deep ocean bottom pressure records bprs in between the crozet islands and kerguelen are compared with grace gravity recovery and climate experiment equivalent water heights an analysis of the correlation is performed for four time series 1 monthly averages of the equivalent water height at the crozet islands 2 the same near the kerguelen islands 3 the mean of the two preceding series and 4 the difference between the two locations expressed in terms of geostrophic transport we find that smoothed grace solutions are strongly correlated with the bpr data with correlation coefficients in the order of 0 7 0 8 consequently grace measures real oceanic mass variations in this region

cone morphologies associated with shallow marine eruptions east pico island azores eruptions in shallow water typically produce cones of volcaniclastic material in order to identify any systematic effects of water depth and other environmental parameters on cone morphology we have measured the heights and widths of cones in multibeam echo sounder data from a submarine ridge extending southeast from pico island azores xrf analyses of dredged samples show that lavas here vary compositionally from alkali basalt to trachybasalt and trachyandesite cones in deeper water are generally steep sided with upper flanks close to 30 the dip of talus at the angle of repose however height width ratios of cones vary more in shallow water 200 400 m summit depth with extreme values below 0 1 while some shallow water cones are steep sided as in deep water others are much flatter three such cones lie on a bench at 300 m depth immediately east of pico island and have flank slopes of only 10 20 we speculate that exceptionally shallow cone slopes here were produced by forced spreading of the erupting columns on reaching the water air density barrier

cretaceous sea surface temperature evolution constraints from tex86 and planktonic foraminiferal oxygen isotopes it is well established that greenhouse conditions prevailed during the cretaceous period 145 66 ma determining the exact nature of the greenhouse gas forcing climatic warming and climate sensitivity remains however an active topic of research quantitative and qualitative geochemical and palaeontological proxies provide valuable observational constraints on cretaceous climate in particular reconstructions of cretaceous sea surface temperatures ssts have been revolutionised firstly by the recognition that clay rich sequences can host exceptionally preserved planktonic foraminifera allowing for reliable oxygen isotope analyses and secondly by the development of the organic palaeothermometer tex86 based on the distribution of marine archaeal membrane lipids here we provide a new compilation and synthesis of available planktonic foraminiferal 18o 18opl and tex86 sst proxy data for almost the entire cretaceous period the compilation uses ssts recalculated from published raw data allowing examination of the sensitivity of each proxy to the calculation method e g choice of calibration and places all data on a common timescale overall the compilation shows many similarities with trends present in individual records of cretaceous climate change for example both sst proxies and benthic foraminiferal 18o records indicate maximum warmth in the cenomanian turonian interval our reconstruction of the evolution of latitudinal temperature gradients low 48 palaeolatitudes reveals temporal changes in the valanginian aptian the low to higher mid latitudinal temperature gradient was weak decreasing from 10 17 c in the valanginian to 3 5 c in the aptian based on tex86 ssts in the cenomanian santonian reconstructed latitudinal temperature contrasts are also small relative to modern 14 c based on low latitude tex86 and 18opl ssts minus higher latitude 18opl ssts compared with 20 c for the modern in the mid campanian to end maastrichtian latitudinal temperature gradients strengthened 19 21 c based on low latitude tex86 and 18opl ssts minus higher latitude 18opl ssts with cooling occurring at low middle and higher palaeolatitude sites implying global surface ocean cooling and or changes in ocean heat transport in the late cretaceous these reconstructed long term trends are resilient regardless of the choice of proxy tex86 or 18opl or calibration this new cretaceous sst synthesis provides an up to date target for modelling studies investigating the mechanics of extreme climates

sea level changes from topex poseidon altimetry and tide gauges and vertical crustal motions from doris sea level difference topex poseidon minus tide gauge time series have been computed over 1993 1997 at 53 selected gauges sites comparison of these sea level differences with vertical crustal motions derived from the doris space geodesy system at 6 colocated sites shows good consistency at one site the socorro volcanic island a striking correlation is reported between sea level differences and doris height time series the observed trend likely reflects a post eruptive deformation associated with a volcanic eruption that occurred in early 1993 at several other gauge sites the sea level differences present large linear trends possibly bearing evidence of land motion this may be the case at rabaul papua new guinea where a volcanic eruption took place in autumn of 1994 the sea level differences at rabaul show a negative trend from this date likely related to the volcanic event

evolution of submarine eruptive activity during the 2011 2012 el hierro event as documented by hydroacoustic images and remotely operated vehicle observations submarine volcanic eruptions are frequent and important events yet they are rarely observed here we relate bathymetric and hydroacoustic images from the 2011 to 2012 el hierro eruption with surface observations and deposits imaged and sampled by rov as a result of the shallow submarine eruption a new volcano named tagoro grew from 375 to 89 m depth the eruption consisted of two main phases of edifice construction intercalated with collapse events hydroacoustic images show that the eruptions ranged from explosive to effusive with variable plume types and resulting deposits even over short time intervals at the base of the edifice rov observations show large accumulations of lava balloons changing in size and type downslope coinciding with the area where floating lava balloon fallout was observed peaks in eruption intensity during explosive phases generated vigorous bubbling at the surface extensive ash vesicular lapilli and formed high density currents which together with periods of edifice gravitational collapse produced extensive deep volcaniclastic aprons secondary cones developed in the last stages and show evidence for effusive activity with lava ponds and lava flows that cover deposits of stacked lava balloons chaotic masses of heterometric boulders around the summit of the principal cone are related to progressive sealing of the vent with decreasing or variable magma supply hornitos represent the final eruptive activity with hydrothermal alteration and bacterial mats at the summit our study documents the distinct evolution of a submarine volcano and highlights the range of deposit types that may form and be rapidly destroyed in such eruptions

evidence for geomagnetic excursions recorded in brunhes and matuyama chron lavas from the trans mexican volcanic belt 1 this study presents paleomagnetic data from 59 independent lava flows from the trans mexican volcanic belt tmvb with ages from 6 4 ma to recent 52 being younger than 1 ma and 11 new 40ar 39ar age determinations most remanence carriers are ti poor titanomagnetite of pseudosingle domain magnetic structure nine lavas contain small amounts of titanomaghemite and four lavas additional titano hematite paleosecular variation of lava flows younger than 1 7 ma is consistent with latitude dependent model g and also in agreement with other pleistocene paleomagnetic data from the tmvb the directional record of brunhes and matuyama chrons lavas was correlated to the geomagnetic polarity timescale and there is evidence for at least four geomagnetic excursions one lava flow dated at 592 20 ka has a fully reversed paleodirection and most likely erupted during the big lost excursion another fully reversed flow dated at 671 12 ka gives new volcanic evidence for the delta stage 17 excursion this excursion is supported by a reversed intermediate direction of another flow from a different volcanic field but of very close age of 673 10 ka from the matuyama age lavas one flow with normal polarity magnetization dated at 949 37 ka could either be related to the kamikatsura or the santa rosa excursion and a normal polarity flow dated at 1628 56 ka could have been emplaced during the gilsa excursion the results presented here confirm in one case but disagree in four cases with results presented in two previous studies of the same lava flows and interpreted as geomagnetic excursions

the memory of the accreting plate boundary and the continuity of fracture zones a detailed aeromagnetic anomaly map of the mesozoic seafloor spreading lineations southwest of bermuda reveals the dominant magnetic grain of the oceanic crust and the character of the accreting boundary at the time of crustal formation the magnetic anomaly pattern is that of a series of elongate lobes perpendicular to the fracture zone flowline trends the linear sets of magnetic anomaly peaks and troughs have narrow regions of reduced amplitude anomalies associated with the fracture zones during the period of mesozoic geomagnetic polarity reversals when 1200 km of central north atlantic seafloor formed the atlantic accreting boundary consisted of stationary elongate spreading center cells that maintained their independence even though sometimes only minor spatial offsets existed between cells normal oceanic crustal structure was formed in the spreading center cells but structural anomalies and discontinuities characteristic of fracture zones were formed at their boundaries which parallel flowlines of mesozoic relative plate motion in the central north atlantic we suggest that the memory for a stationary pattern of independent spreading center cells resides in the young brittle lithosphere at the accreting boundary where the lithosphere is weakest here each spreading center cell independently goes through its cylce of stress buildup stress release and crustal accretion after which its memory is refreshed the temporal offset between the peaks of the accretionary activity that takes place within each cell may provide the mechanism for maintaining the independence of adjacent spreading center cells through times when no spatial offset between the cells exists

trace elements in anorthite megacrysts from the kurile island arc a window to across arc geochemical variations in magma compositions abstract anorthite megacrysts from twelve volcanic centers of the kurile island arc were analyzed by the ion microprobe for concentrations of li be b f mg p cl k ti fe co rb sr y zr nb cs ba la ce pr nd sm eu and pb these 1 2 cm megacrysts are mainly found as early phenocrysts in basalts but are also present in andesites rhyolites and hybrid rocks from almost every studied volcanic center both subaerial and submarine electron and ion microprobe studies show that the cores of these megacrysts are homogeneous and unzoned whereas rims often result from overgrowth of sodium richer plagioclase in equilibrium with its host silica richer melt the cores of these megacrysts can be used to see back through fractional crystallization and magma mixing and serve as a window into an earlier basaltic stage of magmatic evolution petrological observations and numerical crystallization modeling indicate that highly anorthitic plagioclase is the first or second after olivine liquidus phase in high al basalts regardless of varying k2o contents across the arc given the narrow compositional range of anorthite megacrysts the across arc chemical variations are well displayed by direct comparison of trace element concentrations in the megacrysts across the arc the concentration of k in megacrysts increases linearly by a factor of 11 ba 13 sr eu 4 be ti p 3 li 2 ree 5 2 whereas concentrations of pb b and halogens and their ratios to hfse either remain constant or decrease with deepening of the benioff zone from 110 to 220 km we used ion microprobe derived plagioclase basalt partition coefficients to convert trace element concentrations in megacrysts to parental magmatic values the reconstructed concentrations of several key trace elements and their correlation with benioff zone depth are consistent with whole rock basalt data but all give systematically more primitive compositions therefore anorthite megacrysts are helpful for retrieving the chemistry of parental basalts which have subsequently suffered variable differentiation and mixing they can be used to assess delicate variations in primary basaltic magma chemistry as a function of benioff zone depth we also report new results for sr nd and pb isotopes for anorthite bearing rocks and other rocks from the same volcanoes across arc zoning with respect to trace element concentrations ratios and isotopic values is largely linear and requires superposition of several petrogenetic processes trace element transfer by fluids variation in degree of partial melting and progressive change of the melting source

apparent polar wander of the mean lithosphere reference frame summary apparent polar wander in the mean lithosphere no net rotation no net torque uniform drag reference frame is compared with apparent polar wander in the hotspot reference frame over the past 100 myr palaeomagnetic poles and plate rotations previously used to determine an apparent polar wander path for the hotspot reference frame are here used to determine an apparent polar wander path in the mean lithosphere reference frame we find that the two paths are similar especially for late cretaceous time when a 10 20 shift of the pole occurred to first order the hotspots and lithosphere as a whole moved in unison relative to the palaeomagnetic axis during late cretaceous time a non dipole field explanation for the apparent shift can probably be excluded however either motion of the time averaged geomagnetic axis relative to the spin axis or polar wandering could have caused this shift the latter being the more likely explanation

ocean bottom pressure signals around southern africa from in situ measurements satellite data and modeling 1 ocean bottom pressure obp variability in the region of the agulhas current off the south african coast is a crucial variable in the understanding of dynamic processes in the ocean but measurements currently available lack either precision or spatial and temporal coverage we provide a quantitative estimate of obp variability throughout the region with the help of a setup of the roms regional ocean model driving the model with boundary conditions from a global ocean model and atmospheric reanalysis data and running it for 8 years we are able to reproduce many characteristic properties of the regional ocean circulation visible in sea surface height and obp fields while the in situ pressure inverted echo sounders pies measuring local obp variations on short time scales are sparse in the region our model provides a comprehensive estimate of obp variations throughout the region which reach values of up to 15 hpa when barotropic agulhas rings reach the cape basin these signals turn out to be difficult to measure with current gravimetry solutions from the grace satellites but estimates of localized noise levels for a grace follow on mission let the search for them in future satellite measurements appear viable

submarine eruption near socorro island mexico geochemistry and scanning electron microscopy studies of floating scoria and reticulite abstract products of an underwater eruption near socorro island in the ne pacific were observed directly on january 29 1993 ten days after precursors were first recorded by sofar sound fixing and ranging hydrophones located in hawaii and tahiti eruptive activity was noticed from boats and ships as small steam plumes rising from the sea at an area centered at 18 48 n 111 05 w 2 4 km nw of punta tosca and 4 6 km ssw of cape henslow on socorro island the observed steam was produced by 1 3 m large blocks of hot dark grey highly vesiculated basalt rising buoyantly to the surface from two submarine shallow vents at 210 and 30 m depth tens of blocks accompanied by bubbles could be observed rising to the surface in irregular pulses these scoriaceous blocks remained floating at the surface until they would crack into smaller pieces by thermal contraction emitting hissing noises from vapourizing seawater in contact with the hot interior of the blocks steam jets several metres in height were produced and occasionally blocks were propelled laterally by the steam jet depending on vesicularity and permeability blocks remained floating and drifting with the surface current for 1 15 minutes before sinking back floating rocks covered an area of about 6000 m2 this intermittent activity has been observed ever since and has not stopped as of april 1994 buoyant scoria and reticulite are indicative of volatile mostly co2 supersaturation and exsolution in the magma prior to rapid quenching which inhibits loss of volatiles by bubble escape a high velocity ascent of low viscosity magma in a relatively narrow conduit is also required to prevent substantial gas escape and allow formation of reticulite the buoyant scoria is most probably ejected by intermittent lava fountaining at fixed vents as a result of changes in eruption velocities due to changes in the exsolved gas content of the lava between january and july 1993 floating blocks of scoria and reticulite were collected on several occasions from the surface of the sea for chemical and mineralogical analyses major and minor element analyses including ree as well as electron microprobe analyses of different phases revealed that the composition of the emitted lava has not changed through time blocks of basalt sio2 45 47 are highly vitric and vesicular with tabular anorthite phenocrysts up to 3 mm in length and minor grains of forsteritic olivine ree and trace element composition of these rocks suggest an anomalous mantle source for the erupted alkali basalt lava the ongoing eruption will either continue in a similar fashion as described and eventually cease or built up a mound that reaches the surface and forms an island with accompanying change in eruptive style

observations of mesoscale eddies in the south atlantic cape basin baroclinic and deep barotropic eddy variability 1 anticyclones and cyclones in the eastern south atlantic are characterized based on data collected during january 2003 to march 2005 along a jason 1 satellite altimeter ground track as part of the agulhas south atlantic thermohaline transport experiment large and small cyclones and anticyclones were ubiquitous in the deep ocean of the eastern south atlantic as well as in the upper ocean eddy structures jointly corotating in the upper and deep water column were common most of the time 94 these were not axially aligned as they copropagated the agulhas rings and cyclones that populate the region generally carry both a steric component baroclinic and a mass loading component deep barotropic structure average translation speeds were 7 5 cm s 1 for baroclinic eddies and twice as fast for barotropic eddies irrespective of polarity translation speeds were higher than advection by the mean background flow field in addition large mixed baroclinic barotropic rings crashed into the agulhas ridge and nearby seamounts and split into two or more parts some ring parts were also observed to fuse together deep cyclones as well as interactions with topography were observed to play a role in the fission process of agulhas rings these processes can increase the population of agulhas rings and their remnant eddies which took three pathways from the agulhas and into the cape basin 1 a deep pathway between the continental slope and erica seamount 2 a shallower pathway over or near the agulhas ridge and schmitt otto seamount and 3 a deep seaward pathway around the agulhas ridge

stability of foams in silicate melts bubble coalescence and the spontaneous disruption of high porosity foams in silicate melts are the result of physical expulsion of interpore melt syneresis leading to bubble coalescence and diffusive gas exchange between bubbles melt expulsion can be achieved either along films between pairs of bubbles or along plateau borders which represent the contacts between 3 or more bubbles theoretical evaluation of these mechanisms is confirmed by experimental results enabling us to quantify the relevant parameters and determine stable bubble size and critical film thickness in a foam as a function of melt viscosity surface tension and time foam stability is controlled primarily by melt viscosity and time melt transport leading to coalescence of bubbles proceeds along inter bubble films for smaller bubbles and along plateau borders for larger bubbles thus the average bubble size accelerates with time in silicate melts the diffusive gas expulsion out of a region of foam is effective only for water and even then only at small length scales as the diffusion of co2 is negligible the results of our analyses are applicable to studies of vesicularity of lavas melt degassing and eruption mechanisms

underwater observations of active lava flows from kilauea volcano hawaii underwater observation of active submarine lava flows from kilauea volcano hawaii in march june 1989 revealed both pillow lava and highly channelized lava streams flowing down a steep and unconsolidated lava delta the channelized streams were 0 7 1 5 m across and moved at rates of 1 3 m s the estimated flux of a stream was 0 7 m 3 s jets of hydrothermal water and gas bubbles were associated with the volcanic activity the rapidly moving channelized lava streams represent a previously undescribed aspect of submarine volcanism

a model for attenuation of backscatter due to sediment accumulations and its application to determine sediment thicknesses with gloria sidescan sonar where the seafloor consists of a highly backscattering surface covered with a drape of fine grained sediment and where the sediment does not contribute significantly to the backscattered signal the sediment thickness can potentially be inferred from the amount of signal attenuation in the sediment layer a first order acoustical model is proposed in which the signal strength is reduced by an amount proportional to the sediment thickness and attenuation rate in the sediments data collected with geological long range inclined azdic gloria are used to develop and test this model at an acoustic frequency of 6 5 khz the range of sediment thickness that the sonar can potentially recover is 0 20 m a range that is often below the resolution of vertical profilers in the first example the signal variation with grazing angle over a uniformly buried surface is analyzed for attenuation which is expected to increase toward shallow grazing angles and longer attenuating paths through the sediment layer the surface is a lava flow north of hawaii covered by 1 2 m of fine grained sediments clague et al 1990 and gloria data analyzed with the model suggest an attenuation coefficient of 0 2 0 4 db m a value consistent with results from laboratory and field measurements of sediment attenuation in the second example a rocky surface is buried by varying amounts of sediment which is analyzed at almost constant grazing angle the surface is the volcanic basement of the southeast indian ridge and gloria data reveal the thickening of the sediment cover with distance away from the spreading center this is used to derive an average sedimentation rate of 6 mm ka which is generally consistent with results from other studies and suggests that this technique may be used to study differences in sedimentation rates between different regions furthermore the model is used to calculate from image amplitude distributions the sediment thickness distribution which represents the accumulation of sediments in ponds and the exposure of abyssal hills these sedimentary processes are also reflected in the standard deviation of sediment thickness which increases with seafloor age i explore the various sources of error in estimating sediment thickness using this technique and propose a model for nonuniform sediment drapes the first order model assumes a uniform thickness at the subfootprint scale

does the vorticity flux from agulhas rings control the zonal pathway of nadw across the south atlantic 1 as part of the global thermohaline circulation some north atlantic deep water nadw exits the atlantic basin to the south of africa observations have shown that there is a quasi zonal pathway centered at 25 s carrying nadw eastward connecting the deep western boundary current to the cape basin however it has been unclear what sets this pathway in particular waters must move southward through the cape basin thereby crossing isolines of planetary vorticity in order to exit the basin here we find that an eddy thickness flux induced by agulhas rings moving northwestward forces a circulation of nadw through the cape basin the pathway at 25 s feeds the southeastward flow of this circulation while conserving potential vorticity using lagrangian floats advected for 300 years in a 1 10 resolution ocean model we show that the most common pathway for nadw in our model lies directly below the agulhas ring corridor by analyzing the velocity and density fields in the model we find that the decay of these rings and their forward tilt with depth results in a southward velocity across isolines of planetary vorticity of 1 to 2 cm s in the deep waters the associated stream function pattern yields a deep circulation transporting 4 sv of nadw from the deep western boundary current at 25 s to the southern tip of africa

crustal processes at spreading centers recent discoveries and research on mid ocean ridges mor s in the last four years have greatly increased our knowledge of this fundamental type of plate boundary the last iugg review of mor research was in 1975 aumento et al 1975 and its omission as a topic in 1978 may have reflected a feeling that spreading centers were reasonably well understood since that time however there has been a surfeit of important new observations and exciting research and there is now a turmoil of innovative ideas about the processes which create and leave their imprint on two thirds of the earth s crust new seismic measurements have been made and interpreted as evidence for the existence of shallow axial magma chambers amc beneath intermediate total opening rate 5 9 cm yr and fast 9 18 cm yr spreading centers fig 1 yet there is controversy as to whether they are steady state whether they persist beneath slow ridges as well in the steady state what the dimensions and shape of the chamber are its relation to oceanic crustal stratigraphy and its relation to observations in ophiolites black smoker hydrothermal vents have been discovered in the pacific spewing forth 350 c fluids blackened by polymetallic sulfide precipitates this discovery is embroiled in debate as to how long the vents last whether they occur at slow spreading rates as well whether the enormous heat flux of the vents freeze the magma chamber even at fast spreading rates their influence on geochemical balances in the ocean and the value of the sulfide deposits commercially it has been found that our initial view of mor s as two dimensional linear structures has served us well as an initial working model but must be significantly modified to account for variations in tectonic volcanic and hydrothermal activity along strike as well as for episodicity of these processes deep sea drilling project dsdp samples have raised fundamental questions about formation of the magnetic source layer hydrothermal circulation and fault mechanisms along the mor spreading centers which penetrate existing lithosphere along strike as propagating rifts have modified our views of how plate boundaries reorient themselves and how v shaped magnetic and topographic trends are formed we have also found that transform faults may modify spreading center processes for some distance away from their intersections with mor s and that rigid plate transform faults may fail to develop at fast spreading centers under certain conditions resulting in en echelon overlapping of spreading centers this review will focus on these developments and several others only example references to these topics will be given in the text and many important topics will not be discussed directly e g petrology and geochemistry mantle convection research on lithosphere older than 1 m y however i have tried to compile and have included an extensive bibliography for u s research on a broader range of mor topics for mid 1978 to mid 1982 which is the main purpose of this report pre 1978 references are occasionally included where appropriate abstracts are not included

crustal structure of an extinct rift axis in the labrador sea preliminary results from a seismic refraction survey abstract a preliminary crustal structure for the extinct rift axis in the labrador sea is presented based on a one dimensional analysis of ocean bottom seismometer obs refraction profiles within the extinct rift low p wave velocities are observed for crustal 3 6 6 3 km s and upper mantle 7 3 8 1 km s refractors the velocity depth structures for obss sampling crust within the extinct rift suggest a marked thinning and variability in crustal thickness from 4 7 km to 2 6 km in comparison to typical thicknesses of 6 4 km observed in the adjacent flanks the crustal structures at the extinct rift resemble those observed at the active and inactive transforms of oceanic fracture zones following arguments for crustal thinning at fracture zones we speculate that the first order thinning of the lower crust is due to a reduction in the supply of partial melt to the accretionary axis at the extinct rift the reduced supply is attributed to changes in the thermal field and spreading rate as rifting ceased this is consistent with thermal models which predict that crustal thickness decreases appreciably for spreading rates less than 20 mm yr when plate spreading predominates over buoyancy in driving mantle upwelling

volcanosedimentary paleoenvironments of ediacaran fossils in newfoundland a new perspective on paleoenvironments of ediacaran fossils of the upper conception group newfoundland comes from geochemical and sedimentological study of volcanic tuffs and sedimentary rocks tuffs in the conception group have major and trace element compositions and u pb ages comparable with those of source volcanics on the nearby burin and bonavista peninsulas and the islands of st pierre and miquelon loss of silica and alkalies in some ashes indicates weathering on land not marine diagenesis volcanic crystal and lapilli tuffs fail to show grading and have lapilli and highly vesicular scoria scattered in a fine grained matrix and so they were deposited on land not in water these as well as block and ash flows volcanic spindle bombs and degassing features are evidence of eruptions from nearby subaerial volcanic edifices the fossiliferous conception group accumulated within a forearc basin formed on continental crust inboard of the holyrood horst and uplifted as part of an ancient subduction complex or accreted terrane like analogous forearc basins in oregon washington southern chile and japan the conception group includes not only marine bay turbidites but also a variety of intertidal and terrestrial tsunamites seismites tempestites and paleosols traditional marine turbidite models explain deposition of the mall bay lower drook and lower briscal formations of the conception group but the gaskiers upper drook upper briscal and mistaken point formations were deposited in coastal plains and intertidal zones paleoenvironments of vendobiont fossils preserved in life position in newfoundland were terrestrial to marginal marine not deep sea

syn and post fragmentation textures in submarine pyroclasts from lō ihi seamount hawai i abstract the extent and timing of vesiculation and microlite formation in ascending basaltic melts are relatively well understood for subaerial eruptions but they have not been studied thoroughly for submarine eruptions this is largely due to a lack of samples of appropriate volume and grain size we present a quantitative textural evaluation of coarse lapilli from two cone forming deposits at 1 km depth on l ihi seamount hawai i collected during a sampling intensive submersible dive series in 2006 lapilli from the deposits named the northern and southern cones have modal bulk vesicularities of 55 and 42 respectively and vesicle populations dominated by sub spherical non interconnected vesicles with number densities on the order of 10 4 to 10 5 cm 3 clasts of modal vesicularity have margin parallel zonation with seawater quenched rims interpreted to preserve syn fragmentation magmatic textures in microlite free sideromelane glass enclosing post fragmentation tachylitic interiors with vesicle and microlite textures that progressively coarsen from rim to core degassing scenarios are linked to syn fragmentation vesicle textures to demonstrate that both magmas degassed in dominantly closed systems diffusion controlled cooling rates of basaltic pyroclasts in contact with water are linked to post fragmentation evolution of vesicle and microlite textures to make inferences about transportation and dispersal of the pyroclasts in low submarine jets the textural analysis shows that the northern and southern cone eruptions were not strictly analogous to any classical subaerial eruption type they were uniquely submarine with ambient seawater playing a crucial role in the processes of degassing vesiculation crystallization and dispersal

prolonged history of silicic peralkaline volcanism in the eastern pacific ocean socorro island mexico is an alkaline and peralkaline volcanic island located in the eastern pacific ocean on a mid ocean ridge spreading center that was abandoned at 3 5 ma silicic peralkaline rocks comprise up to 80 of the surface of the island rendering socorro virtually unique in the pacific ocean precise replicate 40ar 39ar ages of 21 peralkaline trachytes and rhyolites reveal a history of episodic volcanic activity from 540 to 370 ka that may have culminated with caldera formation repose periods between these episodes may have had maximum duration of 30 kyr after up to 200 kyr of quiescence 40ar 39ar ages indicate that postcaldera silicic peralkaline activity commenced by 180 ka forming the cerro evermann formation postcaldera mafic alkaline lavas of the lomas coloradas formation erupted dominantly between 70 and 150 ka based upon relative age relations the dominant lithology of precaldera and syncaldera silicic peralkaline deposits on socorro is nonfragmental and nonvesicular and lacks lithic fragments and fiamme despite this numerous lines of evidence including welding zonation presence of a proximal ignimbrite or co ignimbrite deposit association with a caldera and compositional heterogeneity within eruptive units suggest that they are dominantly ash flow tuffs a change in eruptive style from predominantly explosive to predominantly effusive followed caldera formation and suggests that a change in the efficacy of magma degassing may be linked to caldera formation on the basis of the presence of a caldera the magma chamber associated with socorro island is shallow and probably resides within the upper oceanic crust or the edifice this together with a prolonged history of silicic magmatism indicates that intrusion of mafic magma maintained thermal viability of the magmatic plumbing system the minimum calculated growth rate for the entire volcanic edifice 7 10 4 km3 yr exceeds those of nonhotspot off axis volcanoes in the pacific by almost an order of magnitude eruption rates for subaerial phases on socorro may be several orders of magnitude smaller than this growth rate and are comparable to subaerial eruption rates of isolated ocean islands related to mantle plumes

petrologic evolution of large failed rifts in the eastern pacific petrology of volcanic and plutonic rocks from the mathematician ridge area and the guadalupe trough

northern east pacific rise magnetic anomaly and bathymetric framework the oceanic crust in the eastern pacific between 7on and 30on and east of 127ow contains a fairly complete history of the spreading centers associated with the east pacific rise since 25 m y b p late oligocene in this paper we have summarized the seafloor spreading magnetic anomaly data and the bathymetric data that reflect the record of this tectonic history the well defined magnetic lineations north of the clarion fracture zone in the mouth of the gulf of california and on the east flank of the east pacific rise epr are carefully examined and used to provide a guide for interpreting the spreading pattern between the clarion and clipperton fracture zones southward of the rivera fracture zone over the mathematician ridge and over the entire epr east of the mathematician ridge between the rivera and siqueiros fracture zones the bathymetric data provide a trace of the fracture zone pattern in each of the above mentioned areas the fracture zone bathymetry and the seafloor spreading magnetic lineations on the epr south of the rivera fracture zone have a distinctive fanning pattern caused by close poles of rotation and plate boundary reorganizations all these data provide a good record of the plate reorganizations in the middle miocene at magnetic anomaly 5a time 12 5 to 11 m y b p in the late miocene at magnetic anomaly 3 4 time 6 5 m y b p and in the pliocene at magnetic anomaly 2 3 time 3 5 m y b p several abandoned spreading centers including the mathematician ridge were left behind as a result of these reorganizations the mathematician ridge is shown to be a set of ridges and trough whose origin is related to the tectonic activity associated with each of the above mentioned reorganizations since anomaly 5a

dynamics of degassing at kilauea volcano hawaii at kilauea volcano hawaii the recent long lived eruptions of mauna ulu and pu u o o have occurred in two major stages defining a characteristic eruptive pattern the first stage consists of cyclic changes of activity between episodes of fire fountaining and periods of quiescence or effusion of vesicular lava the second stage consists only of continuous effusion of lava we suggest that these features reflect the dynamics of magma degassing in a chamber which empties into a narrow conduit in the volcano chamber gas bubbles rise through magma and accumulate at the roof in a foam layer the foam flows toward the conduit and its shape is determined by a dynamic balance between the input of bubbles from below and the output into the conduit the foam thickness is proportional to lq e2 l g 1 4 where l and l are the viscosity and density of magma e is the gas volume fraction in the foam g is the acceleration of gravity and q is the gas flux the bubbles in the foam deform under the action of buoyancy and the maximum permissible foam thickness is hc 2 e lgr where is the coefficient of surface tension and r is the original bubble radius if this critical thickness is reached the foam collapses into a large gas pocket which erupts into the conduit foam accumulation then resumes and a new cycle begins the attainment of the foam collapse threshold requires a gas flux in excess of a critical value which depends on viscosity surface tension and bubble size hence two different eruption regimes are predicted 1 alternating regimes of foam buildup and collapse leading to the periodic eruption of large gas volumes and 2 steady foam flow at the roof leading to continuous bubbly flow in the conduit the essential result is that the continuous process of degassing can lead to discontinuous eruptive behavior data on eruption rates and repose times between fountaining phases from the 1969 mauna uiu and the 1983 1986 pu u o o eruptions yield constraints on three key variables the area of the chamber roof must be a few tens of square kilometers with a minimum value of about 8 km2 magma reservoirs of similar dimensions are imaged by seismic attenuation tomography below the east rift zone close to the roof the gas volume fraction is a few percent and the gas bubbles have diameters lying between 0 1 and 0 6 mm these estimates are close to the predictions of models for bubble nucleation and growth in basaltic melts as well as to the observations on deep submarine basalts the transition between cyclic and continuous activity occurs when the mass flux of gas becomes lower than a critical value of the order of 103 kg s in this model changes of eruptive regime reflect changes in the amount and size of bubbles which reach the chamber roof

gravity and structure of the continental margins of southwestern mexico and northwestern guatemala geophysical measurements over the eastern end of the tehuantepec ridge and adjacent continental margins of southern mexico and northern guatemala indicate that the ridge is a fracture zone and that it marks the boundary between two different subduction provinces a positive free air gravity anomaly which extends northwestward along the outer continental shelf of guatemala curves abruptly landward in the gulf of tehuantepec the positive shelf anomaly is on trend with the positive anomaly of the nicoya peninsula costa rica and suggests that rocks genetically related to cretaceous rocks of the nicoya complex extend northwestward along the continental shelf to the gulf of tehuantepec a crustal and subcrustal cross section of the continental margin of guatemala constrained by gravity magnetic and seismic refraction data indicates that the rock strata causing the outer shelf gravity high dip landward consistent with imbricate thrusting of the oceanic crust beneath and into the continental margin a model crustal cross section of the continental margin of southern mexico north of the tehuantepec ridge shows a markedly different margin structure with a relatively small amount of continental accretion and a continental crustal block extending to within approximately 25 km of the trench axis

consideration of ocean tides in an ogcm and impacts on subseasonal to decadal polar motion excitation ocean induced changes of earth s rotation are attributed either to tides or to variations of the general circulation analogously numerical world ocean models can still be divided into ocean general circulation and tidal models although a neglect of nonlinear interactions in favor of a linear superimposition of both components of motion is questionable by means of a simultaneous simulation of the ocean s circulation and tides we estimate the importance of two oceanic effects with respect to excitation of polar motion nonlinearities between circulation and long period tides and the circulation induced potential due to loading and self attraction of a baroclinic ocean comparing the linear superimposition of separate model simulations with the simultaneous calculation of circulation and tides it turns out that these second order effects contribute about 8 to ocean induced changes in earth s orientation

trench parallel fluid flow in subduction zones resulting from temperature differences differences in the thermal state of subducting crust along the trench of a subduction zone cause differences in subduction zone temperature that persist to tens of kilometers down dip of the trench the resulting differences in fluid viscosity permeability and hydraulic conductivity can lead to trench parallel variations in fluid pressure on the plate boundary fault temperature differences in locations with low decollement temperature 75 c at the trench result in large differences in fluid viscosity and fluid flow from the cold to the hot side of the system margins characterized by high decollement temperature at the trench are probably dominated by temperature controlled differences in intrinsic permeability resulting in fluid flow from the hot to the cold side of the system margins with large trench parallel temperature differences support considerable trench parallel fluid flow and the effect is accentuated for cases where compaction driven dewatering is concentrated near the trench type locations for along strike hydrologic differences include margins with subducting crust with differences in plate age along the trench or patchy hydrothermal circulation in such cases the three dimensional pattern of subduction zone fluid flow should be considered when inferring the location of the source of fluids sampled from boreholes or seeps additionally trench parallel differences in fluid pressure control effective stress on the plate boundary fault reduced effective stress on the plate boundary on the low hydraulic conductivity side of a subduction zone reduces the effective friction coefficient which both reduces frictional heating and may delay the onset of frictional instability

the submarine eruption and erosion of surtla surtsey iceland abstract surtla is the site of a short lived submarine vent which built basaltic elastic deposits almost to sea level in 1963 early in the eruption of surtsey since then wave and current activity have eroded the volcanic pile such that in july 1981 its top was a fairly level plateau 45 m below sea level and its surface comprised a lag deposit of sparse blocks of lava in a bed mainly of glass granules this winnowed layer was underlain by a nonreworked poorly sorted and finer deposit of glassy clasts formed by a combination of disruption by magmatic volatiles steam explosions and quench brecciation during the eruption the explosion violence and associated comminution increased as the pile built up to shallower water depths it is argued that at times of continuous effusion a cupola of steam was situated over the vent as indicated by scoriaceous spatter which shows agglutination and bread crust features that can only have developed in conditions more akin to subaerial than hitherto envisaged in a subaqueous eruption

calibration and application of the clumped isotope thermometer to foraminifera for high resolution climate reconstructions the reconstruction of past ocean temperatures is fundamental to the study of past climate changes therefore considerable effort has been invested in developing proxies for seawater temperatures one of the most recent and promising new proxy is carbonate clumped isotope thermometry in particular because it is based on thermodynamic equilibrium and not on biogeochemical proxies here we present a new calibration of the clumped isotope thermometer to foraminifera based on seven species of planktic and benthic foraminifera spanning a growth temperature range of 2 28 c we used a newly developed technique for the measurements of small samples to improve the applicability of this method to paleoceanography our data have a comparable precision 0 005 0 013 and confirm previous calibration studies based on biogenic and inorganic calcite we discuss possible sources of uncertainty such as over underestimation of the calcification temperatures species specific vital effects ph variations between the seawater and the vacuole water of the species and possible kinetic effects on the clumped isotope calibration to validate our calibration study and test the applicability of our measuring technique to paleoclimate and paleoceanographic studies we measured the isotope composition of globigerinoides ruber white at high resolution in a sediment core covering the last 700 years in the gulf of taranto mediterranean sea the results show that it is necessary to average a relatively large number of analyses to achieve a consistent temperature signal for the detection of small sea surface temperature changes although with the current analytical system clumped isotope thermometry is only applicable to the analysis of relatively large sst changes in marine sediments further technical improvements may make this a very powerful technique for paleoceanographic studies

serreta 1998 2001 submarine volcanic eruption offshore terceira azores characterization of the vent and inferences about the eruptive dynamics abstract high resolution bathymetric data and seafloor sampling were used to characterize the most recent volcanic eruption in the azores region the 1998 2001 serreta submarine eruption the vent of the eruption is proposed to be an asymmetric topographic high composed of two coalescing volcanic cones underlying the location where lava balloons had been observed at the sea surface during the eruption the volcanic products related to the 1998 2001 eruption are constrained to an area of 0 5 km2 around the proposed vent position a submarine strombolian style eruption producing basaltic lava balloons ash and coarse scoriaceous materials with limited lateral dispersion led to the buildup of the cones the 1998 2001 serreta eruption shares many similarities with other intermediate depth lava balloon forming eruptions e g the 1891 eruption offshore pantelleria and the 2011 2012 eruption south of el hierro revealing the particular conditions needed for the production of this unusual and scarcely documented volcanic product

intraslab seismicity and thermal stress in the subducted cocos plate beneath central mexico we present a model of the subducting cocos slab beneath central mexico that provides an explanation for stresses causing the occurrence of the majority of the intraslab earthquakes which are concentrated in a long flat segment based on the recently developed thermal models for the central mexico subduction zone the thermal stresses due to non uniform temperature contrast in the subducting slab are calculated using a finite element approach the slab is considered purely elastic but due to high temperature at its bottom the behavior is considered as ductile creep the calculation results show a 20 km slab core characterized by a tensional state of stress with stresses up to 70 mpa on the other hand the top of the slab experiences high compressive thermal stresses up to 110 mpa depending on the elastic constants used and location along the flat part of the subducting plate these compressive stresses at the top of the slab are not consistent with the exclusive normal fault intraslab earthquakes and two different sources of stress are proposed the trenchward migration of the mexican volcanic arc for the last 7 ma indicates an increase of the slab dip through time this observation suggests that the gravity torque might exceed the suction torque considering the flat slab as an embedded plate subject to an applied clockwise net torque of 0 5 10 16 1 5 10 16 n m the upper half would exhibit tensional stresses of 40 110 mpa that can actually balance the compressive thermally induced stresses an alternative stress source might come from the slab pull force caused by the slab positive density anomaly based on our density anomaly estimations 75 20 kg m 3 a 350 km slab length dipping at 20 into the asthenosphere induces a slab pull force of 1 7 10 12 4 6 10 12 n m this force produces a tensional stress of 41 114 mpa sufficient to balance the compressive thermal stresses at the top of the flat slab the linear superposition of the thermally and torque or slab pull induced stresses shows tensile stresses up to 60 180 mpa inside the flat slab core also our results suggest that the majority of the intraslab earthquakes inside the flat slab are situated where the resultant stresses are larger than 40 80 mpa this study provides a reasonable explanation for the existence of exclusively normal fault intraslab earthquakes in the flat slab beneath central mexico and also it shows that thermal stresses due to non uniform reheating of subducting slabs play a considerable role in the total stress field

fluidal clast breccia generated by submarine fire fountaining trooper creek formation queensland australia abstract a distinctive monomictic breccia composed of fluidal and blocky basaltic andesite clasts occurs in a cambro ordovician submarine volcanic succession in northern queensland australia associated with this fluvial clast buccia facies are coherent facies and coarse and fine breccia facies of the same composition the fluidal clast breccia facies is internally massive and 250 m thick varying only in the ratio of fluidal clasts to blocky clasts fluidal clasts range in size from 2 cm to 170 cm and have moderately to highly vesicular cores and thick up to 1 cm non vesicular formerly glassy rims blocky clasts are highly vesicular to non vesicular subaqueous fire fountain breccias are distinguished from subaerial fire fountain breccias by thick glassy margins on fluidal clasts the lack of welding and agglutination and the distinctive association of highly vesicular fluidal clasts with non vesicular angular blocky clasts recognition of submarine fire fountain breccias in volcanic successions constrains the eruption style proximity tens of metres to source and environment of deposition

geophysical study of the east pacific rise 15 n 17 n an unusually robust segment bathymetric side scan sonar magnetic and gravity data from the east pacific rise epr between 15 and 17 n are used to establish the spreading history and examine melt delivery to an unusually robust spreading segment the axial ridge between the orozco transform fault 15 30 n and the 16 20 n overlapping spreading center osc has an average elevation of 2300 m which is 300 m shallower than typical epr depths and its cross sectional area is double the average value for the northern epr the total opening rate is 86 km myr but the inflated segment appears to have spread faster to the east by more than 20 since 0 78 ma the orientation of magnetic isochrons and lineaments in the side scan sonar indicates a 3 counterclockwise rotation of the spreading direction since 1 8 ma c2 and reflects a change in the pacific cocos plate motion the side scan lineaments also show that the percentage of inward facing faults 83 and the spacing between faults 1 5 km are consistent with the spreading rate dependence shown by carbotte and macdonald 1994 however the mean fault length 4 8 km is 1 5 km shorter than expected for the spreading rate and suggests that extensive off axis volcanism has draped the faults gravity analysis shows that the inflated segment has a 12 mgal bull s eye shaped low in residual mantle bouguer anomaly we offer several possible end member models for the anomaly including a prism of 10 partial melt in the mantle and lower crust or a crustal thickness anomaly of 2 25 km kinematic modeling that is based on structure and magnetic data suggests that two large magmatic pulses occurred at approximately 0 8 ma and 0 3 ma and have reshaped the plate boundary geometry and inflated the segment

the mathematician paleoplate the pacific seafloor is littered with small fragments of lithosphere captured from adjacent plates by past plate boundary reorganizations one of the clearest examples of such a reorganization is documented in the mathematician seamounts region where a distinctive geomorphology and well developed magnetic anomalies are present this reorganization involved a short lived microplate between the failing mathematician ridge and a new propagating spreading center the east pacific rise it produced a transfer of a fragment of lithosphere from the farallon to the pacific plate and also created a number of landforms and magnetic patterns within and on the margins of the captured fragment these make up the mathematician paleoplate in many cases two sides of a microplate are active spreading ridges a microplate evolves into a paleoplate when dual spreading ceases and full spreading resumes at the prevailing spreading ridge we define a paleoplate as the area of the seafloor from the axis of a failed rift to the boundary of resumed full spreading it includes a fragment of captured lithosphere and the lithosphere slowly accreted to it during the period of dual spreading prior to complete abandonment of the failed rift the mathematician paleoplate has the following boundaries and components from west to east the axis of the mathematician failed rift the fragment of captured farallon plate a complex rift initiation site at the moctezuma trough a zone of slow spreading and an as yet ill defined eastern boundary where dual spreading stopped and full spreading resumed the northern boundary of the paleoplate is the rivera fracture zone its southeastern boundary a now inactive transform fault the west o gorman fracture zone in this case as well as in other more poorly documented ones relict landforms and magnetic patterns are carried on the aging lithosphere away from the spreading ridge recording a former geometry

mechanisms of lithospheric extension at mid ocean ridges summary we examine the extensional deformation of oceanic plates at mid ocean ridges especially within an axial yield zone where pervasive faulting occurs thermal models of ridges are developed which include the effects of lithospheric thickening on the mantle flow the heat of magmatic crustal accretion at the ridge axis and the hydrothermal cooling due to seawater circulation in the crust when hydrothermal circulation occurs the brittle lithospheric plate at slow spreading ridges could be as thick as 8 9 km thicker than the crust while the plate at fast spreading ridges is only 1 2 km for a typical slow spreading ridge several kilometres of plate thickening are expected within a distance of 15 km from the ridge axis when subjected to the extensional force due to horizontal stretching shear failure by normal faulting will occur pervasively in an axial zone where the lithospheric plate is the thinnest adopting perfectly plastic rheology as a continuum description of deformation on the distributed faults we obtain approximate solutions for the stress distribution in the yield zone within this yield zone sea floor topography increases significantly away from the ridge axis so that the resuiting gravity sliding force balances the differential horizontal extensional force due to the thickening of the lithospheric plate basal stresses induced by the viscously deforming asthenosphere could significantly influence the stresses inside the lithospheric plate only if the mantle viscosity beneath the ridge is on the order of 10 pa s significantly higher than generally accepted values of 10 pa s model calculations reveal that although the seafloor topography at the mid atlantic ridge at 13 15 on is regionally compensated it is locally supported by stresses in the lithospheric plate the deviation of the lithospheric plate from its thermal isostatic equilibrium position can be explained by necking due to plastic stretching in the axial yield zone and elastic deflection of strong plates outside the yield zone the best fit models require the yield zones to have a half width of 10 15 km we find systematic variations in the gravity and topography of the east pacific rise which indicate strong influence of plate spreading rate on the ridge thermal and mechanical structure at the 16 17 on area where the half spreading rate is 4 3 cm yr a prominent axial topographic high and an axial mantle bouguer gravity low exist implying a crustal or sub crustal low density body the gravity low disappears at the 20 21 n area where the half spreading rate is 3 6 cm yr as the plate spreading rate decreases from 4 3 cm yr at the 16 17 on area to 2 7 cm yr at the 22 23 on area axial ridge topography changes from higher than the thermal isostatic equilibrium position to lower than the isostatic position the low axial topography at the 22 23 n area can be explained by the existence of a low amplitude median valley

the 1891 submarine eruption offshore pantelleria island sicily channel italy identification of the vent and characterization of products and eruptive style high resolution bathymetry and seafloor sampling have been used to characterize the 1891 submarine eruption of the pantelleria volcanic complex this submarine eruption has been documented mainly by historical reports describing basaltic scoria bombs floating on the sea surface i e lava balloons in this study the 1891 eruptive vent has been identified as a small cone volume of 700 000 m3 rising 90 m from 350 m w d and located within a newly discovered submarine volcanic field covering a wide area offshore from the nw coast of pantelleria recently kelly et al 2012 confirmed this location by a multibeam and rov survey pyroclasts from the 1891 eruption crop out directly on the seafloor and are fresh scoria clasts i e small bombs bomb fragments and lapilli and glass ash sized grains both have been characterized in their morphology textures and geochemistry the distinctive vesicularity and crystallization characteristics displayed by the scoriaceous pyroclasts reflect modes of degassing in both syn and posteruptive regimes these characteristics along with the distribution of deposits suggest for the strongest eruptive phase of the 1891 eruption a style analogous to hawaiian fountaining glass grains from a buoyant plume were dispersed northward from the vent up to distances of 1 5 km redirected by the levantine intermediate water the identification of the 1891 submarine eruptive vent offshore pantelleria as well as the features of erupted pyroclasts improve our knowledge of submarine explosive eruptions that occur at shallow intermediate depths and among these of the rare eruptions producing lava balloons

satellite observations of an annual cycle in the agulhas current 1 ocean models show an annual cycle in the agulhas current transport which has not yet been confirmed in analyses of in situ or satellite observations a cross stream coordinate approach is used to study the variability of the agulhas current from 18 years of along track altimetry and merged altimetry and close to 7 years of high frequency sea surface temperature sst observations while the position and width of the agulhas current s dynamical core do not display an annual cycle the geostrophic current speed at the current s core exhibits distinct seasonal variations with a stronger flow observed in austral summer the annual cycle dominates the frequency spectra of the current s core geostrophic velocities

the submarine eruption of the bombarda volcano milos island cyclades greece the milos volcanic field includes a well exposed volcaniclastic succession which records a long history of submarine explosive volcanism the bombarda volcano a rhyolitic monogenetic center erupted 1 7 ma at a depth 200 m below sea level the aphyric products are represented by a volcaniclastic apron up to 50 m thick and a lava dome the apron is composed of pale gray juvenile fragments and accessory lithic clasts ranging from ash to blocks the juvenile clasts are highly vesicular to non vesicular the vesicles are dominantly tube vesicles the volcaniclastic apron is made up of three fades massive to normally graded pumice lithic breccia stratified pumice lithic breccia and laminated ash with pumice blocks we interpret the apron beds to be the result of water supported volcaniclastic mass how emplacement derived directly from the collapse of a small volume subaqueous eruption column and from syn eruptive down slope resedimentation of volcaniclastic debris during this eruptive phase the activity could have involved a complex combination of phreatomagmatic explosions and minor submarine effusion the lava dome emplaced later in the source area is made up of flow banded lava and separated from the apron by an obsidian carapace a few meters thick the near vertical orientation of the carapace suggests that the dome was intruded within the apron remobilization of pyroclastic debris could have been triggered by seismic activity and the lava dome emplacement

subaqueous eruption and emplacement of ot2 in the middle miocene iizuka formation noto peninsula japan abstract ot2 okada tuff 2 is one of six rhyolitic tuff beds that occur in a middle miocene marine diatomite lower iizuka formation and are known as the okada tuff ot2 is well sorted and thins and thickens infilling channels 1 3 m across and 0 30 cm deep with a thickness up to 30 cm small channels 10 30 cm across and 1 10 cm deep are commonly superimposed within wider channels a thin diatomite layer is ripped from the substrate up to a level immediately above the small channels the basal part beneath the ripped up diatomite layer is dominated by ash components with vague wavy stratifications above the ripped up diatomite layer diatomite clasts are dispersed and pumice lapilli increase upward in amount and size with local interruption at a middle lower level by a poorly defined pumice lapilli poor thin layer many glass and pumice shards are blocky with curviplanar surfaces and jigsaw cracks presumably produced by phreatomagmatic explosions the poorly defined two layers of ot2 are interpreted to be ash turbidites individually produced by discrete phreatomagmatic explosions lenticular bedforms and channels indicate that the currents were fully turbulent and unsteady fed by the submarine eruptions upward coarsening of pumice probably represents settling lag of hot pumice ot2 perhaps provides potential diagnostic features to identify eruption fed turbidites

microearthquake activity on the orozco fracture zone preliminary results from project rose we present preliminary hypocenter determinations for 52 earthquakes recorded by a large multi institutional network of ocean bottom seismometers and ocean bottom hydrophones in the orozco fracture zone in the eastern pacific during late february to mid march 1979 the network was deployed as part of the rivera ocean seismic experiment also known as project rose the orozco fracture zone is physiographically complex and the pattern of microearthquake hypocenters at least partly reflects this complexity all of the well located epicenters lie within the active transform fault segment of the fracture zone about half of the recorded earthquakes were aligned along a narrow trough that extends eastward from the northern rise crest intersection in the approximate direction of the cocos pacific relative plate motion these events appear to be characterized by strike slip faulting the second major group of activity occurred in the central portion of the transform fault the microearthquakes in this group do not display a preferred alignment parallel to the direction of spreading and several are not obviously associated with distinct topographic features hypocentral depth was well resolved for many of the earthquakes reported here nominal depths range from 0 to 17 km below the seafloor

the 2011 2012 submarine eruption off el hierro canary islands new lessons in oceanic island growth and volcanic crisis management forty years after the eruption of the teneguia volcano on la palma 1971 the last volcanic event in the canary islands a submarine eruption took place in 2011 off shore el hierro the smallest an

temperature and productivity influences on u37k and their possible relation to solar forcing of the mediterranean winter to assess the extent and nature of human induced climate change we need to understand natural climate variability this heavily relies on proxy based climate reconstructions calling for excellent understanding of the conditions represented by the proxies here we report on an alkenone based u37k sea surface temperature sst proxy record originating from south italian marine sediments and covering 1305 a d to 1979 a d with a 3 87 year resolution the shallow water cores extracted from the gallipoli terrace in the gulf of taranto were dated by the torino group with high accuracy over the last two millennia using radiometric and tephroanalysis methods on the basis of comparison with historical and satellite derived climate and productivity data we propose that u37k reflects mainly sst of the cooler part of the year and is modified by variation in magnitude and timing of peak alkenone production on the seasonal temperature cycle comparison of the u37k record with the record of atmospheric 14c a proxy for solar energy variability shows a high correlation r 0 73 for the period between 1420 a d and before human interference with the 14c record by combustion of fossil carbon and the release of radioactive isotopes by nuclear tests this suggests a centennial scale solar forcing we propose that wind induced mixing resulting from mediterranean climate dynamics plays a crucial role in translating solar activity to alkenone productivity and sst during the cooler part of the year

the morphology of propagating spreading centers new and old recent seabeam investigations over the east pacific rise in the northeast pacific have shown the existence of a propagating spreading center at 18 n the propagating and dying spreading centers are joined by a zone of orthogonal lineations which appear to be the traces of a continuously migrating transform fault similar orthogonal lineations occur at 16 n 107 30 w on the western side of the trace of a 5 m y old propagating spreading center the amplitude and the complexity of the topographic signature over the earlier propagating spreading center are higher and greater than over the presently active center suggesting that the relief of the earlier center may owe its origin to differential subsidence of thermally contrasting segments of lithosphere analogous to fracture zone topography of similar origin traces of former propagating spreading centers are thus a combination of two sets of landforms those resulting from propagation and those resulting from the juxtaposition of two blocks of lithosphere of different thickness

the structure of basaltic scoria and reticulite and inferences for vesiculation foam formation and fragmentation in lava fountains in this investigation pyroclast structures are used to constrain degassing in basaltic lava fountains vesicle size shape number density interconnectedness and packing character are quantified and related to 1 the kinetics of bubble nucleation and growth 2 the structural evolution of magmatic foams and 3 the influence of vesiculation rate on magma fragmentation measurements made on a diverse suite of pyroclasts from kilauea volcano indicate that basaltic foams evolve through an initially disordered closed celled spherical state to a well ordered open celled polyhedral state as the vesicularity rises from 75 to 98 the structural changes occur rapidly 10 s in the conduit and fountain in response to an intense vesiculation burst vesicle size distribution systematics indicate bubble nucleation rates 2 104 events cm 3s 1 that are approximately three orders of magnitude greater than those found for effusive eruptive activity bubble growth rates 9 10 4 cm s exceed effusive estimates by a factor of 3 the observed runaway rate of bubble production indicates strong supersaturations at the onset of nucleation we speculate that the rise speed of the magma as it reaches the level where significant volatile exsolution begins determines the intensity of the vesiculation burst and hence the vigor of the eruption rapid expansion and acceleration of the magma under these conditions may provide the impetus for fragmentation

potential volcanological applications of the doris system a geodetic study of the socorro island mexico coordinate time series summary we analysed doris data from socorro island mexico providing weekly coordinates in itrf2000 from 1900 to 2006 careful analysis of this time series shows horizontal and vertical movements related to recent activity of the mt evermannn volcano a deflation of the volcano between 1993 and 1996 94 mm yr 1 in the n23 w direction directly pointing in the direction of the crater summit is observed we identify also a clear horizontal discontinuity of 81 mm and a significant change in velocity on 2002 october 3 we use a mogi formalism to provide constraints from our geodetic results on the successive locations and size of the depletion and inflation sources our model is consistent with a deflation source for the period 1993 1996 located beneath mt evermannn at a depth of about 6 2 km below sea level however our data constrain only the azimuth and dip angles of the source which could be located elsewhere along the azimuth of the summit depth and volume varying linearly with the distance

comparison of a stochastic seafloor model with seamarc ii bathymetry and sea beam data near the east pacific rise 13 15 n a recent seamarc ii survey along the hanks and crest of the east pacific rise between 13 and 15 n edwards et al 1988 provides us with a unique opportunity to compare stochastic models of the seafloor against bathymetric data stochastic models can be used to quantitatively characterize intermediate to small scale 100 km seafloor features such as abyssal hills in a recent paper goff and jordan 1988 introduced a model for the two point covariance function having five free parameters which describe the amplitude orientation characteristic wavelengths and hausdorff fractal dimension of seafloor topography these parameters can be estimated from multibeam data such as sea beam by inverting auto and cross covariance functions for the beams the stochastic model can be used to generate synthetic topography to arbitrary resolution and scale by assuming appropriate sea beam noise and response characteristics we can use the synthetic topography to produce synthetic data sets comparison of real and synthetic data is an important means of assessing the performance of the stochastic model unfortunately the limited spatial extent of sea beam in the across track direction and lack of 100 coverage over large areas severely limit our ability to compare the full two dimensional stochastic model with real seafloor morphology especially with respect to abyssal hill length characteristics the recent seamarc ii survey included sufficient off axis topography over 100 km 70 000 km2 to permit a comparison of a complete two dimensional synthetic topographic field with a region of abyssal hill terrain that has close to 100 data coverage we have estimated stochastic models for various regions of the seamarc ii survey area from several sea beam tracks which cross it synthetic fields are generated from these models using a fourier method these comparisons indicate the extent to which the model succeeds in characterizing seafloor topography and the directions we will need to take to improve our modeling a change in ridge complexity at 14 n and the location of a propagator pseudofault appear to be correlated with changes in stochastic character modeled in this region such correlations may provide important new constraints on the understanding of ridge processes

deep ocean warming assessed from altimeters gravity recovery and climate experiment in situ measurements and a non boussinesq ocean general circulation model 1 observational surveys have shown significant oceanic bottom water warming but they are too spatially and temporally sporadic to quantify the deep ocean contribution to the present day sea level rise slr in this study altimetry sea surface height ssh gravity recovery and climate experiment grace ocean mass and in situ upper ocean 0 700 m steric height have been assessed for their seasonal variability and trend maps it is shown that neither the global mean nor the regional trends of altimetry slr can be explained by the upper ocean steric height plus the grace ocean mass a non boussinesq ocean general circulation model ogcm allowing the sea level to rise as a direct response to the heat added into the ocean is then used to diagnose the deep ocean steric height constrained by sea surface temperature data and the top of atmosphere toa radiation measurements the model reproduces the observed upper ocean heat content well combining the modeled deep ocean steric height with observational upper ocean data gives the full depth steric height adding a grace estimated mass trend the data model combination explains not only the altimetry global mean slr but also its regional trends fairly well the deep ocean warming is mostly prevalent in the atlantic and indian oceans and along the antarctic circumpolar current suggesting a strong relation to the oceanic circulation and dynamics its comparison with available bottom water measurements shows reasonably good agreement indicating that deep ocean warming below 700 m might have contributed 1 1 mm yr to the global mean slr or one third of the altimeter observed rate of 3 11 0 6 mm yr over 1993 2008

madagascar a pacemaker for the agulhas current system 1 western boundary currents are driven by zonally integrated wind stress curl over the width of subtropical basins this cross basin integration is interrupted in the south indian ocean where madagascar presents a formidable barrier nevertheless a western boundary current has been thought to exist in the mozambique channel the mozambique current recent observations have however shown that no such current exists and that the flow in the channel instead consists of a train of eddies is this western boundary anomaly due to the presence of madagascar we have used a primitive equations model to investigate the flow in the south west indian ocean as if there were no madagascar we show that a normal continuous western boundary current is then formed that constitutes a continuum with the agulhas current the presence of madagascar is shown to affect the frequency of inter ocean exchange events south of africa

geochemistry of hydrothermal fluids from an intraplate ocean island everman volcano socorro island mexico abstract socorro island mexico still volcanically active is an alkaline and peralkaline volcanic island in the eastern pacific ocean located on a mid ocean ridge spreading center that was abandoned at 3 5 ma gas and water samples collected on the island in 1999 from the dome fumaroles and hot springs of everman volcano have been analyzed for major components h o c s he and ar isotopic composition and noble gas abundances the boiling point temperature steam vents and low salinity thermal springs discharge cl free meteoric water and are typical surface manifestations of a high temperature hydrothermal system high h 2 and ch 4 contents up to 14 and 3 5 vol in dry gas respectively and an enrichment of ch 4 in 13 c 13 c ch 4 18 may suggest that these reduced gases were at least partially produced by the interaction of ultramafic rocks with water inside volcanic edifice due to serpentinization of olivine the 3 he 4 he ratios 6 0 6 6 r a which are lower than the morb values and the high he ne ratios 4000 and he ar ratios up to 4 indicate a contribution of radiogenic he from the oceanic crust or shallow magma chamber like at a low 3 he hot spot the maximum of the 40 ar 36 ar values 311 is close to the maximum value found in the socorro volcanic rocks noble gas abundances are close to values for the partially degassed air saturated water however with some excess ne concentrations an enrichment in 132 xe or depletion in 84 kr of the least air contaminated gas sample he ne 4000 relative to air saturated water can be related to the contribution of noble gases from a non atmospheric source probably oceanic crustal component assimilated in a shallow magma reservoir

structure and eruptive mechanisms at surtsey volcano iceland glassy basalt tuff was the primary material cored in 1979 from a 181 m deep drill hole on the east tuff ring of surtsey volcano despite the fact that the hole extends 122 m below sea level all the core is similar to the exposed tephra composing the two tuff rings of the island the tuff includes abundant accretionary lapilli and tuff vesicles indicating that it was all deposited subaerially during the growth of the tuff rings repeated hydromagmatic explosion cycles began with a series of intermittent tephra finger explosions leading up to continuous uprush explosions which lasted for several minutes to several hours this nozzle like continuous activity produced eruption columns 100 250 m in diameter and 500 2000 m in height which probably quarried several hundred metres below the ground surface the continuous uprush explosion type provides a reasonable mechanism to excavate a diatreme from the top down during construction of the tuff rings concentric faults repeatedly downdropped a funnel like structure 400 800 m in diameter several hundred metres thus accounting for the presence of subaerially deposited tephra in the drill core far beneath sea level ring dykes later intruded upward along these faults and fed small lava flows heat in the surface tephra probably originated primarily from these shallow intrusions