fluid flow and mass transport at the valentine wollastonite deposit adirondack mountains new york state the valentine wollastonite skarn in the north west adirondack mountains new york is a seven million ton deposit which resulted from channellized infiltration of hzo rich silica bearing fluids the wollastonite formed by reaction of these fluids with non siliceous calcite marble the skarn formed at the contact of the syenitic diana complex and was subsequently overprinted by grenville age granulite facies metamorphism and retrograde hydrothermal alteration during uplift calcite marbles adjacent to the deposit have generally high 6 o values c 21 0 typical of grenville marbles which have not exchanged extensively with externally derived fluids carbon isotopic fractiona tions between coexisting calcite and graphite in the marbles indicate equilibration at 675 c consistent with the conditions of regional metamorphism oxygen isotopic ratios from wollastonite skarn are lower than in the marbles and show a 14 variation 1 to 13 some isotopic heterogeneity is preserved from skarn formation and some represents localized exchange with low4 0 retrograde fluids detailed millimetre to centimetre scale isotopic profiles taken across skarn marble contacts reveal steep 8i8o gradients in the skarn with values increasing towards the marble the gradients reflect isotopic evolution of the fluid as it reacted with high 6 o calcite to form wollastonite calcite in the marble preserves high 6 o values to within 5 mm of the skarn contact the preservation of high 6 o values in marbles at skarn contacts and the disequilibrium fractionation between wollastonite skarn and calcite marble across these contacts indicate that the marbles were not infiltrated with significant quantities of the fluid thus the marbles were relatively impermeable during both the skarn formation and retrograde alteration skarn formation may have been episodic and fluid flow was either chaotic or dominantly parallel to lithological contacts although these steep isotope gradients resemble fluid infiltration fronts they actually represent the sides of the major flow system because chromatographic infiltration models of mass transport require the assumption of pervasive fluid flow through a permeable rock such models are not applicable to this hydrothermal system and by extension to many other metamorphic systems where low permeability rocks restrict fluid migration pathways minimum time integrated fluid fluxes have been calculated at the valentine deposit using oxygen isotopic mass balance reaction progress of fluid buffering reactions and silica mass balance all three approaches show that large volumes of fluid were necessary to produce the skarn but silica mass balance calculations yield the largest minimum flux and are hence the most realistic

boron isotopes and b ca in benthic foraminifera proxies for the deep ocean carbonate system abstract accurate records of the state of the ocean carbonate system are critical for understanding past changes in pco 2 ocean acidification and climate the chemical principles underlying the proxy of oceanic ph provided by the boron isotope ratio of foraminiferal carbonate are relatively well understood but the proxy s reliability has been questioned we present 76 new multi collector inductively coupled plasma mass spectrometry mc icpms 11 b measurements on a range of benthic foraminifera from 23 late holocene samples from the atlantic that reaffirm the utility of the 11 b ph proxy our boron isotope measurements on 10 benthic foraminifera tests typically yield a precision of 0 25 at 2 s d equivalent to 0 03 ph units 11 b values of epifaunal species are within analytical uncertainty of those predicted from a simple model assuming sole incorporation of b oh 4 from seawater and no vital effects using the independently determined fractionation factor of 1 0272 between 11 b 10 b of aqueous boron species infaunal foraminifera are consistent with this model but record the combined effects of lower pore water 11 b and ph no influence of partial dissolution or shell size on 11 b is observed we have also measured the b ca ratios of the same samples for individual cibicidoides species b ca shows a good correlation with co 3 2 but the b ca of different co occurring species morphotypes varies considerably these effects are not seen in 11 b which may therefore provide a more robust proxy of the ocean carbonate system whilst in theory 11 b and b ca can be combined to provide a quantitative reconstruction of alkalinity and dissolved inorganic carbonate dic in practice this is precluded by propagated uncertainties 11 b data give significant constraints on foraminifera calcification mechanisms and seem most simply explained by incorporation of b oh 4 into a hco 3 pool which is then completely incorporated in foraminiferal caco 3 our demonstration of the predictable variation of 11 b with ph across a wide range of species and locations provides confidence in the application of mc icpms measurements of foraminiferal 11 b to reconstruct past changes in the ocean carbonate system

seawater ph pco2 and co2 3 variations in the caribbean sea over the last 130 kyr a boron isotope and b ca study of planktic foraminifera abstract here a new analytical methodology is described for measuring the isotopic composition of boron in foraminifera using multicollector inductively coupled plasma mass spectrometry mc icpms this new approach is fast 10 samples analysed in duplicate per analytical session and accurate to better than 0 25 at 95 confidence with acceptable sample size requirements 1 3 mg of carbonate a core top calibration of several common planktic and two benthic species from geographically widespread localities shows a very close agreement between the isotopic composition measured by mc icpms and the isotopic composition of b oh 4 in seawater as predicted using the recently measured isotopic equilibrium factor of 1 0272 at the depth of habitat a down core and core top investigation of boron concentration b ca ratio shows that the partition coefficient is influenced by co2 3 complicating the application of this proxy nevertheless it is demonstrated that these two proxies can be used to fully constrain the carbonate system of surface water in the caribbean sea odp site 999a over the last 130 kyr this reconstruction shows that during much of the holocene and the last interglacial period surface water at site 999a was in equilibrium with the atmosphere with respect to co2 during the intervening colder periods although the surface water pco2 was lower than the holocene it was a minor to significant source of co2 to the atmosphere possibly due to either an expansion of the eastern equatorial atlantic upwelling zone or a more local expansion of coastal upwelling in the southern caribbean such reorganisation of the oceanic carbonate system in favour of a larger source of co2 to the atmosphere from the equatorial ocean may require mechanisms responsible for lowering atmospheric co2 during glacial periods to be more efficient than previously supposed

petrography and genesis of spicular and columnar geyserite from the whakarewarewa and orakeikorako geothermal areas north island new zealand domal masses of geyserite which surround many geyser vents in the whakarewarewa geothermal area are formed largely of spicules spicule columns and shrub columns the non branching spicules and the branches of branching spicules individually up to 3 cm high and 1 mm diameter have a laminated core encased by siliceous cortex silicified microbes are rare in the core but common in the cortex silicified microbial mats and pseudodendrites are found in the crevices between neighbouring spicules shrub columns up to 5 cm high and 1 5 cm in diameter are formed of opal a that was precipitated around a three dimensional branching shrub like microbial structure the shrub branches which are hollow with scalloped walls do not contain any evidence of the original microbes or minerals that formed them silicified microbial mats are present between the columns microbial boring and etching by acidic steam led to local diagenetic degradation of these columns in the geyserite mounds at whakarewarewa spicules and spicule columns are common whereas shrub columns are rare interbedding and intercalation of spicular geyserite with shrub columnar geyserite indicate however that these different morphologic entities probably formed under similar environmental conditions on the mounds around the geyser vents petrographic evidence shows that the spicules spicule columns and shrub columns grew through a combination of biotic and abiotic processes

a study of cleaning procedures used for foraminiferal mg ca paleothermometry the various cleaning steps required for preparation of foraminiferal samples for mg ca and sr ca analysis are evaluated for their relative importance and effects on measured elemental ratios it is shown that the removal of silicate contamination is the most important step for the measurement of mg ca ratios in an example bulk sample mg ca decreases from 10 5 to 2 5 mmol mol 1 during clay removal oxidation of organic material causes a lowering of sample mg ca in the order of 10 or approximately 1 c when converted to temperature use of dilute acid leaching to remove adsorbed contaminants causes partial dissolution of the sample carbonate and a corresponding decrease in mg ca reductive treatment also causes dissolution of the sample and a decrease in the mg ca ratio of up to 10 15 sample preparation for sr ca analysis does not require the same degree of rigor as is necessary for mg ca work the within run reproducibility of the method described here for g ruber in a core top sample from the arabian sea was 1 8 mean sample ratio was 4 72 mmol mol 1 when converted to temperature this becomes 28 0 2 c the equivalent result for sr ca was 0 5 mean ratio 1 44 mmol mol 1

accuracy of stable mg and ca isotope data obtained by mc icp ms using the standard addition method the standard addition method is evaluated to verify the accuracy and precision of mg and ca isotope data with complex matrices using the standard sample bracketing technique and analysis by mc icp ms the 44ca 42ca ratio of seawater expressed as 44 42ca relative to srm915a was determined as 0 93 0 03 95 confidence in agreement with estimates obtained by the double spike method using standard addition the seawater 26mg 24mg ratio expressed as 26mg relative to the dsm3 standard was determined as 0 80 0 06 95 confidence in agreement with previous estimates four terrestrial silicate rocks morb flood basalt glacial flour and granodiorite and olivine mineral separates from an island basalt are shown to exhibit no scatter within the error of the method averaging a 26mg of 0 20 0 05 95 confidence although a number of silicate rock data for mg isotope ratios have already been reported this is the first detailed effort to validate the accuracy of such data and test for residual analytical artifact after chemical purification of samples data regressions were evaluated statistically using the mean square weighted deviate mswd demonstrating that the uncertainty on individual data points are generally over estimated the external two standard deviation uncertainty on individual data points is estimated by monte carlo simulation as b0 075 about a factor of two improvement on early publications of mg isotope data the consistency of the standard addition estimates of 26mg in silicate rocks imply that if any residual matrix effects are present then they must be less than the spread of the data 0 11 given the diverse range of matrices in each of the samples the 26mg values of the silicate rocks suggest that mg isotope ratios in silicate material may only have a very restricted range the 26mg values of silicate material in the present study falls between the average values reported by teng et al teng f z wadhwa m helz r t 2007 investigation of magnesium isotope fractionation during basalt differentiation implications for a chondritic composition of the terrestrial mantle earth and planetary science letters 261 84 92 doi 10 1016 j epsl 2007 06 004 and wiechert and halliday wiechert u halliday a n 2006 non chondritic magnesium and the origins of the inner terrestrial planets earth and planetary science letters 256 360 371 doi 10 1016 j epsl 2007 01 007 and given the spread of published 26mg values for chondritic material a chondritic composition for terrestrial mg cannot be ruled out we suggest that some of the small discrepancies between our data and analysis of the same samples in earlier studies may have arisen because the chemical purification of mg prior to analysis can easily induce analytical artifact this method could be expanded to the isotope ratios of other elements which also rely on correcting for mass bias using the standard sample bracketing method where similar analytical discrepancies may also exist

empirical relationship between ph and the boron isotopic composition of globigerinoides sacculifer implications for the boron isotope paleo ph proxy culture experiments were carried out with the planktonic foraminifera globigerinoides sacculifer at three different ph values 7 6 0 05 8 20 0 05 8 60 0 05 in order to establish the ph dependence of the boron isotopic fractionation between the calcitic tests of g sacculifer and seawater additional objectives were to compare the ph dependences of boron isotopic composition of g sacculifer with that of orbulina universa and inorganic calcite in order to evaluate the robustness of the boron isotope paleo ph proxy the relationship between the boron isotopic composition of g sacculifer and the culture water ph shows heavier boron isotopic values at higher ph this empirical relationship is similar but slightly offset by 3 per mil from that of o universa indicating the presence of biogenic influence vital effect on the boron isotopic composition of foraminifera however comparison of the boron isotopic composition of o universa and g sacculifer with that of inorganic calcite grown over a common ph range shows that inorganic processes are the dominant control on the boron isotopic composition of foraminifera and the vital effects play a small but not insignificant role the results imply that in spite of a small biogenic influence the variation in boron isotopic composition of foraminiferal shells with ph at least for o universa and g sacculifer is consistent with that for inorganic calcite demonstrating the potential of this isotopic signature in foraminifera as a reliable paleo ph proxy

correlation of geoid and depth anomalies over the agulhas plateau abstract correlations of geoid anomalies with bathymetry are an important source of information in the compensation of submarine features we have used this technique to constrain models of the density structure of the lithosphere beneath the agulhas plateau south of africa we explain the correlation in terms of a two layer airy isostatic model the plateau is compensated in part by crustal thickening and in part by an underlying layer of anomalously low density mantle the anomalous mantle layer extends to a depth of about 50 km and has a density 1 2 lower than normal mantle crustal thickening alone is insufficient to explain the observed correlation between bathymetry and the geoid the anomalous mantle region may be the source for the excessive volumes of basalt which have intruded and thickened the crust of the plateau there is strong evidence that depleted mantle rock is lighter than undepleted mantle rock a similar model may hold for the walvis ridge

saturated and subcooled hydrothermal boiling in groundwater flow channels as a source of harmonic tremor the potential of hydrothermal boiling in groundwater flow channels for generating harmonic tremor a relatively monochromatic ground vibration associated with volcanic activity is examined we use simple organ pipe theory of normal mode fluid vibration and fundamental energy considerations to develop a first order analytical model of a hydrothermal boiling source of harmonic tremor we use this model to estimate order of magnitude groundwater flow channel lengths and boiling heat transfer rates required to produce harmonic tremor with dominant frequencies in the range 0 5 5 hz and surface wave reduced displacements of up to 100 cm2 depending on groundwater sound speed flow channel lengths of the order of 1 1000 m are required to produce fluid vibration eigenfrequencies in the range 0 5 5 hz the boiling heat transfer rate required to produce tremor with a given surface wave reduced displacement depends on the tremor frequency and on whether saturated boiling or subcooled boiling is the cause of the tremor saturated boiling produces groundwater vibration via steam bubble growth whereas subcooled boiling produces groundwater vibration via steam bubble collapse we find that subcooled hydrothermal boiling is from 102 to 104 times more efficient than saturated boiling in converting boiling thermal power to seismic power for example the boiling heat transfer rates required to produce 1 hz tremor with reduced displacements of up to 100 cm2 via subcooled boiling are generally less than a few thousand megawatts for saturated boiling the required boiling heat transfer rates are several orders of magnitude larger than this the highest values of heat flow reported in the literature for volcanic crater lakes and terrestrial and ocean floor geothermal areas are of the order of 1000 mw taking this value as a first order estimate of an upper limit on possible boiling heat transfer rates in volcanic hydrothermal systems our results suggest that saturated hydrothermal boiling is capable of generating only low amplitude harmonic tremor with surface wave reduced displacements no higher than a few square centimeters however subcooled hydrothermal boiling could potentially generate high amplitude harmonic tremor with reduced displacements as large as several hundred square centimeters as a specific application of our model we evaluate the potential of hydrothermal boiling for generating harmonic tremor at recently active mount st helens and nevado del ruiz volcanoes we conclude that subcooled boiling likely could have produced the tremor episodes considered at both volcanoes saturated boiling also could explain the nevado del ruiz tremor but probably not the more powerful mount st helens tremor

oceanic ph control on the boron isotopic composition of foraminifera evidence from culture experiments culture experiments were carried out with the species orbulina universa at four different ph values 7 70 0 05 8 15 0 05 8 60 0 05 and 9 00 0 10 in order to establish the ph dependence of boron isotope fractionation between seawater and foraminifera a clear relationship between the boron isotopic composition of the foraminifera and the ph of the seawater culture solutions was found showing heavier boron isotopic composition at higher ph this finding supports the viability of boron isotopes as a paleo ph tool it is important to note that orbulina universa cultured in natural seawater as well as those obtained from coretop samples have a significantly lighter boron isotopic ratio than globigerinoides sacculifer from coretop samples suggesting that at least for this species a vital effect is active

bubble collapse as the source of tremor at old faithful geyser old faithful geyser yellowstone was used as a natural laboratory for fluid flow induced seismic activity pressure measurements within the geyser s water column obtained simultaneously with seismic measurements on the surface demonstrated that the tremor observed at old faithful results from impulsive events in the geyser tremor intensity is controlled by the rate of occurrence of these impulsive events there is no resonance observed within the water column the impulsive events are modeled by a collapse of a spherical bubble including the effects of residual non condensible gas and damping the pressure data can be explained by a collapse of a 5 cm radius bubble driven by a pressure difference of p 0 3 10 5 pa and effective viscosity e 0 04 m 2 s using a quasi static geyser model we treat the individual bubble collapses as cooling events that occur when the water column reaches a critical temperature their rate of occurrence is controlled by the heating rate dt dt of the water column as a result the intensity of the hydrothermal and seismic activities is determined by the heat and mass input rate into the geyser it is demonstrated that a sharp widening of the conduit can cause the number of events per unit time to drop as observed while the water level is still rising and heat is being input and thus the tremor intensity can be modulated by variations in the conduit shape

surface ocean ph response to variations in pco2 through two full glacial cycles abstract knowledge of past atmospheric pco 2 is important for evaluating the role of greenhouse gases in climate forcing ice core records show the tight correlation between climate change and pco 2 but records are limited to the past 900 kyr we present surface ocean ph and pco 2 data reconstructed from boron isotopes in planktonic foraminifera over two full glacial cycles 0 140 and 300 420 kyr the data co vary strongly with the vostok pco 2 record and demonstrate that the coupling between surface ocean chemistry and the atmosphere is recorded in marine archives allowing for quantitative estimation of atmospheric pco 2 beyond the reach of ice cores

boron and magnesium isotopic composition of seawater the isotopic systems of boron and magnesium are increasingly being used as proxies for a number of environmental variables and processes the isotopic composition of seawater for both systems plays a central role in these studies and is an important interlaboratory standard given the long residence times of both elements 10 7 years it is commonly assumed that seawater is isotopically homogenous for these systems yet no systematic studies currently exist here we present the b and mg isotopic composition of 26 28 seawater samples from a number of ocean basins that encompass a significant range in salinity 32 to 38 psu temperature 0 3 to 25 9c and water depth 0 to 1240 m we find no significant or systematic variation for either system in accordance with their long residence times we recommend that the mean values we report 11 b 39 61 0 04 2 s e n 28 25 mg 0 43 0 01 2 s e n 26 26 mg 0 82 0 01 2 s e n 26 be used in future studies involving mg and b isotopes copyright 2010 by the american geophysical union

absolute and relative plate motions and hypotheses on the origin of five aseismic ridges in the indian ocean abstract paleopositions of the african indian and antarctic plates are used as an independant set of data to test various hypotheses derived from geophysical data on the origin emplacement mechanism and evolution of five aseismic structures of the western and southern indian ocean crozet and kerguelen plateaus marion dufresne lena and ob seamount chain madagascar and mascarene ridges except for a continental fragment of very limited extent bearing the seychelles islands these structures together with the madagascar and mascarene ridges are probably due to anomalous volcanic episodes at or near active plate boundaries rather than to intraplate emplacement their creation processes are therefore mainly related to the relative motions between the antarctic african and indian plates the volcanic episodes are synchronous with major changes in the relative motions between these plates and thus substantiate the role played by frequent irregularities in the kinematic pattern of the western and southern indian ocean such as ridge jumps asymmetric spreading and rapid variations in spreading velocity or direction finally we think that thermal anomalies located not far from active spreading boundaries may have played a role in the physical processes creating the aseismic ridges

seawater ph control on the boron isotopic composition of calcite evidence from inorganic calcite precipitation experiments experiments involving boron co precipitation with calcite have been carried out inorganically under controlled ph conditions 7 9 0 05 8 3 0 05 and 8 6 0 05 to determine the dependence of the boron isotopic composition 11b of calcite on the ph of seawater another purpose of these experiments was to estimate the magnitude of the biogenic influence on the 11b value of foraminifera by comparing their boron isotopic composition with that of the inorganic calcite over a common ph range the results show a clear relationship between 11b of inorganic calcite and the ph of artificial seawater the variation of boron isotopic fractionation between seawater and calcite with ph estimated from these experiments is similar to that estimated for cultured o universa and the theoretically predicted trend the results also support the hypothesis that b oh 4 is the dominant species incorporated into the calcite structure however the boron isotopic fractionation between seawater and inorganic calcite is lower than that estimated for o universa indicating the presence of a biogenic effect on the boron isotopic composition at least of this species of foraminifera most importantly the results imply that in spite of a small biogenic influence on the boron isotopic composition of foraminifera the variation in 11b of foraminiferal shells with ph at least for o universa is comparable to that for inorganic calcite supporting the potential of this isotopic signature in foraminifera as a reliable paleo ph proxy

rapid in situ silicification of microbes at loburu hot springs lake bogoria kenya rift valley microbial mats located along the margins of hot spring pools and outflow channels at lake bogoria kenya are commonly silicified forming friable laminated crusts columnar microstromatolites composed of silica and calcite are also forming at several springs in sites of oscillating water level or spray silicification of the microbes involves impregnation of organic tissue by very fine amorphous silica particles and encrustation by small 2 m silica spheroids rapid silicification of the microbes which may begin while some are still alive can preserve sheaths and in some examples the filaments capsules and cells although this provides evidence of their general morphology the biological features that are required for taxonomic identifications are commonly poorly preserved the silica precipitation results mainly from evaporative concentration and rapid cooling of spring waters that have been drawn upward through the mats and microstromatolites by capillary processes almost all the silica at the loburu springs nucleates on microbial substrates this affinity of silica for functional groups on microbial surfaces contributes to the rapid silicification of the microbes and their preservation in modern and ancient cherts

measuring pressure in the source region for geysers geyser valley kamchatka abstract liquid water and steam that erupt at geysers are provided from deeper reservoirs whose location geometry and pressure are in general poorly known here we report measurements at two geysers from a field experiment in geyser valley kamchatka designed to measure pressure in the reservoir providing water to the geysers water level in the geyser conduit was controlled and the recharge to the geyser conduit was measured by monitoring the discharge needed to maintain the controlled water level recharge is not constant but depends on water level in the geyser from the relationship between water level in the conduit and recharge to the conduit we can estimate the hydraulic head in the reservoir that supplies water to the geyser hydraulic head in the two studied reservoirs is within a couple of meters of the elevation of the geyser vents and the adjacent rivers pressures in the reservoirs are low enough that the reservoirs should not be prone to hydrothermal explosions and explain why flooding of geysers in geyser valley terminated their eruptions

emplacement of afanasy nikitin seamount based on transfer function analysis of gravity and bathymetry data abstract the nature of compensation of the afanasy nikitin seamount in the central indian ocean lithosphere is examined through transfer function analysis of gravity and bathymetry data along five profiles across the seamount the seamount appears to be regionally compensated by the bending of an elastic plate of only 2 5 km thickness from this low elastic thickness it is inferred that the seamount is formed near a spreading axis transform junction

the origin of harmonic tremor at old faithful geyser volcanic tremor the seismic signal which is often associated with eruptions is among the least understood phenomena in seismology large variations in the physical properties of volcanic rocks and fluids along with the complex geological structure make it difficult to obtain reliable source models old faithful geyser yellowstone national park was chosen as a laboratory for studying fluid flow induced seismicity the geyser s cyclic behavior on an hourly time scale and its accessibility make it suitable for a detailed study of seismic behavior between eruptions simultaneously with underwater pressure measurements inside the geyser we observe that sharp pressure pulses inside the water column attributed to steam bubble collapse are followed by distinct seismic events with a sharp onset and a harmonic horizontal motion whose frequency varies spatially but not temporally a superposition of these events creates the appearance of continuous harmonic tremor since the pressure in the water column exhibits no sign of resonance the harmonic motion must be caused by elastic waves reverberating in the solid medium a near surface soft layer is demonstrated to be a possible elastic model the harmonic tremor observed at old faithful seems analogous to some examples of shallow volcanic tremor

analysis of satellite gravity and bathymetry data over ninety east ridge variation in the compensation mechanism and implication for emplacement process 1 we investigate the mode of compensation emplacement history and deep density structure of the ninety east ridge indian ocean using spectral analyses and forward modeling of satellite gravity and bathymetry data we find that the northern 0 10 n and the southern 20 30 s parts of the ridge are flexurally compensated with an effective elastic thickness 15 km whereas the central part 0 20 s is locally compensated furthermore we find that for a part of central block 10 20 s over osborn knoll the compensation depth is unreasonably very high 30 40 km therefore we favor a model with subsurface loading and interpret this to be due to underplating of mafic material at the base of the crust a hypothesis that is supported by seismic results and direct modeling of gravity data along some profiles these results suggest that the northern and southern parts of ninety east ridge were emplaced off to a ridge axis compared to the central one which might have been emplaced on or near a spreading center locally compensated large topography thick underplated crust in the central part near osborn knoll might result from an interaction of a hot spot with the extinct wharton spreading ridge

timescales and mechanisms of fluid infiltration in a marble an ion microprobe study using a recently developed ion microprobe technique a detailed oxygen isotope map of calcite grains in a coarse grained marble has been constructed supported by trace element mn sr fe analysis and cathodoluminescence cl imaging in order to constrain scales of oxygen isotope equilibrium timescales and mechanisms of metamorphic fluid infiltration and fluid sources and pathways results are compared with a previous study of this sample wada 1988 carried out using a cryo microtome technique and conventional oxygen isotope analysis the marble from the high temperature low pressure hida metamorphic belt in north central japan underwent granulite facies followed by amphibolite facies metamorphic events the latter associated with regional granite intrusion the cl imaging indicates two types of calcite a yellow luminescing ylc and a purple luminescing plc variety the ylc which occupies grain boundaries fractures replacement patches and most of the abundant deformation twin lamellae post dates the dominant plc calcite and maps out fluid pathways systematic relationships were established between oxygen isotope and trace element composition calcite type and texture based on 74 18o 16o and 17 trace element analyses with 20 30 m spatial resolution the ylc is enriched in mn and fe and depleted in 18o and sr compared to plc and is much more 18o depleted than is indicated from conventional analyses results are interpreted to indicate infiltration of 18o depleted metamorphic or magmatic fluid initial 18o 9 10 5 along grain boundaries fractures and deformation twin lamellae depleting calcite grains in sr and enriching them in mn and fe the sample is characterised by gross isotopic and elemental disequilibrium with important implications for the application of chromatographic theory to constrain fluid fluxes in metacarbonate rocks

the japan trench and its juncture with the kuril trench cruise results of the kaiko project leg 3 this paper presents the results of a detailed survey combining seabeam mapping gravity and geomagnetic measurements as well as single channel seismic reflection observations in the japan trench and the juncture with the kuril trench during the french japanese kaiko project northern sector of the leg 3 on the r v jean charcot the main data acquired during the cruise such as the seabeam maps magnetic anomalies pattern and preliminary interpretations are discussed these new data cover an area of 18 000 km2 and provide for the first time a detailed three dimensional image of the japan trench combined with the previous results the data indicate new structural interpretations a comparative study of seabeam morphology single channel and reprocessed multichannel records lead to the conclusion that along the northern japan trench there is little evidence of accretion but instead a tectonic erosion of the overriding plate the tectonic pattern on the oceanic side of the trench is controlled by the creation of new normal faults parallel to the japan trench axis which is a direct consequence of the downward flexure of the pacific plate in addition to these new faults ancient normal faults trending parallel to the n65 oceanic magnetic anomalies and oblique to the japan trench axis are reactivated so that two directions of normal faulting are observed seaward of the japan trench only one direction of faulting is observed seaward of the kuril trench because of the parallelism between the trench axis and the magnetic anomalies the convergent front of the kuril trench is offset left laterally by 20 km relative to those of the japan trench this transform fault and the lower slope of the southernmost kuril trench are represented by very steep scarps more than 2 km high slightly south of the juncture the erimo seamount riding on the pacific plate is now entering the subduction zone it has been preceded by at least another seamount as revealed by magnetic anomalies across the landward slope of the trench deeper future studies will be necessary to discriminate between the two following hypothesis about the origin of the curvature between both trenches is it due to the collision of an already subducted chain of seamounts or does it correspond to one of the failure lines of the america eurasia plate boundary

evolution of the southwest indian ridge from the late cretaceous anomaly 34 to the middle eocene anomaly 20 abstract the determination of the motion of antarctica relative to africa is particularly important when considering the breakup of gondwana two models have been proposed that describe the pattern of seafloor spreading between africa and antarctica during the late cretaceous starting at chron 34 84 ma through to the middle eocene chron 20 46 ma in the first model the motion of antarctica relative to africa can be simply described by a rotation about a single pole of rotation in the second model which we favor the relative motion of antarctica and africa is more complex and a major change in spreading direction between chron 32 74 ma and chron 24 56 ma times is required in this paper we present ten plate tectonic reconstructions of the southwest indian ridge that were produced using a new compilation of magnetic bathymetric and satellite altimetry data in combination with interactive computer graphics these reconstructions illustrate that spreading directions started to change at chron 32 74 ma between chrons 31 and 28 69 64 ma spreading was very slow

variations of elastic plate thickness at continental thrust belts after examining 15 reliable estimates of the elastic thickness of the lithosphere where it underthrusts mountain belts in europe asia and the americas we conclude that the strength of continental lithosphere as a function of depth must be very different from that of oceanic lithosphere depending on the state of stress continental lithosphere may be either very much stronger or very much weaker than would be predicted for oceanic lithosphere of the same thermal age we explain this observation as the result of two competing effects a thicker thermal plate beneath the continents which allows a deeper elastic ductile transition for plates more than 100 million years old and a low activation energy for ductile creep in the continental crust which leads to massive failure of the upper elastic lithosphere at high bending stresses the 100 km values for elastic plate thickness of billion year old lithosphere require a thermal plate thickness of at least 250 km the magnitude of apparent thinning of the elastic plate at high stress is consistent with that calculated from a simple yield envelope which includes a ductile zone in the lower crust the low yield strength of continental materials leads to correlations between effective elastic plate thickness and all of the following dip of the underthrust plate radius of curvature of the thrust front in map view and total length of the mountain belt using insight derived from laboratory experiments on the buckling of spherical shells and drawing analogies with the geophysical characteristics of oceanic subduction zones we arrive at the following sequence of causes and effects the primary variable is the dip of the downgoing plate presumably prescribed by its own excess mass and pressure exerted by dynamic flow in the mantle where this dip is steep the rigidity of the plate is weakened due to inelastic yielding the weakened plate can then buckle at small wavelengths into short arcuate mountain belts thus we believe that the mantle ultimately controls the pattern of surface tectonics but that the nonelastic rheology of the lithosphere is the mechanism by which that control is imposed

pattern of shallow ground water flow at mount princeton hot springs colorado using geoelectrical methods abstract in geothermal fields open faults and fractures often act as high permeability pathways bringing hydrothermal fluids to the surface from deep reservoirs the mount princeton area in south central colorado is an area that has an active geothermal system related to faulting and is therefore a suitable natural laboratory to test geophysical methods the sawatch range front normal fault bordering the half graben of the upper arkansas valley is characterized by a right lateral offset at mount princeton hot springs this offset is associated with the chalk cliffs of hydrothermally altered quartz monzonite because fault identification in this area is complicated by quaternary deposits including glacial and fluvial deposits we use dc electrical resistivity tomography and self potential mapping to identify preferential fluid flow pathways the geophysical data over 5600 resistivity and 2700 self potential measurements provide evidence of the existence of a dextral strike slip fault zone fault b responsible for the offset of the sawatch fault a segment of this dextral strike slip fault termed u1 is acting as the dominant vertical flow path bringing thermal waters to a shallow unconfined aquifer upwelling of the thermal waters is also observed at two zones u2 and u3 of an open fracture called fault a this fault is located at the tip of the sawatch fault and is likely associated with an extensional strain regime in this area self potential measurements are used to estimate the flux of upwelling thermal water the upflow estimate 4 1 10 3 m 3 day for the open segment of the fault b and 2 1 10 3 m 3 day for fault a from the geophysical data is remarkably consistent with the downstream mt princeton hot water production 4 3 4 9 10 3 m 3 day at approximately 60 86 c a temperature map indicates that a third upwelling zone termed u4 may exist at the southern tip of the sawatch fault

analogue modeling of instabilities in crater lake hydrothermal systems 1 we carried out analogue experiments on two phase boiling systems using a porous vertical cylinder saturated with water the base of the cylinder was heated and the top was cooled as in a natural hydrothermal system previous work had shown that once the two phase zone reached a certain level thermal instabilities would develop we made measurements of the acoustic energy related to boiling and we found that high levels of acoustic noise were associated with the part of the cycle in which there was upward water movement we repeated our experiments with a cooling water tank at the top of the system representing a crater lake this showed that periodic thermal instabilities still developed in this situation we then compared our analogue measurements to two natural systems known to exhibit periodic behavior there is good agreement between the thermal and acoustic cycling seen in our model and the observations made at inferno crater lake in the waimangu geothermal area new zealand whose level cycles by nearly 10 m with a typical period of 38 days particularly notable is how in both systems high levels of acoustic noise are associated with rising water level the much larger ruapehu crater lake also in new zealand cycled with a period of several months to a year for over a decade prior to the 1995 eruption strong acoustic and seismic energy usually occurred just before the lake temperature started to rise this suggests a slightly different model in which the increasing two phase flow zone triggers more general convection once it reaches the base of the lake

coprecipitation and isotopic fractionation of boron in modern biogenic carbonates abstract the abundances and isotopic composition of boron in modern biogenic calcareous skeletons from the gulf of elat israel the great barrier reef australia and in deep sea sediments have been examined by negative thermal ionization mass spectrometry the selected species foraminifera pteropoda corals gastropoda and pelecypoda yield large variations in boron concentration that range from 1 ppm in gastropod shells to 80 ppm in corals the boron content of the biogenic skeletons is independent of mineralogical composition and is probably related to biological vital effects the 11 b values of the carbonates range from 14 2 to 32 2 relative to nbs srm 951 and overlap with the 11 b values of modern deep sea carbonate sediments 11 b 8 9 to 26 2 the variations of 11 b may be controlled by isotopic exchange of boron species in which 10 b is preferentially partitioned into the tetrahedral species and coprecipitation of different proportions of trigonal and tetrahedral species in the calcium carbonates carbonates with low 11 b values 15 may indicate preferential incorporation of tetrahedral species whereas the higher 11 b values 30 may indicate 1 1 uptake of both boron species assuming equilibrium with seawater 2 2 preferential incorporation of b oh 4 from in situ high ph internal fluids of organisms that are isolated from seawater the b content and 11 b values of deep sea sediments foraminifera tests and corals are used to estimate the global oceanic sink of elemental boron by calcium carbonate deposition as a result of enrichment of b in corals a substantially higher biogenic sink of 6 4 0 9 10 10 g yr is calculated for carbonates this is only slightly lower than the sink for desorbable b in marine sediments 10 10 10 g yr and approximately half that of altered oceanic crust 14 10 10 g yr thus carbonates are an important sink for b in the oceans being 20 of the total sinks the preferential incorporation of 10 b into calcium carbonate results in oceanic 11 b enrichment estimated as 1 2 0 3 10 12 per mil g yr the boron isotope composition of authigenic well preserved carbonate skeletons may provide a useful tool to record secular boron isotope variations in seawater at various times in the geological record the potential use of boron isotope geochemistry in skeletons as a tracer for palaeoenvironments is demonstrated in ostracoda and foraminifera from the gulf of carpentaria australia the 11 b values of glacial age buried skeletons 4 0 and 4 9 respectively are lower than that of their modern equivalents 17 6 and 13 3 respectively this may reflect a terrestrial boron isotope signature of the water in the gulf during the late quaternary when it was isolated from the ocean

an unusual modern silica carbonate sinter from pavlova spring ngatamariki new zealand a silica carbonate deposit is forming from the dilute alkali chloride waters of pavlova spring a small thermal pool and outflow channel 85 to 10 times lower accumulation rates than typical siliceous sinters in the tvz and deposition of both silica and calcite is controlled by microchemical conditions and local temperature gradients rather than by bulk spring water chemistry

the evolution of pco2 ice volume and climate during the middle miocene abstract the middle miocene climatic optimum 17 15 ma mco is a period of global warmth and relatively high co2 and is thought to be associated with a significant retreat of the antarctic ice sheet ais we present here a new planktic foraminiferal 11b record from 16 6 to 11 8 ma from two deep ocean sites currently in equilibrium with the atmosphere with respect to co2 these new data demonstrate that the evolution of global climate during the middle miocene as reflected by changes in the cyrosphere was well correlated to variations in the concentration of atmospheric co2 what is more within our sampling resolution 1 sample per 300 kyr there is no evidence of hysteresis in the response of ice volume to co2 forcing during the middle miocene contrary to what is understood about the antarctic ice sheet from ice sheet modelling studies in agreement with previous data we show that absolute levels of co2 during the mco were relatively modest 350 400 ppm and levels either side of the mco are similar or lower than the pre industrial 200 260 ppm these new data imply the presence of either a very dynamic ais at relatively low co2 during the middle miocene or the advance and retreat of significant northern hemisphere ice recent drilling on the antarctic margin and shore based studies indicate significant retreat and advance beyond the modern limits of the ais did occur during the middle miocene but the complete loss of the ais was unlikely consequently it seems that ice volume and climate variations during the middle miocene probably involved a more dynamic ais than the modern but also some component of land based ice in the northern hemisphere

calibration of the boron isotope proxy in the planktonic foraminifera globigerinoides ruber for use in palaeo co2 reconstruction abstract the boron isotope ph proxy applied to mixed layer planktic foraminifera has great potential for estimating past co2 levels which in turn is crucial to advance our understanding of how this greenhouse gas influences earth s climate previous culture experiments have shown that although the boron isotopic compositions of various planktic foraminifera are ph dependent they do not agree with the aqueous geochemical basis of the proxy here we outline the results of culture experiments on globigerinoides ruber white across a range of ph 7 5 8 2 and analysed via multicollector inductively coupled plasma mass spectrometry mc icpms and compare these data to core top and sediment trap samples to derive a robust new species specific boron isotope ph calibration consistent with earlier culture studies we show a reduced ph dependency of the boron isotopic composition of symbiont bearing planktonic foraminifera compared to borate ion in seawater we also present evidence for a size fraction effect in the 11b of g ruber finally we reconstruct atmospheric co2 concentrations over the last deglacial using our new calibration at two equatorial sites odp site 999a and site geob1523 1 these data provide further grounding for the application of the boron isotope ph proxy in reconstructions of past atmospheric co2 levels

a core top assessment of proxies for the ocean carbonate system in surface dwelling foraminifers we have assessed the reliability of several foraminifer hosted proxies of the ocean carbonate system 11b b ca and u ca using holocene samples from the atlantic and pacific oceans we examined chemical variability over a range of test sizes for two surface dwelling foraminifers globigerinoides sacculifer and globigerinoides ruber measurements of 11b in g ruber show no significant relationship with test size in either atlantic or pacific sites and appear to provide a robust proxy of surface seawater ph likewise there is no significant variability in the 11b of our atlantic core top g sacculifer but we find that 11b increases with increasing test size for g sacculifer in the pacific these systematic differences in 11b are inferred to be a consequence of isotopically light gametogenic calcite in g sacculifer and its preferential preservation during postdepositional dissolution the trace element ratio proxies of ocean carbonate equilibria u ca and b ca show systematic increases in both g ruber and g sacculifer with increasing test size possibly as a result of changing growth rates this behavior complicates their use in paleoceanographic reconstructions in keeping with several previous studies we find that mg ca ratios increase with increasing size fraction in our well preserved atlantic g sacculifer but not in g ruber in contrast to previous interpretations we suggest that these observations reflect a proportionally larger influence of compositionally distinct gametogenic calcite in small individuals compared to larger ones as with 11b this influences g sacculifer but not g ruber which has negligible gametogenic calcite

subaqueous silicification of the contents of small ponds in an early devonian hot spring complex rhynie scotland the early devonian rhynie and windyfield cherts of northeast scotland originated as siliceous sinters deposited by hot springs silicification affected both subaerial and subaqueous environments preserving a diverse terrestrial and freshwater biota cherts originally deposited in small shallow pools can be recognised on the basis of both texture and fossil content textures comprise clotted and microcoprolitic textures bacterial coatings on plant axes that can be stromatolitic and a variety of bacterial and fungal meshworks the crustacean lepidocaris the euthycarcinoid heterocrania the charophyte alga palaeonitella and chytrid fungi are typical elements of the aquatic biota observations of modern hot springs in yellowstone national park wyoming u s a demonstrate that shallow ponds and streams on low angle outwash areas and dormant vent orifices provide suitable environmental analogues textures comparable to those described from rhynie are recorded from yellowstone sinters but examples of the

experimental measurement of boron isotope fractionation in seawater abstract the boron isotopic composition of marine carbonates is considered to be a tracer of seawater ph use of this proxy benefits from an intimate understanding of chemical kinetics and thermodynamic isotope exchange reactions between the two dominant boron bearing species in seawater boric acid b oh 3 and borate ion b oh 4 however because of our inability to quantitatively separate these species in solution the degree of boron isotope exchange has only been known through theoretical estimates in this study we present results of a spectrophotometric procedure wherein the boron isotope equilibrium constant 11 10 k b is determined empirically from the difference in the dissociation constants of 11 b oh 3 and 10 b oh 3 in pure water 0 6 mol kg 1 h 2 o kcl and artificial seawater within experimental uncertainty our results show no dependence of 11 10 k b on temperature but 11 10 k b at 25 c in pure water was statistically different than results obtained in solutions at high ionic strength 11 10 k b of the seawater s 35 b t 0 01 mol kg 1 h 2 o at 25 c is 1 0272 0 0006 this result is significantly larger than the theoretical value used in numerous paleo ph studies 11 10 k b 1 0194

self potential and passive seismic monitoring of hydrothermal activity a case study at iodine pool waimangu geothermal valley new zealand abstract iodine geyser located in the waimangu geothermal valley new zealand has been studied by both passive electrical and seismic methods the activity of the geyser was monitored at various distances from the vent using self potential method the self potential signals display cyclic negative variations with respect to a baseline drawn when the geyser is quiet the minimum in the self potential signals coincides with the maximum overflow we provide a numerical model able to explain both the polarity and magnitude of the observed signal this model is based on the fluctuations of the hydraulic head in the conduit of the geyser the divergence of the streaming current density is created at the interface between the pipe and the surrounding rock passive seismic experiments were used to localize ambient noise sources these signals have been processed with the so called matched field processing technique mfp a dominant source emerged from this processing that we characterized in range and depth with a good accuracy

multiscale matched field processing for noise source localization in exploration geophysics abstractin the study we investigate the use of ambient noise data to locate microseismic sources at the exploration scale we develop a multiscale matched field processing mfp approach to localize seismic sources at frequencies below 10 hz an application to an actual data set acquired over a hydrocarbon field is presented to determine the reliability of the mfp procedure the data used were continuously recorded over five days at a total of 397 stations on a 1 km per side square seismic network the mfp results show 1 a dominant and stable surface source associated with human activities road and exploration platforms around the reservoir and 2 weaker sources at depth below the seismic network that are related to the injection extraction process

geoid height versus topography for oceanic plateaus and swells oceanic plateaus and swells are a major component of the seafloor topography yet they remain among the most poorly understood features this is especially true of the oceanic plateaus which show large variations in crustal thickness to determine the depth and mode of compensation for 53 of the largest plateaus and swells we analyzed the relationship between geoid height and topography in polygonal areas containing each feature both geoid height and topography were first band pass filtered 400 km i 4000 km to isolate the signal associated with local compensation from flexural and deep mantle signals the ratio of geoid height to topography was then determined by fitting a straight line to the data except for nine of the smaller features there is a high correlation between geoid height and topography that is positive in accordance with airy and thermal compensation models eighteen features have high geoid topography ratios that cannot be explained by the airy compensation model of crustal thickening these features thermal swells are partially supported by thermal buoyancy forces in the lower half of the lithosphere the ratios are highest for active hot spot swells and decay with the thermal age of the swell to values consistent with airy compensation of the enduring volcanic edifice the remaining features plateaus have lower geoid topography ratios in agreement with the airy compensation model those plateaus with average height greater than 4 km are thought to be continental fragments the shorter plateaus tend to be volcanic features modified continental plateaus presumably small fragments of extended and intruded continental margin crust cluster around heights of n3 km overlapping the range associated with oceanic plateaus since the origin of many plateaus is poorly understood this global geoid topography analysis provides a new technique for comparing the deep structure of oceanic plateaus and swells

precise location of unsurveyed seamounts in the austral archipelago area using seasat data summary seasat altimetric geoid data are used to detect uncharted seamounts in the austral archipelago area the various physical parameters which affect the geoid signature of a seamount are inspected to analyse their influence on the precision of the location these parameters are the shape of the seamount its density the crustal model the effective elastic thickness of the lithosphere and the distance between the seamount and sub satellite tracks respectively the results of the tests performed on synthetic data as well as on charted structures show that when a seamount is detected on at least two neighbouring tracks it is possible to locate it with good confidence and to give an estimation of its height if the correct elastic thickness is assumed the precision on the location is order 15 km ten previously unsurveyed seamounts have been located in the austral archipelago it appears that they are emplaced along two well defined azimuths n 1 10 e and n 150 e and that two parallel distinct volcanic chains form the austral archipelago

a 23 000 year record of surface water ph and pco2 in the western equatorial pacific ocean the oceans play a major role in defining atmospheric carbon dioxide co2 levels and although the geographical distribution of co2 uptake and release in the modern ocean is understood little is known about past distributions boron isotope studies of planktonic foraminifera from the western equatorial pacific show that this area was a strong source of co2 to the atmosphere between approximately 13 800 and 15 600 years ago this observation is most compatible with increased frequency of la nina conditions during this interval hence increased upwelling in the eastern equatorial pacific may have played an important role in the rise in atmospheric co2 during the last deglaciation

transient self potential anomalies associated with recent lava flows at piton de la fournaise volcano reunion island indian ocean self potential signals are sensitive to various phenomena including ground water flow streaming potential thermal gradients thermoelectric potential and potentially rapid fluid disruption associated with vaporization of water we describe transient self potential anomalies observed over recent 9 years lava flows at piton de la fournaise volcano reunion island indian ocean repeated self potential measurements are used to determine the decay of the self potential signals with time since the emplacement of a set of lava flow we performed a 9 km long self potential profile in february 2004 in the grand brule area this profile was repeated in july august 2006 the second repetition of this profile crossed eight lava flows emplaced between 1998 and 2005 during seven eruptions of piton de la fournaise volcano the self potential data show clear positive anomalies up to 330 mv and spatially correlated with the presence of recent lava flows the amplitude of the self potential anomalies decreases exponentially with the age of the lava flows with a relaxation time of not vert similar 44 months we explain these anomalies by the shallow convection of meteoric water and the associated streaming potential distribution but we cannot exclude possible contributions from the thermoelectric effect and the rapid fluid disruption mechanism this field case evidences for the first time transient self potential signals associated with recent volcanic deposits it can be also a shallow analogue to understand the variation of self potential signals in active geothermal areas and transient self potential signals associated with dike intrusion at larger depths the empirical equation we proposed can also be used to diagnose the cooling of recent lava flow on shield volcanoes

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

microbial silicification in iodine pool waimangu geothermal area north island new zealand implications for recognition and identification of ancient silicified microbes silicified microbes provide evidence for some of the earliest life forms on earth they are extremely important to understanding the early development of life and the conditions that allowed its development such discussions commonly rely on comparisons with extant taxa and therefore depend upon the preservation style of the microbes and in particular the preservation of the taxonomically important features silicified microbes are deceptive they commonly appear to be well preserved even though their taxonomically critical features have been destroyed by silicification an understanding of the early taphonomic processes that influence microbial silicification can be obtained by studying extant microbes that are being silicified in modern hot spring pools iodine pool located in the waimangu geothermal area on the north island of new zealand is ideal for this purpose the spring water has a temperature of 69 100 c a ph of 8 3 9 0 and 440 457 ppm sio 2 glass slides left in the shallow marginal waters of this pool for 90 h became covered with thin layers of opaline silica discrete opal a spheres unsilicified microbes pseudofilaments partly silicified microbes and silicified microbes the rapidly silicified microbes appear well preserved with their general morphology diameter length and presence or absence of septa being readily apparent most of the silicified microbes however lack the key features that would allow accurate comparisons with extant taxa only two of the silicified microbes can be tentatively allied with the unsilicified forms despite being found side by side on the same glass slide these problems in identifying modern rapidly silicified microbes suggest that identifications of ancient silicified microbes can be problematic and must be treated with caution

the plumbing of old faithful geyser revealed by hydrothermal tremor 1 old faithful geyser in yellowstone national park usa has attracted numerous scientific investigations for over two centuries to better understand its geological structure the physics of its eruptions and the controls of its intermittency using data acquired with a seismic array in 1992 we track the sources of hydrothermal tremor produced by boiling and cavitation inside the geyser the location of seismic sources identifies a previously unknown lateral cavity at 15 m below the surface on the sw side of the vent and connected to the conduit this reservoir is activated at the beginning of each geyser eruption cycle and plays a major role in the oscillatory behavior of the water level in the conduit before each eruption

locating hydrothermal acoustic sources at old faithful geyser using matched field processing in 1992 a large and dense array of geophones was placed around the geyser vent of old faithful in the yellowstone national park to determine the origin of the seismic hydrothermal noise recorded at the surface of the geyser and to understand its dynamics old faithful geyser ofg is a small scale hydrothermal system where a two phase flow mixture erupts every 40 to 100 min in a high continuous vertical jet using matched field processing mfp techniques on 10 min long signal we localize the source of the seismic pulses recorded at the surface of the geyser several mfp approaches are compared in this study the frequency incoherent and frequency coherent approach as well as the linear bartlett processing and the non linear minimum variance distorsionless response mvdr processing the different mfp techniques used give the same source position with better focalization in the case of the mvdr processing the retrieved source position corresponds to the geyser conduit at a depth of 12 m and the localization is in good agreement with in situ measurements made at old faithful in past studies

the accuracy of δ11b measurements of foraminifers values of 11b reported in the literature for holocene samples of the same foraminiferal species show large variability 6 relative to cited precision 1 this is indicative of significant inter laboratory biases and raises concerns about the accuracy of foraminiferal proxy ph records to investigate this problem we have measured the 11b of modern ocean carbonates using several different analytical procedures we have used total evaporation negative thermal ionisation mass spectrometry te ntims with various sample preparation and loading procedures and multi collector inductively coupled plasma mass spectrometry mc icpms the 11b of pure boric acid solutions measured by te ntims and mc icpms agree well demonstrating no fundamental biases between the techniques yet the 11b values measured by te ntims for non foraminiferal carbonates are about 2 lighter than those by mc icpms whereas foraminifers measured by the same standard te ntims procedure are 2 to 6 heavier than those by mc icpms significantly we found that foraminiferal samples kept in acidic solution over several months yielded lower 11b values up to 5 lower and better reproducibility when re measured by te ntims we infer that organic material released on foraminiferal dissolution causes biases in ntims measurements no residual organics should be present in the mc icpms measurements as matrix is separated from sample before analysis moreover we demonstrate by standard addition the absence of any matrix influence in the mc icpms procedure beyond the inherent uncertainties in the standard addition approach 0 35 2sigma degradation of the inferred organic residue either by long term storage in acidic solution or by loading samples in 30 h2o2 reduces but does not totally remove the variable bias between te ntims and mc icpms 11b measurements despite these problems careful analysis of similar samples by ntims may permit data to be obtained with consistent relative differences that still yield valuable proxy records but there is clearly considerable potential for inaccuracies to result in such an approach from hard to detect changes in matrix

an integrated geophysical study east of the southern chagos laccadive ridge complex central indian ocean basin implications for the rodriguez triple junction dynamics in the late cretaceous abstract the central indian ocean basin east of the southern chagos laccadive ridge complex has evolved due to seafloor spreading in the n s direction since the late cretaceous the trace of the rodriguez triple junction rtj has been inferred in this basin but its exact location and dynamics prior to c29o are not known about 20 000 km of total magnetic intensity data along with satellite gravity mosaic were used to understand the dynamics of the rtj in the late cretaceous synthetic seafloor spreading models endorse the presence of late cretaceous to early palaeogene magnetic anomalies c34y to c24o in the study area half spreading rates are initially low 32 mm yr between c34y to c33y and increase up to 78 mm yr by c28y thereafter a decreasing trend is seen 57 mm yr towards the west in the study area higher spreading rates 120 mm yr between c33y and c32ay and distorted anomalies between chrons c32ay and c30y are observed detailed geophysical analysis suggests that the higher spreading rates are due to the presence of transferred crust and the distorted anomalies are caused by the presence of fragmented tectonic blocks depicting microplate formation and rotation plate reconstruction models suggest that the rtj at c34y time lay southwest of comorin ridge which coincided with the northern edge of the conrad rise until c33y the rtj left a linear trace parallel to the fracture zone trend on both the indian and antarctic plates while it gradually moved eastwards on the african plate the rtj migrated westwards until c31y causing the transfer of a considerable amount of crust from the african plate to both the indian and antarctic plates thereafter the rtj rapidly reversed its migration direction towards the east until c29o causing the formation and rotation of a microplate on the indian plate and lengthening of the southwest indian ridge the present study suggests that this episodic migration of the rtj may be due to ridge propagation under the influence of the marion hotspot which was in the vicinity during that period this rtj migration resulted in microplate formation lithospheric transfer and thickened lithosphere similar to the inferences made at many triple junctions in the pacific ocean thus the origin of anomalous structures such as the crozet bank may be related to triple junction dynamics under the influence of marion hotspot further the counter clockwise rotation of the southwest indian ridge between c33y and c29y may have a causal link with the formation and rotation of the microplate which occurred during the same period the present study thus facilitated the inference of the dynamics of the rtj trace between c34y and c29o on the three plates

hydroacoustic noise precursors of the 1990 eruption of kelut volcano indonesia abstract the results of a hydroacoustic monitoring experiment in the kelut crater lake indonesia prior to its 1990 eruption are presented with the benefit of hindsight indeed the underwater noise levels in three widely separated frequency bands together with the lake water temperature was radio transmitted and almost continuously recorded from a period of quiescence of the volcano till the onset of its 10 february 1990 eruption which destroyed the monitoring buoy the comparative analysis of the noise variations in the three bands together with seismic and temperature data have shed light on the mechanisms underlying the pre eruptive activity the three acoustic levels had shown conspicuous yet distinctive changes prior to the eruption acoustic level in the low frequency 1 50 hz band which increased one year before the resumption of seismic activity and the lake warming up is interpreted as the result of boiling at depth the source of high acoustic level in the audiometric 500 5000 hz range is clearly the bubbling of volcanic gases occurring as a strong convective column in the middle of the lake from the variations of this audiometric level we have estimated that the degassing rate in the lake increased by a hundred fold during the pre eruptive period variations of ultrasonic 20 100 khz frequency acoustic level seem to be related with pressure and thermal changes within the hydrothermal system and its rock matrix beneath the lake in conclusion this experiment demonstrates the potential of hydroacoustic monitoring as an early warning system at crater lake volcanoes

a case study of resistivity and self potential signatures of hydrothermal instabilities inferno crater lake waimangu new zealand 1 inferno crater lake waimangu one of the largest hot springs in new zealand displays vigorous cyclic behavior in lake level and temperature it provides a natural small scale laboratory for investigating the geo electrical signature of fluid flows we measured self potential and electrical resistivity to see whether the huge variations of fluid volume approximately 60 000 m3 during a mean cycle period of 40 days could be detected electrical resistivity measurements revealed spectacular changes over time with the medium becoming more conductive as the lake receded this result is consistent with analog models where the vapor phase is replaced by liquid at recession the self potential survey did not detect temporal changes related to fluid movements this can be explained by the ph of the pore water 2 3 which is close to the point of zero charge of silica

geyser eggs from te whakarewarewatangaoteopetauaawahiao north island new zealand waikorohihi and mahanga two geysers located on geyser flat at te whakarewarewatangaoteopetauaawahiao new zealand discharge chloride waters with a ph of 8 5 9 25 at 100 c many large up to 10 8 5 cm coated grains composed of amorphous silica opal a that are informally termed geyser eggs surround the vent of waikorohihi whereas smaller 3 cm geyser eggs are present in the proximal outflow channel of mahanga the geyser eggs have nuclei of detrital sinter or rhyolitic tuff their cortices are formed of thin concentric laminae and or spicular geyserite most cortical laminae contain a low diversity biota that includes filamentous microbes three types and scattered spherical microbes silicified grains of pinus radiata pollen are common in some laminae multiple phases of silica encrustation around the filamentous microbes disguised most features that are needed to confidently identify them most filaments are probably cyanobacteria variations in the microporosity and or microbe concentrations define individual cortical laminae growth of the geyser eggs in the pool around waikorohihi vent was controlled by the eruptive behavior of the geyser the eggs are submerged by boiling water during eruptions during dormant phases the geyser eggs are subaerially exposed while exposed their surfaces may remain moist from steam or rainwater or they may desiccate the low microbial diversity in the geyser eggs is probably related to their periodic inundation by boiling water and the highly variable conditions the abundant microbial laminae in the geyser eggs confirm that microbial templates are found in siliceous coated grains from almost all environmental settings in siliceous thermal spring systems

modelling of the gravity effects of flexurally compensating thin layers under the afanasy nikitin seamount abstract modelling of the gravity data over the afanasy nikitin seamount is carried out to estimate the variation of the crustal and flexural parameters along the strike of the seamount it is inferred that the compensation of the seamount is due to both the bending of a very thin elastic plate and the occurence of low density material at depth the elastic plate thickness is inferred as 2 km and does not change significantly along the strike of the seamount furthermore the crustal thicknesses range within 12 22 km not showing any systematic variation along the strike of the seamount it is suggested that the afanasy nikitin seamount was emplaced on thin and weak lithosphere very near to a slowly migrating ridge which trapped a hot rising plume resulting in a higher crustal thickness

hot spring and geyser sinters the integrated product of precipitation replacement and deposition complex ornate sinter deposits are found in many hot spring and geysers systems throughout the world including those located in the taupo volcanic zone on the north island of new zealand those sinters are formed of opal a that replaced microbes opal a precipitated as cement accessory minerals e g kaolinite jarosite calcite biological detritus e g leaves wood pollen grains and lithic detritus the opal a is compositionally variable because of the amount of water oh and h2o and in some cases accessory elements e g au ag bound into its structure the composition and fabric of the siliceous sinter found at any locality reflect the relative balance among the processes of replacement precipitation and deposition the microbes that inhabit these systems are of critical importance because they are commonly replaced by and or encrusted with opal a in many settings copious amounts of opal a are precipitated as cement around the frameworks of silicified filaments the cementation pro

the geoid roughness a scanner for isostatic processes in oceanic areas maps of bandpass filtered geoid height anomalies deduced from seasat altimetry data which are referred to as geoid roughness maps are contoured over the indian ocean both in the short 30 90 km bandwith and in an intermediate one 90 230 km interpretation of these maps essentially provides a guide as to where further detailed studies should be conducted the roughness maps can thus be used as a fast and efficient scanner for isostatic processes at the scale of an entire ocean additionally roughness maps provide a qualitative appraisal of the emplacement mechanisms of intraplate features this can for example be appreciated by comparing the interpretations of the maps with the results of admittance studies over such features

relative positions of africa and antarctica in the upper cretaceous evidence for non stationary behaviour of fracture zones abstract new bathymetric and magnetic data have been collected in the enderby basin southern indian ocean during cruise md34 of the n o marion dufresne january february 1983 one can identify in this basin oceanic crust created along the same portion of the southwest indian ridge as the crust of the mozambique basin two sets of magnetic lineations with different amplitude split by a large offset fracture zone are localized in each basin and enable us to obtain accurate reconstructions of the africa antarctica plate boundary at the time of anomalies 34 33 and 32 the data west of prince edward fracture zone are taken into account to adjust the parameters of rotation these reconstructions of isochrons allow us to discuss the evolution of the geometry of accretionary plate boundaries and to emphasize the unstable behaviour of fractures zones in particular we give evidence for an important change in the spreading direction along prince edward fracture zone which demonstrates that stability of a pole of rotation is not guarantueed by major offset fracture zones

isostatic response of the lithosphere beneath the mozambique ridge sw indian ocean and geodynamic implications summary the sw indian ocean is characterized by the presence of several aseismic features the mozambique ridge an elongated feature lying roughly parallel to the se coast of africa is by far the least known of those structures mainly due to the scarcity of marine data to date the crustal nature and the origin of the ridge are still controversial points since knowledge of the origin of the mozambique ridge is important for a better understanding of the evolution of the sw indian ocean the isostatic response of the lithosphere beneath the ridge is analysed in order to characterize its effective elastic thickness and the emplacement process of the feature two different approaches are applied the direct computation of the geoid anomaly over the ridge by means of a 2 5 dimensional method and the computation of the admittances between the bathymetry and both the geoid and gravity anomalies both approaches point to a local isostatic response of the lithosphere the crustal thickness beneath the mozambique ridge ranges from 17 to 30 km and the average density from 2 78 to 2 7 103 kg m 3 varying with locality in good agreement with the few existing refraction profiles based on our results on the geochemical similarity between the basalts cored at the dsdp site 249 and the mor basalts erlank reid 1974 thompson et al 1982 and on the present knowledge of the sw indian ocean kinematics martin hartnady 1986 we propose an on ridge origin for the mozambique ridge the ridge would have been formed by the anomalous activity of a spreading axis linking the northern mozambique and the transkei basins accretion centres between m10 and m2 times at the m2 epoch a ridge jump event would have caused the spreading to cease the astrid ridge a poorly known aseismic structure lying close to the coast of antarctica near 15 e may be the antarctic counterpart of the mozambique ridge formed by the same accretionary phenomenon the two ridges breaking apart at m2 time due to the ridge jump event however any admittance computation at such high latitudes is impossible at present due to the very poor quality of both bathymetry and altimetry and so this hypothesis cannot be verified

pressure changes associated with the ascent and bursting of gas slugs in liquid filled vertical and inclined conduits at basaltic volcanoes the sources of long period and very long period seismicity and acoustic signals are frequently described in terms of fluid dynamic processes in particular the formation and ascent of gas slugs within the magma column and their bursting at the surface to investigate pressure changes associated with these processes two phase flow experiments have been carried out in vertical and inclined pipes with both single gas slugs and a continuously supplied gas phase the ascent of individual gas slugs is accompanied by strong dynamic pressure variations resulting from the flow of liquid around the slug these dynamic transients generate sub static pressures below the ascending slug in viscosity controlled systems and produce super static pressures when the slug reaches the surface and motion ceases in inertia dominated systems conduit inclination promotes a change of regime from bubbly to slug flow and favours an increase in size and velocity of the slugs at the expense of their frequency of occurrence during continuously supplied two phase flow the experimental pressure data support previous theoretical analyses of oscillatory sources in ascending slugs as the slugs approach the surface and burst pressure oscillations are also observed during the release of gas slugs and in their wake region

origin and compensation of chagos laccadive ridge indian ocean from admittance analysis of gravity and bathymetry data abstract the chagos laccadive ridge clr is a prominent aseismic volcanic ridge in the northern indian ocean the ridge together with the southern mascarene plateau smp to which it is genetically related is considered as a volcanic trace of the reunion hotspot we have examined the isostatic compensation of the clr through transfer function analysis of gravity and bathymetry data along seven profiles the analysis suggests that the clr is compensated locally with an airy crustal thickness t c of 20 km the rather low elastic plate thickness t e of about 4 km implies that the volcanism of the ridge took place very near a spreading centre the proximity of the chagos fracture zone indicates that the emplacement was probably near a spreading centre transform junction

emplacement of the marion dufresne lena and ob seamounts south indian ocean from a study of isostasy abstract three gravity and topographic profiles over the ob lena and marion dufresne seamount chain southwest indian ocean are analysed in terms of the isostatic response of the lithosphere these seamounts are regionally supported by both the bending of an elastic plate which varies significantly in thickness along the chain and by the presence of light material at depth this variation is correlated with the age of the lithosphere at the time of loading by the seamounts which varies between 6 and 40 m y these ages when combined with the identification of magnetic anomalies yield absolute emplacement ages for each seamount these results favour a creation of the marion dufresne and lena seamounts near the active southeast indian ridge during the upper cretaceous their emplacement due to intraplate volcanism as the result of the displacement of the antartic plate over a hot spot is therefore considered very unlikely interpretation of the results over ob seamount are somewhat less conclusive but support a similar upper cretaceous emplacement near the southwest indian ridge

the equivalent elastic thickness te seismicity and the long term rheology of continental lithosphere time to burn out creme brulee insights from large scale geodynamic modeling abstract depending on the conditions and time scale the lithosphere exhibits elastic brittle plastic or viscous ductile properties as suggested by rock mechanics experiments a large part of the long term lithospheric strength is supported in the ductile regime unfortunately these data validated for strain rates 10 6 s 1 small scales few cm and simplified conditions cannot be univocally interpolated to geological time and spatial scales strain rates 10 17 10 13 s 1 100 1000 km spatial scales complex conditions without additional parameterization an adequate parameterization has to be based on real time observations of large scale deformation indeed for the oceanic lithosphere the goetze and evan s brittle elastic ductile yield strength envelopes derived from data of experimental rock mechanics were successfully validated by a number of geodynamic scale observations such as the observations of plate flexure and the associated t e equivalent elastic thickness estimates yet for continents the uncertainties of flexural models and of the other data sources are much stronger due to the complex structure and history of continental plates for example in one continental rheology model dubbed jelly sandwich the strength mainly resides in the crust and mantle while in another dubbed creme brulee the mantle is weak and the strength is limited to the upper crust these models have arisen because of conflicting results from distributed earthquake elastic thickness t e and rheology data we address these problems by examining the plausibility of each rheological model from general physical considerations we review the elastic thickness t e estimates and their relationship to the seismogenic layer thickness t s to show that these two quantities have no direct physical relation we also show that some of small t e must be artifacts of inconsistent formulation of the mechanical problem in some free air anomaly admittance models we point out that this does not necessarily detract from the admittance method itself but refers to its incorrect application in the continental domain we then explore by analytical and numerical thermo mechanical modeling the implications of a weak and strong mantle for tectonic structural styles we conclude that rheological models such as creme brulee which invoke a weak lithosphere mantle are generally incompatible with observations the jelly sandwich is in better agreement and we believe provides a useful first order explanation for the long term support of the earth s main surface features

seafloor spreading magnetic anomalies south off sri lanka abstract results obtained from compilation and reinterpretation of about 21 200 line km of bathymetry magnetic and satellite gravity data between 10 s to 10 n latitudes and 75 to 90 e longitudes south off sri lanka are presented here magnetic data and the synthetic seafloor spreading model reveal the presence of mesozoic anomaly sequence m11 through m0 south of sri lanka the oldest magnetic anomaly m11 134 ma occurs between 110 and 140 km away from the sri lankan coast the seafloor created during the early cretaceous is estimated to have evolved with variable half spreading rates ranging from 5 5 to 1 53 cm yr the trends of the fracture zones inferred from the offsets in the magnetic anomalies have been constrained using the satellite gravity mosaic the cretaceous magnetic quiet zone cmqz 121 84 ma crust between the isochrons m0 and a34 has an unequal width and widens from about 170 km in the west 80 e longitude to about 500 km towards east 85 e longitude plate reconstruction models for 160 fit 134 m11 121 m0 and 84 ma a34 are generated under the constraints of newly identified magnetic anomaly isochrons and fracture zones the mismatch in the a34 reconstruction is attributed to the emplacement of the ob lena and marion dufresne seamounts at the spreading ridge axis and or the frequent ridge jumps in the middle cretaceous during the major plate reorganization

origin and tectonic evolution of the caroline ridge and the sorol trough western tropical pacific from admittance and a tectonic modeling analysis abstract an independent caroline sea plate surrounded by prominent seismotectonic features is said to exist in the western tropical pacific the caroline sea plate is a transient feature existing only when the caroline philippine sea plate boundary is jumping to the caroline pacific boundary the origin and the tectonic evolution of the caroline ridge sorol trough region which forms the northern part of the caroline pacific plate boundary are examined using a standard admittance analysis a tectonic modeling study and additional direct gravity and magnetic analyses the observed admittance function is fitted well by shallow airy model in the 250 125 km 0 025 k 1 wavelength region whose average crustal thickness t c is 20 km and also by thin plate flexure model of isostasy within the diagnostic waveband of 250 80 km 0 025 k 1 with an effective elastic thickness t e of 5 7 km a simple interpretation of the observed admittance function is that the local airy model of isostasy is the main support mechanism in the study region for the longer wavelength more fundamental topography corresponding to that of the caroline ridge the caroline ridge is inferred to have formed in a near ridge tectonic setting on top of oceanic lithosphere which was young thin and weak at the time of emplacement the tectonic model study after an explanation for rifting along the mendana fracture zone in the nazca plate suggests that the sorol trough formed in the 17 7 ma interval splitting the caroline ridge into two longitudinal halves the rifting along the sorol trough which took place in the manner described in weissel and karner 1989 flexural uplift of rift flanks due to mechanical unloading of the lithosphere during extension j geophys res 94 13 919 13 950 necking of unbroken lithosphere post dated the formation of the caroline ridge by 10 15 m y during which time t e in the region increased to 5 10 km the rifting necking along the sorol trough represents a second loading event in the region the combined gravity effects of the rifting as well as the increase in the t e in the region from essentially nil to 5 10 km is reflected in the agreement between the observed admittance and the regional model with a 5 7 km t e in the wavelength region 80 km these results are consistent with the findings and the suggestions of the previous investigators in the region in that 1 the caroline ridge was constructed in the late oligocene by hot spot volcanism on top of oligocene seafloor of the caroline sea back arc basin 2 the eauripik rise genetically related to the caroline ridge formed on young and weak caroline basin lithosphere and is supported by airy manner crustal thickening and that 3 the lithosphere of the caroline ridge was young at the time of rifting necking along the sorol trough on the basis of the current results and the findings of the previous workers a tentative hypothetical rheological model for the crustal density structure of the caroline ridge sorol trough is also given the caroline sea plate appears to be bounded in the north and south by e w trending boundaries that deform diffusely while more normal narrow n s trending boundaries exhibiting more localized albeit slow deformation bound the plate in the east and west

application of acoustic noise and self potential localization techniques to a buried hydrothermal vent waimangu old geyser site new zealand a seismo acoustic and self potential survey has been performed in the hydrothermal area of the old waimangu geyser new zealand which was violently erupting a century ago nowadays no surface activity is visible there we set up an array of 16 geophones and recorded a high and steady acoustic ambient noise we applied the matched field processing mfp approach to the acoustic data to locate the sources responsible for the ambient noise the white noise constraint processor reveals the presence of a unique and well focused acoustic source at a depth of 1 5 m below the seismic array for this very shallow source the application of mfp enabled the determination of both the source location and the dispersion curve of seismic velocity the study was completed by self potential sp measurements on several profiles around the acoustic noise source which displayed a large positive anomaly above it the results of the sp inversion gave an electric streaming current density source very close to the acoustic one both sources likely belong to a shallow hydrothermal structure interpreted as a small convective cell of boiling water beneath an impermeable layer the joint application of these methods is a promising technique to recognize hydrothermal structures and to study their dynamics

reconstructing past ocean ph depth profiles measurement of boron isotope compositions in species of planktonic foraminifera that calcified their tests at different depths in the water column are used to reconstruct the ph profile of the upper water column of the tropical ocean results for five time windows from the middle miocene to the late pleistocene indicate ph depth profiles similar to that of the modern ocean in this area which suggests that this method may greatly aid in our understanding of the global carbon cycle

mechanical and thermal isostatic response of the del cano rise and crozet bank southern indian ocean from altimetry data an analysis of the relationship between the bathymetry and the gravity field has been carried out both in the frequency domain admittance technique and in the space domain over the del cano rise and the crozet bank southern indian ocean a bathymetric map contoured from original shipboard data and a geoid anomaly map computed from a merged set of seasat and geos3 data were used the inversion of admittances for water depth crustal thickness crustal density and elastic plate thickness shows that the isostatic response of the lithosphere supporting the del cano rise differs very much from that of the one situated beneath the crozet bank the del cano rise appears to have been formed near the active southwest indian ridge before early eocene and is locally supported by a thickened crust shallow thermal processes contribute to the present topography of the crozet bank emplaced further away from the southeast indian ridge on older crust

seismicity at old faithful geyser an isolated source of geothermal noise and possible analogue of volcanic seismicity abstract old faithful geyser in yellowstone national park u s a is a relatively isolated source of seismic noise and exhibits seismic behavior similar to that observed at many volcanoes including bubblequakes that resemble b type earthquakes harmonic tremor before and during eruptions and periods of seismic quiet prior to eruptions although old faithful differs from volcanoes in that the conduit is continuously open that rock fracturing is not a process responsible for seismicity and that the erupting fluid is inviscid h2o rather than viscous magma there are also remarkable similarities in the problems of heat and mass recharge to the system in the eruption dynamics and in the seismicity water rises irregularly into the immediate reservoir of old faithful as recharge occurs a fact that suggests that there are two enlarged storage regions one between 18 and 22 m the base of the immediate reservoir and one between about 10 and 12 m depth transport of heat from hot water or steam entering at the base of the recharging water column into cooler overlying water occurs by migration of steam bubbles upward and their collapse in the cooler water and by episodes of convective overturn an eruption occurs when the temperature of the near surface water exceeds the boiling point if the entire water column is sufficiently close to the boiling curve that the propagation of pressure release waves rarefactions down the column can bring the liquid water onto the boiling curve the process of conversion of the liquid water in the conduit at the onset of an eruption into a two phase liquid vapor mixture takes on the order of 30 s the seismicity is directly related to the sequence of filling and heating during the recharge cycle and to the fluid mechanics of the eruption short 0 2 0 3 s monochromatic high frequency events 20 60 hz resembling unsustained harmonic tremor and in some instances b type volcanic earthquakes occur when exploding or imploding bubbles of steam cause transient vibrations of the fluid column the frequency of the events is determined by the length of the water column and the speed of sound of the fluid in the conduit when these events occur damping is controlled by the characteristic and hydraulic impedances which depend on the above parameters as well as on the recharge rate of the fluid two periods of reduced seismicity of a few tens of seconds to nearly a minute in duration occur during the recharge cycle apparently when the water rises rapidly through the narrow regions of the conduit causing a sudden pressure increase that temporarily suppresses steam bubble formation a period of decreased seismicity also precedes preplay or an eruption this appears to be the time when rising steam bubbles move into a zone of boiling that is acoustically decoupled from the wall of the conduit because of the acoustic impedance mismatch between boiling water c 10 3 g cm 2 s 1 and rock c 3 10 5 g cm 2 s 1 sustained harmonic tremor occurs during the first one to one and a half minutes of an eruption of old faithful but is not detectable in the succeeding minutes of the eruption the eruption tremor is caused by hydraulic transients propagating within a sublayer of unvesiculated water that underlies the erupting two phase liquid vapor mixture the resonant frequencies of the fluid column decrease to about 1 hz when all of the water in the conduit has been converted to a water steam mixture surges are observed in the flow at this frequency but the resonance has not been detected seismically possibly because the two phase erupting fluid is seismically decoupled from the rock on which seismometers are placed if old faithful is an analogue for volcanic seismicity this study shows that because the frequency of tremor depends on the acoustic properties of the fluid and on conduit dimensions both properties must be considered in analysis of tremor in volcanic regions because magma sound speed can vary over nearly two orders of magnitude as it changes from an undersaturated liquid into a saturated two phase mixture tremor frequency might vary by this magnitude and very broad band seismographs may be required if tremor is to be monitored as magma goes from an undersaturated liquid to a vesiculated froth cessation of fluid induced seismicity may indicate that the processes that drive the transients cease but it is also possible that the processes that drive the transients continue but the fluid properties change so that the fluid becomes acoustically decoupled from the rock on which seismometers are placed

ground truthing the boron isotope paleo ph proxy in planktonic foraminifera shells partial dissolution and shell size effects 1 sediment samples from the ontong java plateau in the pacific and the 90 east ridge in the indian ocean were used to investigate whether shell size and early diagenesis affect 11b of the symbiont bearing planktonic foraminifer globigerinoides sacculifer in pristine shells from both study locations we found a systematic increase of 11b and mg ca with shell size shells in the sieve size class 515 865 m revealed 11b values 2 1 to 2 3 higher than shells in the 250 380 m class this pattern is most likely due to differences in symbiont photosynthetic activity and its integrated effect on the ph of the foraminiferal microenvironment we therefore suggest smaller individuals must live at approximately 50 100 m water depth where ambient light levels are lower using the empirical calibration curve for 11b in g sacculifer only shells larger than 425 m reflect surface seawater ph partial dissolution of shells derived from deeper sediment cores was determined by shell weight analyses and investigation of the shell surface microstructure by scanning electron microscopy the 11b in partially dissolved shells is up to 2 lower relative to pristine shells of the same size class in agreement with a relatively higher weight loss in smaller shells samples from the ontong java plateau show a more pronounced dissolution effect than larger shells on the basis of the primary size effect and potential postdepositional dissolution effects we recommend the use of shells that are visually pristine and in the case of g sacculifer larger than 500 m for paleoreconstructions