late quaternary climate change from δ18o records of multiple species of planktonic foraminifera high resolution records from the anoxic cariaco basin venezuela seasonal trade wind induced upwelling along the southern margin of the caribbean sea occurs in response to the annual migration of the intertropical convergence zone laminated high deposition rate sediments of the cariaco basin a small anoxic basin on the venezuelan continental shelf clearly record large changes in the past intensity of this upwelling because sediments of the cariaco basin are largely unbioturbated they offer a natural opportunity to study the stable isotopic records of multiple planktonic foraminiferal taxa and to evaluate their sensitivity to both the modern hydrography and temporal changes in upwelling intensity and climate oxygen isotope data 18o from four dominant foraminiferal taxa are presented for the time period covering the last 28 kyr the 18o data from globigerina bulloides after correction for nonequilibrium precipitation are used as a monitor of sea surface conditions during the winter spring upwelling season the 18o data from white globigerinoides ruber are used as a measure of annual average conditions in the near surface while pink g ruber data are consistent with use as an index of endmember conditions during the summer fall nonupwelling season data from the deeper dwelling neogloboquadrina dutertrei yield information on conditions near the base of the local thermocline during the last glacial 18o data from g ruber and generally reduced interspecific differences indicate cooling of surface waters over the cariaco basin by up to 4 c this longer term cooling does not appear to be related to changes in upwelling intensity along the coast but may instead reflect more regional cooling of the larger caribbean superimposed on this pattern between 12 6 and 10 ka is a convergence of 18o data between g bulloides and n dutertrei implying much stronger upwelling during the last deglaciation this scenario is consistent with other evidence for high productivity at this time at 14 ka a sharp 18o depletion event observed in all taxa seems to have been produced by increased freshwater discharge to the southern caribbean suggesting either higher regional rainfall or the influence of glacial melting in the andes minimum 18o values of pink g ruber around 6 7 ka record warmer summer sea surface temperatures and or decreased salinity in the mid holocene

radiocarbon based carbon source quantification of anomalous isotopic foraminifera in last glacial sediments in the western north pacific a previous study interpreted extremely 13c depleted excursions of planktonic and benthic foraminifera in last glacial sediments 17 500 to 25 400 cal years b p of the core retrieved from off shimokita peninsula and off hokkaido japan as evidence for periodic releases of methane arising from the dissociation of methane hydrate to better understand the formation process of the 13c depleted excursions we conducted high resolution natural radiocarbon measurements and biogeochemical analyses we found highly depleted 13c excursions ranging from 10 2 to 1 6 and 6 8 to 1 6 in planktonic and benthic foraminifera respectively most of the foraminiferal tests in these horizons were brown most likely as a result of postdepositional alteration reflecting the formation of authigenic carbonate on the surface of tests these alterations were also supported by high levels of mg calcite and the acid leaching test for anomalous foraminifera to evaluate the carbon sources in the altered foraminifera tests we quantified the relative contributions of 14c free methane derived carbon sources to the formation of authigenic carbonates in foraminifera with depleted 13c excursions using a coupled mass balance isotopic model 14c c and 13c 12c the radiocarbon ages of both planktonic and benthic 13c depleted foraminifera were approximately 600 to 2000 years older than those of normal tests from nearby horizons the relative contributions of authigenic carbonates derived from the methane oxidizing process reached to 22 wt for planktonic foraminifera and 15 wt for benthic foraminifera the 13c values of methane calculated from the mass balance model were between 29 and 68 for planktonic foraminifera and between 40 and 108 for benthic foraminifera consistent with 13c values reported for thermogenic and abiogenic methane in global methane hydrate reservoirs these data consistently suggest that methane related drastic environmental change occurred in the horizons that included 13c anomalies this study provides important information for interpreting geological records of the methane hydrate instability associated with climate

helium and carbon fluxes in lake nyos cameroon constraint on next gas burst on 21 august 1986 a lethal gas burst issued from lake nyos in cameroon western africa killing at least 1700 people although the consensus is that the victims died of asphyxiation by co2 of magmatic origin the frequency of such catastrophic degassing events is still unknown the co2 flux at the bottom of lake nyos is estimated based on the measurement of3he4he and4he20ne ratios and the chemical composition of gases exsolved from the lake water the calculated co2 flux at the bottom of lake nyos is 4 4 2 3 1013cm3stp year combined with the estimated release in the august 1986 event of 8 2 1014 cm3 stp the co2 flux suggests that the gas burst may happen about once in18 10 years although the significant uncertainty should be taken into account for the frequency resulting from assumptions such as steady state fluxes in the lake and small fractionization of the c he ratio during the degassing event several yearly measurements of hypolimnetic fluxes and seasonal measurements of epilimnetic fluxes are needed to constrain better the recurrence interval in addition the results of a regional3he4he survey of carbonated mineral springs in northwestern cameroon are discussed in the context of the regional geotectonics

origin of the lethal gas burst from lake monoun cameroun abstract on 15 august 1984 a lethal gas burst issued from a submerged 96 m deep crater in lake monoun in cameroun western africa killing 37 people the event was associated with a landslide from the eastern crater rim which slumped into deep water waters below 50 m are anoxic dominated by high fe 2 600 mg l and hco 3 1900 mg l anoxic and supersaturated with siderite which is a major component of the crater floor sediments the unusually high fe 2 levels are attributed to reduction of laterite derived ferric iron gradually brought into the lake as loess and in river input sulfur compounds are below detection limits in both water and gas gases effervescing from depressurized deep waters are dominantly co 2 with minor ch 4 having 13 c of 7 18 and 54 8 per mil respectively bacterial decomposition of organic matter may account for the methane but 14 c of lake water indicates that only 10 of the carbon is modern giving an apparent age of 18 000 years the dominant source of carbon is therefore attributed to long term emission of co 2 as volcanic exhalation from vents within the crater which led to gradual build up of hco 3 in the lake the density stratification of the lake may have been upset by an earthquake and underwater landslide on 15 august which triggered overturn of the lake and caused nucleation of co 2 in the deep water the resultant ebullition of co 2 from deep lake waters led to a gas burst at the surface and locally generated a water wave up to 5 m high people travelling through the gas cloud were asphyxiated presumably from co 2 and suffered skin discoloration from unidentified components

controls on methane concentration and stable isotope δ2h ch4 and δ13c ch4 distributions in the water columns of the black sea and cariaco basin methane ch4 concentration and stable isotope 2h ch4 and 13c ch4 depth distributions show large differences in the water columns of the earth s largest ch4 containing anoxic basins the black sea and cariaco basin in the deep basins the between basin stable isotope differences are large 83 for 2h ch4 and 9 for 13c ch4 and the distributions are mirror images of one another the major sink in both basins anaerobic oxidation of ch4 results in such extensive isotope fractionation that little direct information can be obtained regarding sources recent measurements of natural 14c ch4 show that the ch4 geochemistry in both basins is dominated 64 to 98 by inputs of fossil radiocarbon free ch4 from seafloor seeps we derive open system kinetic isotope effect equations and use a one dimensional vertical stable isotope box model that along with isotope budgets developed using radiocarbon permits a quantitative treatment of the stable isotope differences we show that two main factors control the ch4 concentration and stable isotope differences 1 the depth distributions of the input of ch4 from seafloor seeps and 2 anaerobic oxidation of ch4 under open system steady state conditions in the black sea and open system non steady state conditions in the cariaco basin copyright 2006 by the american geophysical union

determining the flux of methane into hudson canyon at the edge of methane clathrate hydrate stability methane seeps were investigated in hudson canyon the largest shelf break canyon on the northern us atlantic margin the seeps investigated are located at or updip of the nominal limit of methane clathrate hydrate stability the acoustic identification of bubble streams was used to guide water column sampling in a 32 km2 region within the canyon s thalweg by incorporating measurements of dissolved methane concentration with methane oxidation rates and current velocity into a steady state box model the total emission of methane to the water column in this region was estimated to be 12 kmol methane per day range 6 24 kmol methane per day these analyses suggest this methane is largely retained inside the canyon walls below 300 m water depth and that it is aerobically oxidized to near completion within the larger extent of hudson canyon based on estimated methane emissions and measured oxidation rates the oxidation of this methane to dissolved co2 is expected to have minimal influences on seawater ph this article is protected by copyright all rights reserved

fossil methane source dominates cariaco basin water column methane geochemistry natural radiocarbon measurements on methane 14c ch4 extracted from the cariaco basin water column show that 98 of the methane in cariaco basin waters is derived from fossil radiocarbon free sources previous work on cariaco basin methane ch4 considered only a diagenetic sediment source similar measurements of sediment 14c ch4 indicate that sediment ch4 is produced from modern particulate material thus the sediment and water column ch4 have distinct sources using time dependent ch4 geochemical box models which include a fossil seep ch4 source term we estimate 1 0 024 0 028 tg y 1 of seep ch4 are added to the cariaco basin water column 2 the water column ch4 will reach a steady state concentration by the year 2065 and 3 the seep ch4 inputs possibly began in 1967 following the july 30 caracas earthquake oxidizing this ch4 to dissolved inorganic carbon does not appear to affect cariaco basin 14c chronologies copyright 2005 by the american geophysical union

wavelet analysis of a bathymetric profile reveals anomalous crust a 200 km anomalous topographic zone was discovered using wavelet scale analysis in a 1600 km linear bathymetric profile taken northeast of hawaii a spectral analysis of the zone shows that the power in wavelengths shorter than 25 km averages 5 10 times lower than the surrounding topography conversely wavelengths longer than 25 km have 2 3 times greater power than nearby topography further research reveals that this low frequency zone correlates with the eastern flank of a regional bathymetric high which has been described by j mammerickx 1981 details of the features suggest that this zone is a small abandoned slow spreading rift overprinted by a regional thermal swell the magnitude of the feature is smaller than other known abandoned spreading centers making it easy to overlook without the application of the wavelet transform

ocean island densities and models of lithospheric flexure estimates of the effective elastic thickness te of the oceanic lithosphere based on gravity and bathymetric data from island loads are commonly significantly lower than those based on the wavelength of plate bending at subduction zones the anomalously low values for ocean islands have been attributed to the finite yield strength of the lithosphere to erosion of the mechanical boundary layer by mantle plumes to pre existing thermal stresses and to overprinting of old volcanic loads by younger ones a fifth possible contribution to the discrepancy is an incorrect assumption about the density of volcanic loads we suggest that load densities have been systematically overestimated in studies of lithospheric flexure potentially resulting in systematic underestimation of effective elastic thicknesses and overestimation of the effects of hotspot volcanism we illustrate the effect of underestimating load density with synthetic examples and an example from the marquesas islands this effect combined with the other effects listed above in many cases may obviate the need to invoke hotspot reheating to explain low apparent elastic thicknesses

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

long wavelength gravity and topography anomalies it is shown that gravity and topography anomalies on the earth s surface may provide new information about deep processes occurring in the earth such as those associated with mantle convection two main reasons are cited for this the first is the steady improvement that has occurred in the resolution of the long wavelength gravity field particularly in the wavelength range of a few hundred to a few thousand km mainly due to increased coverage of terrestrial gravity measurements and the development of radar altimeters in orbiting satellites the second reason is the large number of numerical and laboratory experiments of convection in the earth including some with deformable upper and lower boundaries and temperature dependent viscosity the oceans are thought to hold the most promise for determining long wavelength gravity and topography anomalies since their evolution has been relatively simple in comparison with that of the continents it is also shown that good correlation between long wavelength gravity and topography anomalies exists over some portions of the ocean floor

thermal and mechanical properties of the cape verde rise the cape verde rise represents an extreme case of the process of oceanic midplate volcanism the regional depth and geoid anomalies relative to the plate model are the largest observed peaking at 2 4 km and 12m respectively the late tertiary volcanoes rest on mesozoic aged seafloor indicating that the lithosphere was about 130 m y old when loaded depth and geoid anomalies in the cape verde region are examined using both forward filtering and admittance techniques to determine the thermomechanical properties of the swell the effective elastic thickness for the plate supporting the volcanoes is 28 4 km and the compensation depth for the broad swell is 69 10 km these two results combined with heat flow observations from the rise are consistent with a model for reheating of the lithosphere beneath cape verde such that the average thermal gradient is increased by 35 while there is no doubt that the cape verde lithosphere has been reheated it is unlikely that the entire depth anomaly is explained by thermal anomalies confined to the lithosphere a linear programming inversion of the geophysical data places an upper bound of 1 7 km on the portion of the depth anomaly due to lithospheric reheating there are no permissible thermal models fitting all of the geophysical data that do not require either dynamic support from or elevated temperature within the asthenosphere

gas emission from diffuse degassing structures dds of the cameroon volcanic line cvl implications for the prevention of co2 related hazards abstract in the mid 1980s lakes nyos and monoun violently released massive gas mainly magmatic co 2 killing about 1800 people subsequent geochemical surveys and social studies indicate that lakes nyos and monoun event is cyclic in nature and may occur anywhere in the about 37 other volcanic lakes located in the corridor of the cameroon volcanic line cvl this potential threat motivated us to check if alike nyos and monoun the internal dynamic of the other lakes is also controlled by inputs of deep seated derived co 2 and attempt to measure and provide comprehensive insights on the passive gas emission along the cvl this knowledge shall contribute to the prevention of volcanic lake related hazards in cameroon and the refinement of the global carbon cycle we used in situ fixation and dry gas phase sampling methods to determine co 2 origin and the concentration and the accumulation chamber technique to measure diffuse co 2 emission from nine lakes and on soil at nyos valley and mount manenguba caldera the results suggest that although in minor concentrations compared to nyos and monoun ranging from 0 56 mmol kg 1 to 8 75 mmol kg 1 the bottom waters of some lakes also contain measurable magmatic co 2 with 13 c varies from 4 42 to 9 16 vs pdb that finding implies that under certain circumstances e g increase to volcanic and or tectonic activities along the cvl the concerned lakes could develop a nyos type behavioural scheme the diffuse gas emission results indicate that the nine surveyed lakes release approximately 3 69 0 37 kt km 2 yr 1 of co 2 to the atmosphere extrapolation to the approximately 39 volcanic lakes located on the cvl yields an approximate co 2 output of 27 37 0 5 kt km 2 yr 1 representing 0 023 of the global co 2 output from volcanic lakes in addition to the precedent value the gas removal operation in lakes nyos and monoun released approximately 2 52 0 46 10 8 mol km 2 yr 1 co 2 to the atmosphere from january 2001 to march 2013 more than double the per area co 2 released by the yellowstone volcanic system the co 2 emission from soil was estimated to be 4 57 1 30 kt km 2 yr 1 the soil gas geochemistry of the mount manenguba caldera also indicates a dominant magma derived co 2 mean 13 c 8 6 vs vpdb

extent of the south pacific superswell 1 the south pacific superswell is known as a broad area beneath french polynesia characterized by numerous volcanic chains and very shallow seafloor compared to the depth predicted for its age by classical seafloor subsidence models so far its exact extent has not yet been established thanks to better bathymetric coverage and a specially adapted filtering method we present here a new complete and precise mapping of the superswell we show that the superswell covers a region broader than previously expected it extends between latitudes 10 n and 30 s and longitudes 130 w and 160 w and has a maximum amplitude of 680 m it is composed of two branches that display different characteristics when compared to other geophysical observations under the southern branch a dip in the geoid is observed which could be linked to upwelling in a convective mantle where the low velocity zone is located immediately below the lithosphere as indicated by tomography models under the northern branch there is a 12 m high in the geoid surface anomaly that may be explained by isostatic compensation of a mass deficiency located in the mantle at 300 km depth the northern branch could also be related to another superplume that is now going down but which was rising up 30 90 m y ago when it initiated the secondary plumes which created the line islands

the gas cloud of lake nyos cameroon 1986 results of the italian technical mission abstract on august 21 1986 a gas cloud issued from lake nyos in cameroon killed over 1700 people an italian technical mission reached the area seven days later and obtained the first field evidences of the catastrophe on the basis of observations and measurements in the field and of samples collected the origin of the gas outburst is attributed to a phreatic explosion from beneath the bed of the lake this interpretation appears to fit well the observed and reported phenomena and seems perfectly consistent with the geological geothermal conditions of the area

forearc slope deformation above the japan trench megathrust implications for subduction erosion abstract subduction erosion is a commonly invoked model that is used to explain the tectonic subsidence of the japan trench forearc slope although other models have explained the morphology and history of the margin new multichannel seismic reflection and bathymetric data collected after the 2011 tohoku earthquake provide the opportunity to investigate the detailed structure of the overriding plate near the earthquake epicenter and obtain new constraints on tectonic models we use regional residual separation of the local bathymetry to constrain fault scarp extents and local landward dipping forearc basins seismic images of these basins clearly show landward dipping horizons in the shallow section the strata in these basins imply a different mechanism for formation than the surrounding forearc slope and we propose that these basins formed from local uplift a regional basal unconformity mapped 150 km along trench has highly variable relief indicating that forearc slope subsidence occurs at multiple wavelengths in response to multiple different sources we characterize the upper to middle slope transition and propose that this region may be the landward limit of major subduction erosion and also the main region for large mass wasting normal faults found in this setting have maximum lengths of 20 km limiting their role in margin processes our results place constraints on the extent of major subduction erosion at the japan trench margin and indicate that subduction erosion should be revisited as the sole model of formation to include additional tectonic processes

the 1967 caracas earthquake fault geometry direction of rupture propagation and seismotectonic implications the fault plane orientation of the july 30 1967 caracas earthquake mw 6 6 has been a source of controversy for several years this strike slip event was originally thought to have occurred on an east west oriented fault plane reflecting the relative motion between the caribbean and south american plates more recently however the complex seismic radiation from this event was interpreted as being indicative of a north south striking fault that ruptured along three en echelon segments in this study we synthesize evidence based on the intensity and damage reports the distribution of aftershocks and the results of a joint formal inversion of the p and sh waves and show that these data clearly indicate that the rupture of the 1967 earthquake occurred on an east west trending fault system using a master event technique the largest aftershock which occurred 40 min after the main event is shown to lie 50 km east of the epicenter of the mainshock the epicentral distances of small aftershocks registered in caracas based on the s p arrival time differences and the polarizations of the p waves are also consistent with these events occurring on an east west oriented fault system north of caracas a joint inversion of the teleseismic p and sh waves recorded on long period seismographs of the world wide standardized seismographic network shows that in a time frame of 65 s four distinct bursts of seismic moment release subevents occurred with a total seismic moment of 8 6 1018 n m the first three subevents triggered sequentially from west to east in a direction that is almost identical to the east west trending nodal planes of the source mechanisms the average depth of these three subevents is 14 km the fourth and last identifiable subevent of the sequence shows a reverse faulting mechanism with the nodal planes oriented roughly east west it occurred at a 21 km depth about 50 km to the north of the fault zone defined by the strike slip subevents this fourth subevent appears to reflect compressional deformation of the southern caribbean possibly related to underthrusting along the proposed curacao trench the complexity of the fault system causing the 1967 earthquake suggests that the relative motion along the caribbean south america plate boundary in central venezuela is taken up over a broad highly faulted and highly stressed zone of deformation and not by a simple major throughgoing fault

modal depth anomalies from multibeam bathymetry is there a south pacific superswell a region west of the southern east pacific rise sepr between the marquesas and austral fracture zones has previously been found to exhibit anomalous depth age behavior based on gridded bathymetry and single beam soundings since gridded bathymetry has been shown to be unsuitable for some geophysical analysis and since the area is characterized by unusually robust volcanism the magnitude and regional extent of depth anomalies over the young eastern flank of the so called south pacific superswell are re examined using a mode seeking estimation procedure on data obtained from several recent multibeam surveys the modal technique estimates a representative seafloor depth based on the assumption that bathymetry from non edifice and edifice populated seafloor has a low and a high standard deviation respectively flat seafloor depth values are concentrated in a few bins which correspond to the mode this method estimates a representative seafloor value even on seafloor for which more than 90 of coverage is dominated by ridge and seamount clusters where the mean and median estimates may be shallow by hundreds of meters where volcanism related bias is moderate the mode mean and median estimates are close depth age results indicate that there is only a small anomaly 200 m over is 35 ma pacific plate seafloor with little age dependent shallowing suggesting that the lithosphere east of the main hot spot locations on the superswell is normal an important implication is that in sparsely surveyed areas depths from etopoj are significantly different from true depths even at large scales 1000 km and thus are unsuitable for investigations of anomalies associated with depth age regressions we find that seafloor slopes on conjugate profiles of the pacific and nazca plates from i5 to 35 ma are both slightly lower than normal but are within the global range proximate to the sepr seafloor slopes are very low 218 m myr 2 on the pacific plate o 22 ma and slightly high 410 m myr i21 on the nazca plate o 8 ma slopes for older pacific seafloor 22 37 ma are near normal 399 m myr i seafloor slopes are even lower north of the marquesas fracture zone but are highly influenced by the marquesas swell we find that the low subsidence rate on young pacific seafloor cannot be explained by a local hot spot or a small scale convective model exclusively and a stretching thickening model requires implausible crustal thickness variation 30

terrestrial methane seeps and mud volcanoes a global perspective of gas origin abstract a global database of gas composition and methane stable isotopes of 143 terrestrial mud volcanoes from 12 countries and 60 seeps independent from mud volcanism from eight countries was compiled and examined in order to provide the first worldwide statistics on the origin of methane seeping at the earth s surface sixteen seep data were coupled with their associated subsurface reservoirs the surface seepage data indicate that at least 76 of the mud volcanoes release thermogenic gas with only 4 biogenic and 20 with mixed character the average 201 data of methane concentration and methane carbon isotope ratios 13 c 1 of mud volcanoes are 90 v v and 46 4 respectively the other types of seeps which are independent from mud volcanism have an average 13 c 1 value that is slightly higher 42 9 gases from mud volcanoes are generally lighter more methane less ethane and propane than their associated reservoir gases suggesting a molecular fractionation during advective fluid migration other types of seeps especially dry seeps maintain the reservoir c 1 c 2 c 3 bernard ratio mud volcanoes behave like a natural refinery and the origin of gas more isotopically enriched than 50 and with c 1 c 2 c 3 500 should be attributed to a thermogenic source rather than partial oxidation of biogenic gas some data that appear biogenic in the bernard diagram can be explained by molecular fractionation of mixed gas consequently the bernard parameter may be misleading when applied to mud volcanoes since it does not always reflect the original gas composition the mechanisms of the molecular advective segregation should be studied quantitatively by specific models and experiments

14c activity and global carbon cycle changes over the past 50 000 years a series of 14c measurements in ocean drilling program cores from the tropical cariaco basin which have been correlated to the annual layer counted chronology for the greenland ice sheet project 2 gisp2 ice core provides a high resolution calibration of the radiocarbon time scale back to 50 000 years before the present independent radiometric dating of events correlated to gisp2 suggests that the calibration is accurate reconstructed 14c activities varied substantially during the last glacial period including sharp peaks synchronous with the laschamp and mono lake geomagnetic field intensity minimal and cosmogenic nuclide peaks in ice cores and marine sediments simulations with a geochemical box model suggest that much of the variability can be explained by geomagnetically modulated changes in 14c production rate together with plausible changes in deep ocean ventilation and the global carbon cycle during glaciation

water and gas chemistry of lake nyos and its bearing on the eruptive process abstract the isotopic and chemical composition of water samples collected from lake nyos some two and eight weeks after the eruption of august 21 1986 point to the existence of three distinct mixing regimes involving three water components an essentially homogeneous unmixed body of water at depths 100 m overlain by water increasingly affected by surface evaporation and a 5 10 m layer containing recent but pre eruption rain water the cationic constituents na k mg ca mn fe of the lake water correspond to the dissolution of around 0 1 g of local rock the waters are close to saturation with respect to siderite the composition of the gas dissolved in the deep lake waters 0 65 b w of co2 pco2 4 4b corresponds in mmol mol 1 to 996 co2 2 0 ch4 2 0 n2 0 05 ar 0 004 he 0 0002 h2 0 0001 ne co 2 13 c 3 4 and of he rair 5 5 suggest deep magmatic origins the 13c and 2h content of ch4 organic sedimentary origin the presence of aromatic hydrocarbons indicates very high temperatures of hydrocarbon formation exsolution of gas will first lead to the precipitation of siderite then iron hydroxide the chemistry of the lake waters points to a loss of some 240 000 t of co2 from the upper 100 m of the lake their isotopic composition is consistent with the assumption that the eruption was triggered by the accumulation of cold rain waters at the lake surface prior to the eruption inducing partial convective overturn there is no need to invoke addition of chemical or isotopic constituents from deeper levels during the eruption

in situ determination of heat flow in unconsolidated sediments subsurface thermal measurements are the most effective least ambiguous tools for locating geothermal resources measurements of thermal gradient in the upper few tens of meters can delineate the major anomalies but it is also desirable to combine these gradients with reliable estimates of thermal conductivity to provide data on the energy flux and to constrain models of the heat sources responsible for the anomalies problems associated with such heat flow measurements include the economics of casing or grouting holes the long waits and repeated visits necessary to obtain equilibrium temperature values the possible legal liability arising from disturbance of aquifers the hazards presented by pipes protruding from the ground and the security problems associated with leaving cased holes open for periods of weeks to months

screening and ranking of sedimentary basins for sequestration of co2 in geological media in response to climate change sedimentary basins are suitable to different degrees for co2 geological sequestration as a result of various intrinsic and extrinsic characteristics of which the geothermal regime is one of the most important warm basins are less favorable for co2 sequestration than cold basins because of reduced capacity in terms of co2 mass and because of higher co2 buoyancy which drives the upward co2 migration a set of 15 criteria with several classes each has been developed for the assessment and ranking of sedimentary basins in terms of their suitability for co2 sequestration using a parametric normalization procedure a basin s individual scores are summed to a total score using weights that express the relative importance of different criteria the total score is ranked to determine the most suitable basin or region thereof for the geological sequestration of co2 the method is extremely flexible in that it allows changes in the functions that express the importance of various classes for any given criterion and in the weights that express the relative importance of various criteria examples of application are given for canada s case and for the alberta basin in canada

a reexamination of the flexural deformation beneath the hawaiian islands seismically derived depth estimates to the top of the oceanic crust beneath the hawaiian islands indicate that the curvature of the deflected lithosphere is much larger than commonly believed the conservative and model independent curvature estimates exceed 10 7 m 1 and are comparable in magnitude to curvatures at trenches and outer rise systems the depth estimates are used to constrain both two dimensional 2 d and three dimensional 3 d flexural models the curvature constraints require a 2 d variable elastic thickness that decreases from 35 km in areas away from the volcanic load to 25 km directly beneath the load in an attempt to understand the nature of the yielding beneath the hawaiian islands we introduce two new 3 d models the first model combines a realistic yield strength based rheology with a new technique for 3 d flexure calculations in which the elastic plate thickness is curvature dependent the new variable rigidity model predicts an undeformed mechanical plate thickness of 44 km decreasing to 33 km beneath the big island of hawaii the best fitting mechanical thickness corresponds approximately to the depth to the 600 c isotherm in 90 m y old lithosphere the second model uses a broken plate but here the crack is oriented along the weak molokai fracture zone rather than along the island chain trend this unconventional flexure model can explain the observed asymmetry in the depth data across the fracture zone without requiring the excessively large elastic thickness of more conventional broken plate models both the proposed models imply that modeling with constant thickness plates may underestimate the true mechanical plate thickness by being unduly influenced by the weak zone beneath the seamounts

mechanisms of the nyos carbon dioxide disaster and of so called phreatic steam eruptions abstract during the night of august 21 1986 a huge volume of concentrated co 2 was emitted by the crater maar of nyos cameroon it killed more than 1700 people and all animal life as far as 14 km away two hypotheses have been put forward to account for this disaster the chronologically first one imputes it to have been a phreatic eruption exceptionally co 2 rich as had been the case in february 1979 on the dieng plateau in central java indonesia where the erupting crater was lake less the second one claims a limnic origin for the co 2 release through an overturn of the 220 m deep lake nyos the hypolimnium of which was supposed to be oversaturated by dissolved gas of volcanic origin the present paper points to six observed facts for which the eruptive hypothesis easily accounts and which the authors of the limnic one do ignore

the potential for catastrophic dam failure at lake nyos maar cameroon the upper 40 m of lake nyos is bounded on the north by a narrow dam of poorly consolidated pyroclastic rocks emplaced during the eruptive formation of the lake nyos maar a few hundred years ago this 50 m wide natural dam is structurally weak and is being eroded at an uncertain but geologically alarming rate the eventual failure of the dam could cause a major flood estimated peak discharge 17000 m3 s that would have a tragic impact on downstream areas as far as nigeria 108 km away this serious hazard could be eliminated by lowering the lake level either by controlled removal of the dam or by construction of a 680 m long drainage tunnel about 65 m below the present lake surface either strategy would also lessen the lethal effects of future massive co2 gas releases such as the one that occurred in august 1986

effective elastic thickness of the lithosphere along the easter seamount chain bathymetry and gravity data collected during legs 5 6 and 7 of the 1993 gloria expedition and the recently released 2 min altimetry derived global gravity grid are used to determine the effective elastic thickness of the lithosphere along the easter seamount chain esc forward modeling admittance and coherence methods yield consistent restfits with the exception of the eastern and western ends of the esc the effective elastic thickness along the chain is 1 4 km the thin elastic thickness for the majority of the esc seamounts is compatible with a young seafloor age at the time of loading derived from new radiometric ages of the seamounts along the chain and a magnetic isochron age interpretation of the nazca plate seafloor age the elastic thickness southeast of the nazca fracture zone is 6 km apparently because of the seafloor age discontinuity across the fracture zone the elastic thickness near the san felix island at the eastern end of the esc is even greater 11 km which is compatible with the estimated seafloor age at the time of loading a slight increase in the effective elastic thickness of the far western part of the esc suggests dynamic compensation or less thermal weakening of lithosphere above a plume channel versus directly above the plume center these findings combined with published geochemistry support a hotspot origin for the esc complicated by large scale plate boundary reorganizations and channeling of plume material to the east pacific rise

mifil a method to characterize seafloor swells with application to the south central pacific 1 we propose a filtering method to characterize large scale depth anomalies the mifil method for minimization and filtering requires two stages a first one to roughly remove the volcano component by minimizing the depth anomaly and a second one to smooth the shape and totally remove the small spatial length scale remaining topography using a median filter the strength of this method directly applicable on two dimensional grids is that it does not require any assumption on the location amplitude or width of the large scale feature to characterize except its minimal width we only consider the spatial length scale of the features to remove application to volcanic chains of the south central pacific is presented and the results lead to a better understanding of the tectonics and volcanism emplacement of the zone the society is the only classical hot spot that corresponds to the simple interaction of a plume with the lithosphere and for which a buoyancy flux of 1 58 0 15 mg s 1 is obtained the marquesas volcanic chain although quite comparable presents a swell morphology that prevents such interpretation and quantification for the tuamotu and cook austral volcanic chains no reliable quantification can be made because the depth and geoid anomalies are caused by several phenomena occurring at different depths that cannot be separated

crustal structure flexure and subsidence history of the hawaiian islands seismic reflection profile data have been used to estimate the long term 106 years mechanical properties of the oceanic lithosphere underlying the hawaiian islands the data show prominent reflectors associated with the top of the oceanic crust reflector 2 the m discontinuity and the base of the crust reflector 3 the two way travel time to the reflectors has been depth converted using velocity depth functions derived from seismic refraction expanding spread profile data and compared to calculated depths based on simple three dimensional elastic plate models a good fit to the reflector depths was obtained for a model with an effective elastic thickness of the lithosphere te of 40 km the main discrepancies with the refraction data occur beneath oahu and molokai where the predicted depth of the base of the flexed crust is too shallow and beneath the southeast flank of hawaii where it is too deep we attribute the thickened crust beneath oahu to ponding of magma at the base of the crust following the last phases of tholeiitic shield building and the thinned crust beneath the flanks of hawaii to crustal melting beneath the youngest part of the chain the effective te of 40 km is higher than expected for 80 m y oceanic lithosphere possibly due to the effects of subsurface loads that act at or near the base of the flexed crust according to free air gravity anomaly and bathymetry data te does not change significantly along the island chain retaining a similar value for hawaii as it does for oahu we have used this result to estimate the pattern of subsidence and uplift that would be expected along the hawaiian islands because of flexural loading of hawaii the observed depth of the haleakala wave cut terrace northeast of maui can be generally explained by the flexure model the main discrepancies between observed and the predicted depths occur northwest of hawaii where they suggest a somewhat higher value of te possibly due to incomplete adjustment of the lithosphere we suggest that the addition of loads to the end of a volcanic chain generated as a plate moves over a mantle hotspot is accommodated by progressive flexing downstream causing subsidence of preexisting volcanos that lie in the flexural moat and uplift of islands on the flanking bulge the progressive flexure model predicts a characteristic pattern of offlap and onlap in the regions flanking the hawaiian islands that is generally similar to stratigraphic relationships observed along previously published seismic profiles of the moat a progressive flexure model may also apply to the case of a continental plate moving over a mantle hotspot and could explain the observations of 1 a clinoform pattern of stratigraphy in some flood basalt provinces and 2 a seaward dipping reflector sequence at some passive continental margins

global relationship between oceanic geoid and seafloor depth new results a new global transfer function admittance between oceanic geoid and seafloor depth has been derived for the wavelength band 1000 4000 km after removal of lithospheric cooling effects using the most recently published crustal ages of the oceans in this wavelength band the admittance is positive reaches a maximum of 5m km around 2750 km wavelength then decreases to 2 5 m km on the average at shorter wavelengths between 2500 km and 1000 km these values are systematically higher by 30 40 than those we derived in a previous study using crustal ages of poorer quality in view of these new results we cannot exclude that part of the signal at the 2750 km wavelength is of convective origin unlike the conclusions of the previous study on the other hand at shorter wavelengths the observed admittance can still be interpreted in terms of crustal thickening and sediment loading

an inference on the process of gas outburst from lake nyos cameroon abstract a large volume of lethal gas was released from lake nyos in the northwest province of cameroon on august 21 1986 some 50 days after the event a hydro physical and chemical survey of lake nyos was carried out by four japanese scientists including the author vertical density distributions of lake water showed remarkably stable stratification through the whole water column chemical analyses showed high concentrations of dissolved co2 and fe which contribute to stable stratification assuming an initial saturation of dissolved co2 in lake water released gas volume was estimated to be 0 68 km3 as a difference between saturated and remaining gas contents in the present lake temperature drops due to quasiadiabatic expansion of co2 gas exsolved from various water layers were estimated for every 10 m layer thickness assuming a constant temperature distribution and a complete thermal exchange between expanding gas and lake water the calculated temperature profile agreed well with the profile measured by ctd conductivity temperature depth profiler the height of the gas cloud over the lake and the flow velocity of draining gas from the outlet of the rim were evaluated using a simple geometrical flux model assuming a sinusoidal variation of ejection with equal volume mixing with air and head loss coefficient of cd 0 3 the calculated maximum gas height of 110 m and the corresponding drain velocity of 19 m s 1 were obtained

liquid carbon dioxide of magmatic origin and its role in volcanic eruptions natural liquid carbon dioxide is produced commercially from a 2 5 km deep well near the 4 500 yr old maar volcano mount gambier south australia the carbon dioxide has accumulated in a dome that is located on the extension of a linear chain of volcanic activity a magmatic origin for the fluid is suggested by the geological setting 13cpdb of 4 0 for the co2 where pdb represents the carbon isotope standard and a relatively high 3he component of the contained helium and high 3he c ratio 6 4 x 10 10 the 3he 4he and he ne ratios are 3 0 and 1 370 times those of air respectively the co2 as collected at the earth s surface at 29 5 c and 75 bar expands more than 300 fold to form a gas at 1 atm and 22 c we suggest that liquid co2 or high density co2 fluid the critical point is 31 1 c 73 9 bar of volcanic origin that expands explosively from shallow levels in the earth s crust may be a major contributor to phreatic volcanic eruptions and maar formation less violent release of magmatic co2 into crater lakes may cause gas bursts with equally disastrous consequences such as occurred at lake nyos cameroon in august 1986

response to comment on mantle flow drives the subsidence of oceanic plates croon et al challenge our conclusion that sea floor depth variations are driven by the underlying mantle convection we point out that contrary to their claim our data analysis is pertinent and that the sea floor linear trend as the square root of the distance from the ridge is a robust observation the mechanism responsible for this trend is an asthenospheric flow faster than the overlying plate which shapes the lithosphere structure

the evolution of thermal structure and water chemistry in lake nyos abstract we collected a time series of physical and chemical data to gain a better understanding of the dynamics of lake nyos measurements of water and gas chemistry and temperature made during january march and may 1987 are compared to data taken in september 1986 just after the initial co 2 gas release there is no pattern of change in overall heat content of the lake although heat input to bottom waters 185 208 m has occurred at a rate of 1600 mw m 2 this increase in heat content translates to a change from 23 38 to 24 12 c at 200 m and can be explained by geothermal heat flow and addition of thermal spring water concentrations of ca 2 mg 2 na k fe 2 and alkalinity have increased only in bottom waters in situ lake processes such as sulfate and iron reduction are unable to account for the changes in alkalinity observed chemical changes are consistent with a scenario where slightly thermal soda water is being input to the bottom of the lake measurements of p co 2 at depth ranged from 18 to 28 of saturation and exhibited horizontal variability overall recharge of co 2 in bottom waters is negligible mainly because of increasing ion concentrations in bottom water total stability of the water column increased 33 from 48 800 j m 2 in september 1986 to 64 700 j m 2 in may 1987 as long as co 2 concentrations remain the same this level of stability is higher than could be disrupted by common limnologic or meteorologic processes there is thermal and chemical evidence that a buildup of dissolved iron and co 2 in bottom waters must have preceded the august 1986 gas release in addition a survey of all crater lakes in cameroon indicates that only lakes nyos and monoun contain high concentrations of dissolved iron and co 2 thus there is a low probability of any other cameroonian lake releasing a substantial volume of co 2

origin of light hydrocarbons in gases from mud volcanoes and ch4 rich emissions abstract this paper presents new chemical data of hydrocarbon rich gases discharged from mud volcanoes and ch4 rich emissions located in different areas of italy northern apennines and sicily the determination of c5 c10 alkanes cyclics and aromatics was carried out by gc ms while the main gas species and c1 c4 hydrocarbons were analyzed by gc tdc and gc fid respectively methane is by far the most abundant component of all the investigated gas emissions with the exception of part of those discharging close to mt etna volcano which are co2 rich the gas samples collected from the emilia apennine and sicily show d ch4 and 13c ch4 values typical of thermogenic gases most gases from the romagna apennine discharge methane with a biogenic isotopic signature non methane hydrocarbons of biogenic gases are almost exclusively c2 c4 alkanes with minor amounts fraction of mol mol of c4 compounds including few aromatics and cyclics likely deriving from minor thermogenic contribution not evidenced by the 13c ch4 and d ch4 values the etnean gases whose r ra and 13c co2 values indicate a strong contribution from a hydrothermal fluid source related to the nearby volcanic system show a significant enrichment of aromatics compounds up to 36 of the non methane organic gas fraction which is likely produced by catalytic reforming processes such as dehydrocyclization of alkanes the thermogenic gases from the apennines and southwestern sicily are characterized by the presence of more than 20 different cyclic compounds with concentrations up to several mol mol cyclic compounds are likely formed by i thermal cracking and ii uncompleted aromatization of alkanes occurring at depth 3 km and temperatures not exceeding 120 150 c

temporal variability of in situ methane concentrations in gas hydrate bearing sediments near bullseye vent northern cascadia margin 1 to assess the temporal variability in the methane fluxes from marine sediments that overly gas hydrate bearing sediments and the factors that might control its rate in situ methane concentrations were measured near bullseye vent on the northern cascadia continental margin a long term sampling device collected overlying water and pore fluid samples from 25 cm above seafloor at the sediment water interface swi and 7 cmbsf centimeters below seafloor over a 9 month period august 2009 may 2010 these samples provide a record at 4 day resolution of in situ methane ethane propane sulfate and chloride concentrations as well as stable carbon isotope ratios of methane 13c ch4 and dissolved inorganic carbon 13c dic we show that pore fluids near the swi are saturated or supersaturated with respect to methane 80 mm and the methane flux from the seabed is variable over time we hypothesized that regional seismic activity controlled this variable ch4 flux in the northern cascadia continental margin setting however we found no direct correlation between earthquakes and ch4 flux we also posited alternative controls on ch4 flux variability such as storms regional oceanography and microbial activity again no direct correlation was seen this study takes first steps toward exploring which physical factors play a role in methane flux from hydrate bearing sediments

origin and age of the lake nyos maar cameroon abstract lake nyos occupies a young maar crater in the precambrian granitic terrane of northwest cameroon the lake is partly surrounded by poorly consolidated ultramafic nodule bearing pyroclastic surge deposits that were explosively ejected from the nyos crater at the time of its formation radiocarbon dates show that the maar probably formed about 400 years ago field evidence suggests that carbon dioxide could have been the principal volatile involved in the formation of the nyos maar and that the role of water may have been minor the formation of the nyos maar was preceded by a brief period of effusive basaltic volcanism but the maar itself may have largely formed by cold dry explosive processes carbon dioxide may still be trapped interstitially in a diatreme inferred to underlie lake nyos its gradual release into the waters of lake nyos may have set the stage for the tragic gas release event of august 21 1986 only young maar lakes such as nyos may pose a danger of future lethal gas releases

directional median filtering for regional residual separation of bathymetry 1 we present a new spatial filter designed to separate short length scale bathymetric features from regional bathymetry the directional median dim filter divides a given filter circle into n bow tie sectors allocates data points inside the filter circle to each sector based on their relative position within the circle estimates a median for each sector and returns the lowest of these n medians this approach prevents dim filtering from choosing the biased medians near the features on a sloping regional trend which is a serious artifact of standard median filtering as the bathymetry comprises diverse length scale features the separated results vary with the choice of filter width such variations are spatially distributed using a finite range of filter widths we evaluate the spatial distribution of variations in the dim based separation by estimating their median absolute deviation mad at each data point the distribution of mad values is indicative of uncertainties inherent in the separation we demonstrate the effectiveness of dim filtering with both synthetic and actual bathymetric data

gas buildup in lake nyos cameroon the recharge process and its consequences the gases dissolved in lake nyos cameroon were quantified recently december 1989 and september 1990 by two independent techniques in situ measurements using a newly designed probe and laboratory analyses of samples collected in pre evacuated stainless steel cylinders the highest concentrations of co2 and ch4 were 0 30 mol kg and 1 7 mmol kg respectively measured in cylinders collected 1 m above lake bottom probe measurements of in situ gas pressure at three different stations showed that horizontal variations in total dissolved gas were negligible total dissolved gas pressure near the lake bottom is 1 06 mpa 10 5 atm 50 as high as the hydrostatic pressure of 2 1 mpa 21 atm comparing the co2 profile constructed from the 1990 data to one obtained in may 1987 shows that co2 concentrations have increased at depths to below 150 m based on these profiles the average rate of co2 input to bottom waters was 2 6 108 mol a increased deep water temperatures require an average heat flow of 0 32 mw into the hypolimnion over the same time period the transport rates of co2 heat and major ions into the hypolimnion suggest that a low temperature reservoir of free co2 exists a short distance below lake bottom and that convective cycling of lake water through the sediments is involved in transporting the co2 into the lake from the underlying diatreme increased ch4 concentrations at all depths below the oxycline and a high14c content 41 modern in the ch4 4 m above lake bottom show that much of the ch4 is biologically produced within the lake the ch4 production rate may vary with time but if the co2 recharge rate remains constant co2 saturation of the entire hypolimnion below 50 m depth would require 140a given present day concentrations

basin wide estimates of the input of methane from seeps and clathrates to the black sea numerous methane emitting bottom features such as seeps methane clathrate hydrates clathrates and mud volcanoes have been identified recently in the black sea the fluxes of methane from these sources averaged over large spatial scales are unknown here we take advantage of the fact that the black sea is a semi enclosed basin with restricted deep water circulation to establish first order estimates of basin wide fluxes of methane from these sources to the water column and atmosphere first we measured the natural radiocarbon content of methane 14c ch4 dissolved in the water column and emitted from seeps the 14c ch4 results showed that the dominant source of methane to the water column is emitted from seeps and a smaller source is diagenetically produced in relatively modern sediments the 14c ch4 results were then used to partition a basin wide total methane budget this analysis estimated the basin wide flux of methane from seeps and clathrates to the water column to be 3 60 to 4 28 tg yr 1 second a geochemical box model was used to calculate possible distributions of methane inputs from seeps and clathrates as well as provide additional estimates of the basin wide flux of methane from seeps and clathrates to the water column 4 95 to 5 65 tg yr 1 2006 elsevier b v all rights reserved

new analytic solutions for modeling vertical gravity gradient anomalies modern processing of satellite altimetry for use in marine gravimetry involves computing the along track slopes of observed sea surface heights projecting them into east west and north south deflection of the vertical grids and using laplace s equation to algebraically obtain a grid of the vertical gravity gradient vgg the vgg grid is then integrated via overlapping flat earth fourier transforms to yield a free air anomaly grid because of this integration and associated edge effects the vgg grid retains more short wavelength information e g fracture zone and seamount signatures that is of particular importance for plate tectonic investigations while modeling of gravity anomalies over arbitrary bodies has long been a standard undertaking similar modeling of vgg anomalies over oceanic features is not commonplace yet here we derive analytic solutions for vgg anomalies over simple bodies and arbitrary 2 d and 3 d sources we demonstrate their usability in determining mass excess and deficiency across the mendocino fracture zone a 2 d feature and find the best bulk density estimate for jasper seamount a 3 d feature the methodologies used herein are implemented in the generic mapping tools available from gmt soest hawaii edu

flexure modelling at seamounts with dense cores summary the lithospheric response to seamounts and ocean islands has been successfully described by deformation of an elastic plate induced by a given volcanic load if the shape and mass of a seamount are known the lithospheric flexure due to the seamount is determined by the thickness of an elastic plate te which depends on the load density and the age of the plate at the time of seamount construction we can thus infer important thermomechanical properties of the lithosphere from te estimates at seamounts and their correlation with other geophysical inferences such as cooling of the plate whereas the bathymetry i e shape of a seamount is directly observable the total mass often requires an assumption of the internal seamount structure the conventional approach considers the seamount to have a uniform density e g density of the crust this choice however tends to bias the total mass acting on an elastic plate in this study we will explore a simple approximation to the seamount s internal structure that considers a dense core and a less dense outer edifice although the existence of a core is supported by various gravity and seismic studies the role of such volcanic cores in flexure modelling has not been fully addressed here we present new analytic solutions for plate flexure due to axisymmetric dense core loads and use them to examine the effects of dense cores in flexure calculations for a variety of synthetic cases comparing analytic solutions with and without a core indicates that the flexure model with uniform density underestimates te by at least 25 per cent this bias increases when the uniform density is taken to be equal to the crustal density we also propose a practical application of the dense core model by constructing a uniform density load of same mass as the dense core load this approximation allows us to compute the flexural deflection and gravity anomaly of a seamount in the wavenumber domain and minimize the limitations recognized from the analytic tests then the dense core model is applied to predict the lithospheric flexure beneath howland island in the tokelau seamount chain these results are compared with the predictions of the uniform density model based on age dating of howland and the age of the seafloor traditional te versus age curves predict the elastic plate thickness beneath the seamount to be around 20 km which is comparable to the best dense core model of te 26 km however the best uniform density model is found at te 12 km which is significantly less than the predicted from our investigations of synthetic and real seamount cases we conclude that the dense core model approximates the true mass distribution of a seamount better than the uniform density model finally we suggest that the role of underplating in flexure modelling may need to be reexamined because the dense core model predicts substantially less deflections than the uniform density model without requiring additional buoyancy caused by underplated material

an estimate of the effective elastic thickness of the cape verde rise application of the response function technique to the load of the cape verde archipelago on the cape verde rise suggests the effective elastic thickness of the rise is only 15 3 km this is thinner than would be expected for 130 ma crust from previous observations worldwide which suggest a value of 30 10 km this thinning is considered to be substantial evidence that the lithosphere of the cape verde rise has undergone thermal rejuvenation comparison of this study with previous work at hawaii suggests that reduction of the effective elastic thickness over swells with large depth anomalies is related to plate motion over the mantle heat source

methanotrophic bacteria occupy benthic microbial mats in shallow marine hydrocarbon seeps coal oil point california 1 microbial mats composed of giant sulfur bacteria are observed throughout the benthos along continental margins these communities serve to oxidize dissolved sulfides to sulfate and are typically associated with the recent exposure of sulfide rich sediments such mats are also ubiquitous in areas of hydrocarbon seepage where they are thought to consume sulfide generated in underlying sediment despite the high abundance of dissolved methane in hydrocarbon seeps few studies have considered the importance of methanotrophy in mat communities to assess the importance of methanotrophs in microbial mats from hydrocarbon seeps an approach involving lipid biomarkers stable isotopes and enrichment culturing was applied microbial mat samples were collected from benthic surfaces at two hydrocarbon seeps located in the coal oil point seep field offshore from goleta california both samples display a high abundance of 16 1 fatty acids including two isomers specific to type i methanotrophic bacteria 16 1 8 and 16 1 6 depleted values of 13c found in 16 1 fatty acids suggests methane assimilation into biomass whereas three separate investigations of sulfide oxidizing bacteria yield fractionation factors too small to account for these values on the basis of these observations and experiments an isotope mass balance was applied to fatty acids present in the microbial mat samples which indicates methanotrophs contribute up to 46 of total fatty acids these results implicate methanotrophy as an important function for microbial mats in seep areas despite the visual appearance of these mats as being composed of giant sulfur bacteria

dynamics of lake eruptions and possible ocean eruptions dissolved gas in liquid is able to power violent eruptions two kinds of such gas driven eruptions are known in nature explosive volcanic eruptions driven by dissolved h2o in magma at high temperatures and lake eruptions driven by dissolved co2 in water at low temperatures there are two known occurrences of lake eruptions one in 1984 lake monoun and one in 1986 lake nyos both in cameroon africa the erupted co2 gas asphyxiated 1700 people in the lake nyos eruption and 37 people at lake monoun here we review experimental simulations of co2 driven water eruptions and dynamic models of such eruptions and a bubble plume theory is applied to the dynamics of lake eruptions field evidence experimental results and theoretical models show that lake eruptions can be violent and theoretical calculations are consistent with the high exit velocities and eruption columns inferred from observations furthermore the dynamics of lake degassing experiments are consistent with theoretical models other kinds of gas driven eruptions are possible and may have occurred in nature in the past a concentrated and large release of methane gas or hydrate from marine sediment may result in an ocean eruption furthermore injection of liquid co2 into oceans might also lead to ocean eruptions if care is not taken the various kinetic and dynamic processes involved are examined and quantified

modal depths from shipboard bathymetry there is a south pacific superswell since the first quantification 10 years ago of a large regional depth anomaly in french polynesia numerous studies have explored the origin and the geophysical and geochemical consequences of this south pacific superswell these efforts would be widely viewed as a waste of time if this superswell were proven not to exist recently levitt and sandwell 1996 have proposed just that based on a modal analysis of multibeam bathymetric data collected on sea floor aged 15 to 35 ma in eastern french polynesia citing a discrepancy between the etopo5 gridded depths and ship data from 4 expeditions in their study area they suggest that the entire superswell could be an artifact of poor etopo5 sampling and gridding here we present a more comprehensive analysis of original ship soundings from 82 oceanographic expeditions with satellite navigation collected since 1967 the soundings span sea floor from less than 30 to more than 110 ma we confirm the conclusion of levitt and sandwell that depth anomalies calculated from etopo5 slightly overestimate the true values from original ship soundings on sea floor aged 30 to 35 million years but a positive depth anomaly of 250 m exists nevertheless on older sea floor we find that etopo5 slightly underestimates the magnitude of the depth anomaly based on a modal analysis but overall the agreement in the mode of the depth age data between etopo5 and the original ship soundings is quite striking especially given the fact that less than 20 of the ship data used in our analysis had been collected at the time that the global bathymetric grid was prepared there indeed is a south pacific superswell and the magnitude and age dependence of the depth anomaly is as originally calculated from the mode of gridded bathymetry

filtering the signature of submerged large woody debris from bathymetry data modeling water velocities and depth for riverine aquatic habitat analysis requires fine scale surveys of river bathymetry the presence of submerged large woody debris lwd distorts the results of acoustic bathymetry surveys and subsequent modeling if the lwd data is not separated from the background river bathymetry submerged lwd typically appears in digital acoustic data for a low gradient sand bed river as impulse spikes of a few data points with a substantially shallower depth than the surrounding data this paper examines the performance of linear and nonlinear filtering algorithms for removing impulse spikes from a digital bathymetry signal a synthesized data set is used for control tests that quantify the error associated with each method the more successful nonlinear filtering techniques median and erosion filters are used to filter single beam echosounder data from the sulphur river in northeastern texas usa median filtering proved to be the best technique for removing lwd impulse spikes while leaving the background bathymetry relatively unchanged efficient automation of spike removal from a data set requires a method for selecting the filter characteristics without recourse to engineering judgment or prior experience a method of a priori selecting the minimum filter order based upon the physical scales of the lwd and the statistics of the data separation in the survey is proposed based on scaling analysis and validated with the study results

satellite monitoring of the vertical temperature profile of lake nyos cameroon abstract the present contribution to lake nyos studies which have been carried out since 1987 by a team of french geophysicists working in close cooperation with the institut de recherches geologiques et minieres irgm of cameroon involves a continuous long duration remote monitoring of the temperature distribution in the depth of the lake the system which was developed for that purpose essentially includes 1 a vertical chain of temperature probes lifted by an anchored buoy called nyossonde and 2 an associated telemetry relayed by the argos transponder on board the noaa meteorological satellites the telemetered data are collected processed and archived both in france lannion toulouse garchy and cameroun yaounde even though the upper part of the chain was damaged the lower part is still in operation analysis of the telemetered data and comparison with other measurements confirm that the presently described monitoring system is well suited for installation in crater lakes

testing techniques to quantify drumlin height and volume synthetic dems as a diagnostic tool glacial bedform height h and volume v likely preserve important information about the behaviour of former ice sheets however large systematic errors exist in the measurement of h and v three semi automated methods to isolate drumlins from other components of the landscape e g trees hills as portrayed by nextmap have recently been devised however it is unclear which is most accurate this paper undertakes the first quantitative comparison of such readily implementable methods illustrating the use of statistically representative synthetic landscapes as a diagnostic tool from this analysis guidelines for quantifying the 3d attributes of drumlins are proposed specifically to avoid obtaining incorrect estimates caused by substantial systematic biases interpreters should currently take three steps declutter the digital elevation model for estimating h but not v remove height data within the drumlin then interpolate across the resultant hole to estimate a basal surface using delaunay triangulation results are demonstrated through analysis of drumlins in an area in western central scotland the guidance arguably represents the best current advice for subglacial bedforms in general highlighting the need for more studies into the quality of mapped data using synthetic landscapes copyright 2014 john wiley sons ltd

the lake nyos gas disaster chemical and isotopic evidence in waters and dissolved gases from three cameroonian crater lakes nyos monoun and wum abstract to better understand the cause of the nyos gas disaster of august 21 1986 we conducted geochemical and limnological surveys in october 1986 of three lakes nyos monoun and wum which are located in the cameroon volcanic zone that is characterized by a prevalence of young alkaline basalts and basanitoids lake wum was studied as a non active control co2 is dissolved in significant concentrations about 1 5 of saturation in gas active lakes nyos and monoun but is virtually absent in lake wum stable isotopic ratios of total dissolved carbon 13c 3 for nyos and 5 5 for monoun and of helium 5 7 ratm for nyos and 3 6 ratm for monoun indicate a mantle origin of these gases however so42 and cl concentrations are found to be very low concentrations of dissolved chemical species like fe2 mg2 ca2 and hco3 are high in the two gas active lakes whereas they are very low in the gas inactive lake high salinities in the gas active lakes are probably due to dissolution of indigenous mafic rocks and transported lateritic soil in acidic co2 rich warm water the gas active lakes are characterized by increasing temperature and salinity with increasing depth indicating an active influx of heat and dissolved materials at the bottom density estimates show that the lake water is stably stratified in spite of the inverse temperature profile of the lakes on account of dissolved chemical species the concentrations of dissolved carbonate species co2 aq and hco3 are positively correlated with those of ionic dissolved species indicating their common occurrence in the bottom water the august 1986 gas bursts from lake nyos were most likely caused by rapid exsolution of dissolved co2 within the lake an explosive process such as a phreatic eruption or a co2 gas jetting from beneath the bottom is unlikely because of low concentrations of cl and so42 no oxygen isotopic shift low turbidity and no reported perturbation of the bottom sediments exsolution of co2 bubbles could occur if co2 saturated bottom water was displaced upwards by an increased influx of high salinity water from the bottom during the rainy season exsolution of co2 at the upper layers was possibly accelerated by upwelling of a two phase fluid co2 bubbles and solution a mechanism known as a pneumatic lift pump resulting in discharge of a large amount of co2 gas the h2s concentration in the gas cloud must have been kept far below the lethal level because of a high fe2 concentration of the lake water

the significance of siderite in the sediments from lake nyos cameroon abstract small amounts 1 3 wt of endogenic siderite feco3 and traces of pyrite fes2 are present in a 1 m long sediment core from lake nyos thermodynamic modeling shows that the deep waters are supersaturated with respect to siderite and pyrite no other carbonates were found to be stable this rare occurrence of siderite is symptomatic of the build up of large concentrations of co2 and fe2 in the lake nyos waters a search for siderite in the deeper sediments could reveal the existence of previous gas burst events that have occurred at lake nyos or in other cameroon lakes

a survey of methane isotope abundance 14c 13c 2h from five nearshore marine basins that reveals unusual radiocarbon levels in subsurface waters methane ch4 in the subsurface ocean is often supersaturated compared to equilibrium with the modern atmosphere in order to investigate sources of ch4 to the subsurface ocean isotope surveys 14c ch4 13 c ch4 2 h ch4 were conducted at five locations skan bay sb santa barbara basin sbb santa monica basin smb cariaco basin cb and the guaymas basin gb depth distributions of ch4 concentration and isotopic abundance were determined for both the sediment and water column at the sb sbb smb and cb sites ch4 emitted from seeps on the continental shelf adjacent to the sbb as well as seeps and decomposing clathrate hydrates in the gb was also collected purified and analyzed methane isotope distributions in the sediments were consistent with known methanogenic and methanotrophic activity seep and clathrate hydrate derived ch4 was found to be depleted in radiocarbon however surprising results were obtained in the water column at all sites investigated in sb the radiocarbon content of the subsurface ch4 concentration maximum was on average 41 less than its suspected sediment ch4 source suggesting ch4 seepage in the bay in the sbb smb and cb the 14c ch4 contents in the subsurface ocean were 1 2 to 3 6 times greater than modem carbon quantities suggesting a source of 14c from atmospheric nuclear weapons testing nuclear power plant effluents or cosmogenic isotope production copyright 2008 by the american geophysical union

flexure and gravity anomalies of the oceanic lithosphere beneath the louisville seamount abstract we have calculated the elastic thickness te flexural deflection and gravity anomaly of the oceanic crust beneath the louisville seamount lsc 03 near the kermadec trench a regional residual separation of the bathymetry was performed to remove the effect of other geologic features e g the trench we used the uniform density and dense core models to approximate the total mass of the seamount which was defined as the surface load required for flexural deformation from the flexure modeling results we found that more flexural depression was predicted by the uniform density model than by the dense core model however the uniform density model predicted a significantly smaller gravity anomaly than observed whereas the dense core model minimized the prediction misfits reasonably the best flexure model was found with a te of 16 km for the uniform density model and 6 km for the dense core model the flexure computed with the dense core model was consistent with the seismically detected moho the flexure modeling for lsc 03 thus indicates that the dense core model better approximates the inner structure of the lsc 03 based on the crustal age and geochronology of the given seamount the age of the oceanic crust at the time of seamount formation t is 20 ma if this is the case however the te estimates from both flexure models require some degree of lithospheric reheating by louisville hotspot activity alternatively considering the tectonic plate motion of the osbourn trough t becomes approximately 4 ma this younger lithosphere model is more consistent with the observed flexural deformation and the te estimate from the dense core model therefore the time that the seamount induced lithospheric deformation occurred may be far earlier than the age dated volcanism

southward migration of the intertropical convergence zone through the holocene titanium and iron concentration data from the anoxic cariaco basin off the venezuelan coast can be used to infer variations in the hydrological cycle over northern south america during the past 14 000 years with subdecadal resolution following a dry younger dryas a period of increased precipitation and riverine discharge occurred during the holocene thermal maximum since approximately 5400 years ago a trend toward drier conditions is evident from the data with high amplitude fluctuations and precipitation minima during the time interval 3800 to 2800 years ago and during the little ice age these regional changes in precipitation are best explained by shifts in the mean latitude of the atlantic intertropical convergence zone itcz potentially driven by pacific based climate variability the cariaco basin record exhibits strong correlations with climate records from distant regions including the high latitude northern hemisphere providing evidence for global teleconnections among regional climates

tidally controlled gas bubble emissions a comprehensive study using long term monitoring data from the neptune cabled observatory offshore vancouver island long term monitoring over one year revealed high temporal variability of gas emissions at a cold seep in 1250 m water depth offshore vancouver island british columbia data from the north east pacific time series underwater networked experiment observatory operated by ocean networks canada were used the site is equipped with a 260 khz imagenex sonar collecting hourly data conductivity temperature depth sensors bottom pressure recorders current meter and an ocean bottom seismograph this enables correlation of the data and analyzing trigger mechanisms and regulating criteria of gas discharge activity three periods of gas emission activity were observed a short activity phases of few hours lasting several months b alternating activity and inactivity of up to several day long phases each and c a period of several weeks of permanent activity these periods can neither be explained by oceanographic conditions nor initiated by earthquakes however we found a clear correlation of gas emission with bottom pressure changes controlled by tides gas bubbles start emanating during decreasing tidal pressure tidally induced pressure changes also influence the subbottom fluid system by shifting the methane solubility resulting in exsolution of gas during falling tides these pressure changes affect the equilibrium of forces allowing free gas in sediments to emanate into the water column at decreased hydrostatic load we propose a model for the fluid system at the seep fueled by a constant sub surface methane flux and a frequent tidally controlled discharge of gas bubbles into the ocean transferable to other gas emission sites in the world s oceans

flexure of the ocean lithosphere from island uplift bathymetry and geoid height observations the society islands summary ln flexure models of the response of the ocean lithosphere to seamount loading three key unknowns must generally be considered the density distribution of the load the degree of fd in of the deflection created moat and the flexural rigidity the models are usually constrained by one of three types of observations gravity or geoicl height anomalies the moat and arch geometry and the amount of uplift of islands on this arch the determination of the model parameters from geoid anomalies is critically dependent on the interpretation of the broad and regional geoid rise that is seen over many of the younger seamount chains and likewise the flexure parameters deduced from the moat and arch geometry are a function of the interpretation of the associated bathymetric swell the uplift data require that the tectonic erosional and sea level history of the region be known although this requirement can be relaxed if rather than the uplift relative to sea level the change in uplift with distance from the load is used as the observed quantity all these types of observations have been used in this paper to constrain the flexure model for the society islands and while any one data type on its own does not lead to reliable flexure parameters a consistent solution is found when the three are taken together the results point to a relatively low load density of 2 5 g m apart from a high density core a moat that is partly fdled in with sediment or volcanic debris and a flexural rigidity of about 3 x dyne cm

a seismic reflection profile study of lithospheric flexure in the vicinity of the cape verde islands 1 seismic reflection profile data are used to determine the stratigraphic architecture of the flexural moat that flanks the cape verde islands the moat region is characterized by upward of 1 2 km of poorly to well stratified material the two lowermost units thicken from west to east and are attributed to sediment loading and flexure at the nearby west africa continental margin during early cretaceous to early miocene the two uppermost units in contrast thicken concentrically around the islands and are attributed to the infilling of a flexural moat that formed by volcanic loading since the early miocene flexure modeling shows that the thickness of the moat infill cannot be explained only by surface loading and requires that the downward flexure due to surface loads is opposed by an upward acting subsurface load the best fit combined surface and buried loading model is for an elastic thickness te of 29 km a load and infill density of 2700 kg m 3 and a ratio of surface to subsurface loads f of 0 2 these results are in accord with spectral studies of free air gravity anomaly and topography data based on the admittance technique the subsurface loads are spatially limited to the islands and their submarine flanks nevertheless they are associated with a broad regional uplift of up to 400 m the uplift is large enough to explain why the moat infill is generally tilted away from rather than toward the islands the uplift is too small however to account for the height of the swell upon which the cape verde islands are superimposed the origin of the cape verde swell is not known however a normal te and a modest heat flow anomaly suggest that the swell cannot be fully explained by uplift due to thermal reheating of the lithosphere by an underlying hot spot and that other deep seated mantle processes must be involved

synchronous radiocarbon and climate shifts during the last deglaciation radiocarbon data from the cariaco basin provide calibration of the carbon 14 time scale across the period of deglaciation 15 000 to 10 000 years ago with resolution available previously only from holocene tree rings reconstructed changes in atmospheric carbon 14 are larger than previously thought with the largest change occurring simultaneously with the sudden climatic cooling of the younger dryas event carbon 14 and published beryllium 10 data together suggest that concurrent climate and carbon 14 changes were predominantly the result of abrupt shifts in deep ocean ventilation

quaternary fault kinematics and stress tensors along the southern caribbean from fault slip data and focal mechanism solutions deformation along the southern caribbean coast as confirmed by the compilation of stress tensors derived from fault plane kinematic indicators microtectonics and further supported by focal mechanism solutions herein presented results from a compressive strike slip transpressional senso lato regime characterized by a nnw sse maximum horizontal stress 1h 11 and or an ene wsw minimum 1h 13 or 12 horizontal stress which is responsible for present activity and kinematics of six sets of brittle features east west right lateral faults nw se right lateral faults synthetic riedel shears ene wsw to east west dextral faults p shears nnw sse normal faults almost north south left lateral faults antithetic riedel shears and ene wsw reverse faults sub parallel to fold axes and mostly in the subsurface the latter ones being associated to ene wsw trending folding in this particular region the brittle deformation obeys the simple shear model although not all the deformation can be accounted for it since partitioning is also taking place regional folding and thrusting is essentially due to the normal to structure component of the partitioned maximum horizontal stress conversely the maximum horizontal stress on

testing 3d landform quantification methods with synthetic drumlins in a real digital elevation model metrics such as height and volume quantifying the 3d morphology of landforms are important observations that reflect and constrain earth surface processes errors in such measurements are however poorly understood a novel approach using statistically valid synthetic landscapes to quantify the errors is presented the utility of the approach is illustrated using a case study of 184 drumlins observed in scotland as quantified from a digital elevation model dem by the cookie cutter extraction method to create the synthetic dems observed drumlins were removed from the measured dem and replaced by elongate 3d gaussian ones of equivalent dimensions positioned randomly with respect to the noise e g trees and regional trends e g hills that cause the errors then errors in the cookie cutter extraction method were investigated by using it to quantify these synthetic drumlins whose location and size is known thus the approach determines which key metrics are recovered accurately for example mean height of 6 8 m is recovered poorly at 12 5 0 6 2 m but mean volume is recovered correctly additionally quantification methods can be compared a variant on the cookie cutter using an un tensioned spline induced about twice 1 79 as much error finally a previously reportedly statistically significant p 0 007 difference in mean volume between sub populations of different ages which may reflect formational processes is demonstrated to be only 30 50 likely to exist in reality critically the synthetic dems are demonstrated to realistically model parameter recovery primarily because they are still almost entirely the original landscape results are insensitive to the exact method used to create the synthetic dems and the approach could be readily adapted to assess a variety of landforms e g craters dunes and volcanoes