carbon geochemistry of cold seeps methane fluxes and transformation in sediments from kazan mud volcano eastern mediterranean sea abstract despite growing concerns about potential enhancement of global warming and slope failure by methane produced by gas hydrate dissociation much uncertainty surrounds estimates of gas hydrate reservoir sizes as well as methane fluxes and oxidation rates at the sea floor for cold seep sediments of the eastern mediterranean sea depth dependent methane concentrations and rates of anaerobic oxidation of methane aom are constrained by modeling the measured pore water sulfate profile the calculated dissolved methane distribution and flux are sensitive to the advective flow velocity which is estimated from the depth distributions of conservative pore water constituents na b near complete anaerobic oxidation of the upward methane flux of 6 0 mol m 2 yr 1 is supported by the depth distributions of indicative biomarkers and the carbon isotopic compositions of organic matter and dissolved inorganic carbon pore water and solid phase data are consistent with a narrow depth interval of aom 14 18 cm below the sediment water interface based on an isotopic mass balance the biomass of the microbial population carrying out oxidation of methane coupled to sulfate reduction at the given methane flux represents 20 of the total organic carbon which is a significant pool of in situ formed organic matter model results indicate that the asymptotic methane concentration is reached a few meters below the sediment surface the predicted asymptotic concentration is close to the in situ saturation value with respect to gas hydrate suggesting that the rate of shallow gas hydrate formation is controlled by the ascending methane flux the proposed model approach can be used to predict the formation of gas hydrate and to quantify methane fluxes plus transformation rates in surface sediments where fluid advection is an important transport mechanism

a kinetic model for the methane hydrate precipitated from venting gas at cold seep sites at hydrate ridge cascadia margin oregon we develop a kinetic model for hydrate crystallization from methane gas venting through shallow sediments at hydrate ridge on the cascadia margin of oregon that predicts how pore water chlorinity temperature and crystallized hydrate evolve after the onset of steady venting predictions are compared to observations at ocean drilling program site 1249 in the preferred model calculated gas hydrate saturation and chloride concentrations reach those observed at depths less than 20m below seafloor bsf under the southern summit of hydrate ridge in similar to 650years and the vertical water flux must be less than 50kg m 2 yr hydrate accumulates more slowly between 20m bsf and the base of the hydrate stability zone where there is no free gas accumulating to observed levels of a few volume percent of hydrate in 10 5 to 10 6 years depending on the water flux that is assumed through this zone this dichotomy means that the presently observed gas venting must have been diverted to this area similar to 650years ago or be episodic and infrequent if the gas venting for the last 650years has been as observed today the latent heat of hydrate precipitation in the upper 20m of sediments would have caused the temperature to increase similar to 0 8 degrees c at similar to 20m bsf and similar to 0 2 degrees c at similar to 100m bsf this would have caused a similar to 5m rise in the elevation of the base of hydrate stability zone and decreased the rate of hydrate crystallization from 1 5kg ch4 m 2 yr 650years ago to 0 7kg ch4 m 2 yr today

the anaerobic oxidation of methane and sulfate reduction in sediments from gulf of mexico cold seeps we determined the geochemical characteristics of sediments and measured rates of the anaerobic oxidation of methane aom and sulfate reduction sr in samples collected near thermogenic structure ii gas hydrate mounds and in areas lacking hydrates along the continental slope in the gulf of mexico we used radiotracer c 14 and s 35 techniques to determine rates of aom and sr over depth in sediment cores abundant mats of white and orange beggiatoa spp were common in areas of active seepage and these sediments were enriched in hydrogen sulfide and methane in cores collected from areas without beggiatoa or hydrate concentrations of redox metabolites showed little variation over depth and these sites were inferred to be areas of low seepage integrated aom rates were low in beggiatoa free cores 0 05 mmol m 2 day 1 and averaged 2 8 4 6 mmol m 2 day 1 in seep cores that contained beggiatoa or gas hydrate integrated sr rates were also low in beggiatoa free cores 1 mmol m 2 day 1 and averaged 54 94 mmol m 2 day 1 in cores with beggiatoa or hydrate rates of sr generally exceeded rates of aom and the two processes were loosely coupled suggesting that the majority of sr at gulf of mexico hydrocarbon seep sites is likely fueled by the oxidation of other organic matter possibly other hydrocarbons and oil rather than by aom

gas hydrate occurrence on the continental slope of the northern south china sea high resolution multi channel seismic data and geological samples were collected during two research cruises of the r v fendou 4 in 1999 and 2000 studies on these data and samples together with results from sites 1143 1145 and 1148 of odp leg 184 suggest that the geological structure on the continental slope of the northern south china sea is favorable for the formation of gas hydrates bottom simulating reflectors bsrs and geochemical anomalies which indicate the existence of gas hydrates have been recognized in sediments of the xisha trough the dongsha rise and the accretionary wedge of the manila subduction zone these gas hydrates are generated by two different mechanisms depending on the tectonic regime and the seismic and geochemical characteristics the first applies to the passive continental margin of the nor them south china sea on the dongsha rise and in the xisha trough the gas hydrates are associated with diapiric structures active faults slumps and gravity flows as well as high late cenozoic sedimentation rates their seismic expression includes bsrs seismic blanking zones and velocity anomalies the second mechanism is operative on the active continental margin along the manila subduction zone especially in the accretionary wedge here gas hydrate occurrence is marked by widespread bsrs and acoustic pull down possibly related to the existence of free gas in the sediments beneath the bsr the thickness of the seismic blanking zones averages 250 m suggesting that the stable gas hydrate zone has about the same thickness c 2005 elsevier ltd all rights reserved

sulfate methane transition as a proxy for average methane hydrate saturation in marine sediments 1 we develop a relationship between the sulfate methane transition smt and average gas hydrate saturation aghs for systems dominated by methane migration from deeper sources the relationship is explained by a one dimensional numerical model that simulates gas hydrate accumulation in marine sediments higher methane fluxes result in shallow smt depths and high aghs while lower methane fluxes result in deep smts and low aghs we also generalize the variation between aghs and scaled smt depth a procedure that aids prediction of aghs at different sites from observations of the smt such as along cascadia margin

geochemical evidence of methane seepage in the sediments of the qiongdongnan basin south china sea abstract in this paper the qs 1 sediments 3 64 m long at 1500 m water depth collected from the qiongdongnan basin were used to analyze the relationship between sedimentary geochemical anomalies and methane seepage mo u covariation ce anomaly and mg ca sr ca end member model were analyzed to identify the variations of redox environment and authigenic carbonate precipitation finally constructing the methane release events in this study area these observations suggest that 1 significant mo u enrichments and ba front indicate the presence of high flux active methane seepage with smi of 260 cmbsf in this study area the depositional environment evolved as oxic anoxic reducing sulfide environment from bottom to top as evidenced by mo u covariation and ce anomaly 2 based on sr ca and mg ca end member model proposed by bayon et al 2007 the high mg calcite was the dominant authigenic carbonate currently which usually precipitated behind the methane seep events and 3 the active cold seep can be divided into three methane release events mres mre i the most intense methane seepage occurred at 22 1 ka bp mre ii three low intensity methane seepage occurred at 19 3 16 5 and 11 3 ka bp respectively and mre iii the continuous methane seep began at 9 3 ka bp until now the synthesis of geochemical indicators has significant implications for tracing methane seepage of varying intensity and duration

remobilization of authigenic uranium in marine sediments by bioturbation uranium behaves as a nearly conservative element in oxygenated seawater but it is precipitated under chemically reducing conditions that occur in sediments underlying low oxygen bottom water or in sediments receiving high fluxes of particulate organic carbon sites characterized by a range of bottom water oxygen bwo and organic carbon flux ocf were studied to better understand the conditions that determine formation and preservation of authigenic u in marine sediments our study areas are located in the mid latitudes of the northeast pacific and the northwest atlantic oceans and all sites receive moderate 0 5 g cm2 kyr to high 2 8 g cm2 kyr ocf to the sediments bwo concentrations vary substantially among the sites ranging from 3 to 270 m a mass balance approach was used to evaluate authigenic u remobilization at each site within each region studied the supply of particulate nonlithogenic u associated with sinking particles was evaluated by means of sediment traps the diffusive flux of u into sediments was calculated from pore water u concentration profiles these combined sources were compared with the burial rate of authigenic u to assess the efficiency of its preservation a large fraction one third to two thirds of the authigenic u precipitated in these sediments via diffusion supply is later regenerated even under very low bwo concentrations 15 m bioturbating organisms periodically mix authigenic u containing sediment upward toward the sediment water interface where more oxidizing conditions lead to the remobilization of authigenic u and its loss to bottom waters

evidence of intense methane seepages from molybdenum enrichments in gas hydrate bearing sediments of the northern south china sea abstract the application of molybdenum mo enrichment to sediments can potentially provide unique constraints on the methane seepage dynamics at continental margins we report herein elemental and isotopic measurements for authigenic carbonates and sediment samples in drill cores 95 m long site gmgs2 08 at 800 m water depth from gas hydrate bearing sediments in the northern south china sea five intervals that were impacted by methane seepages in the sediment profile were identified by the presence of strongly 13 c depleted carbonates 13 c values as low as 56 8 the carbonates often cement chemosynthetic bivalve shell fragments and have anomalous 18 o positive values these observations suggest that methane seepage was intense during those time intervals and that carbonate precipitation occurred in the uppermost sediments probably due to gas hydrate dissociation interestingly the five intervals were all characterized by strong mo enrichments we thus hypothesize that the presence of strong mo enrichments is a good indicator of past episodes of methane seepage such an environment results in sulfidic conditions that occur in a narrow zone in proximity to seawater close to the seafloor leading to resultant mo enrichments in associated sediments in contrast environments with low methane flux would lead to the distribution of mo throughout a wide sulfidic zone and consequently an absence of mo enrichment in the sediments the insights into the environment for mo enrichments at seeps have significant implications for tracing past methane seepages their intensities and even possible occurrences of gas hydrate dissociation

molybdenum and uranium geochemistry in continental margin sediments paleoproxy potential abstract we measured solid phase mo and u concentrations in marine sediments from the california mexico peru and chile margins to ascertain the behavior of mo and u during early diagenesis in continental margin settings at sites along the california mexico and peru margins where there are estimates of mass accumulation rates authigenic u accumulation rates range from 0 50 nmol m 2 day 1 at the california and mexico margin sites mo accumulation rates range from 0 to 134 nmol m 2 day 1 whereas at the peru margin site rates may be as high as 550 nmol m 2 day 1 we observe relationships between metal accumulation rates and the delivery and burial rates of organic carbon corg in the case of mo there appears to be at least two relationships between metal accumulation rate and organic carbon burial for most of the data presented in this manuscript continental margin sediments have a mo corg accumulation rate ratio of 20 mol mol 1 this value is significantly lower however than those reported for anoxic basins 100 mol mol 1 but is consistent with reported mo corg rain ratios from mexico margin sediment traps in contrast to mo there appears to be a single u corg burial ratio of 5 mol mol 1 which includes a range of environments from anoxic basins and open ocean sites we interpret the relationships between metal accumulation and organic carbon to indicate that the reactions that govern authigenic metal accumulation are primarily sensitive to the delivery and burial of organic carbon in these particular settings however we note that any relationship between metal accumulation and organic carbon could be indirect in the particular case of mo based on what is known about mo geochemistry from the literature it is possible that mo authigenesis is coupled to sulfur cycling in many of the environments covered by this study and that the observed association between mo and organic carbon burial is a consequence of the coupling between carbon and sulfur burial using the observed relationships between mo and corg burial as well as constraints from the mo isotope budget we estimate that continental margin sediments are an important sink for mo the magnitude of this sink 0 4 108 mol y 1 or larger may be as much as one quarter of the oceanic removal term and is likely to be larger than the modern anoxic basin sink

controls on marine carbonate cement mineralogy review and reassessment abstract commonly cited models for controls on variations in carbonate mineralogy are only partly successful in accounting for natural mineralogical trends this limited success may result in part from a failure to recognize or address the ramifications of assumptions inherent to such models in general changes from calcite to aragonite or vice versa and variations in mgco3 incorporation in calcite are interpreted as resulting from 1 kinetic rather than thermodynamic effects 2 the effects of fluid chemistry rather than hydrologic biological or substrate effects 3 the effects of single parameters of fluid chemistry rather than the interactive or aggregate effects of several components and or 4 inorganic rather than organic parameters of fluid chemistry recent laboratory data suggest the necessity of a new approach to modeling controls on mineralogy these data indicate that no single fluid parameter is likely to be a sole or dominant control on carbonate mineralogy in natural environments in some cases thermodynamic stability differences may be as important as kinetic factors in determining mineralogy furthermore the effects of substrate hydrologic and biological factors and of organic compounds cannot be ruled out for most natural situations although at present reliable data are lacking to quantify the influence of these factors on mineralogy future models of controls on carbonate mineralogy need to consider the aggregate and interdependent effects of all of these factors

the oaxaca mexico earthquake of 1931 lithospheric normal faulting in the subducted cocos plate a study of the great earthquake in oaxaca mexico on 15 january 1931 shows that it was a normal fault earthquake striking east west the lack of evidence of surface faulting near the source region1 and the focal depth of 40 km inferred from body wave modelling suggest that the earthquake occurred within the subducted slab the large size of the earthquake ms 8 0 strongly suggests that the rupture broke through the entire lithosphere of the subducted cocos plate other large normal fault earthquakes in the japanese2 aleutian3 peruvian4 and sumba5 arcs show lithospheric faulting seawards of the trench or very close to it the 1931 event however occurred farther down dip in the slab 150 km landward of the trench below the bottom of what appears to be the seismogenic plate interface we speculate here that large although infrequent normal fault earthquakes such as the 1931 oaxaca earthquake may have a role in decoupling at depth the subducted slab from the upper part of the slab and perhaps from the overriding mexican continental plate the epicentral location of the 1858 michoacan earthquake6 ms 7 5 suggests that it may represent another example of lithospheric normal faulting in the middle american arc such earthquakes for which the periods of recurrence are unknown constitute an extremely high seismic risk to population centres of mexico

a viscoelastic coupling model for the cyclic deformation due to periodically repeated earthquakes at subduction zones vertical displacements due to periodic reverse faulting events in an elastic plate overlying a viscoelastic maxwell half space are obtained and compared with the observed deformation cycle coseismic strain release postseismic transients interseismic strain accumulation from japan the viscoelastic effects including the influence of buoyant restoring forces are obtained using the method developed by rundle and plate convergence and strain accumulation are incorporated following the procedure suggested by savage the resulting deformation cycle is compared with that of an analogous elastic half space dislocation model in which postearthquake effects are due to transient aseismic slip below the coseismic fault cyclic deformation is similar but not identical for the two models and observations from southwest japan suggest the superiority of the viscoelastic coupling model in particular inclusion of the effects of steady state flow in the asthenosphere overcomes a defect of the elastic half space model and results in agreement with the observed interseismic movement pattern several aspects of the postseismic deformation its landward migration and its transition to the interseismic phase of the cycle are explained as well but the short duration of near trench transients relative to those observed farther inland is not matched the success of a buried slip model in explaining early postseismic near trench movements and asthenospheric flow in accounting for cumulative postearthquake transient motions suggests the existence of a transition zone between lithosphere and asthenosphere whose behavior is brittle elastic in the short term and ductile for longer term deformation and such a modification may reconcile remaining discordant observations however reasonable variations in coupling model parameters cannot account for observed differences in the deformation cycle in other parts of japan and these regional differences remain unexplained

geochemistry of deep sea sediments at cold seep sites in the nankai trough insights into the effect of anaerobic oxidation of methane abstract two types of sediment were recovered from methane seep sites in the nankai trough cold seep sediment affected by anaerobic oxidation of methane aom and unaffected typical oxic marine sediment concentrations of carbon sulfur iron and trace metals were analyzed in these sediments the results showed that the sulfur contents of cold seep sediments are higher than those of typical oxic marine sediments although the organic carbon contents of the two types of sediment are identical the high sulfur contents of seep sediments suggest that aom was the dominant process at the cold seep sites h 2 s produced as a result of aom was fixed as reduced sulfur such as iron sulfide the highest manganese concentrations approx 1000 ppm were found in surface sediments at non seep sites whereas a strong enrichment of mn was not observed in the cold seep sediments manganese might be released from sediments as soluble mn 2 ion in the reduced environment produced by methane bearing seepage the molybdenum content in cold seep sediments up to 28 3 ppm is about 30 times higher than that in the typical oxic marine sediments however a correlation between mo and organic carbon contents which is characteristic of anoxic sediment was not observed in our samples cold seep sediments are a potential sink in the global geochemical cycle of mo

foraminifera from gas hydrate bearing sediments of the northeastern south china sea proxy evaluation and application for methane release activity abstract anomalously low 13c values of foraminifera in the stratigraphy are usually interpreted to reflect activities of paleo methane seepage the application of this proxy however requires to understand the cause of anomalously low 13c signals in foraminifera here we present stable carbon and oxygen isotopes 13c and 18o mg ca ratios and accelerator mass spectrometry ams radiocarbon data from cores gmgs2 08b 0 5 5 m below the seafloor mbsf and 08c 2 23 mbsf at 798 m water depth recovered from the gas hydrate bearing sediments in the northeastern south china sea scs the main objectives are to investigate the primary versus secondary signals of extremely negative 13c values of foraminifera and further recognize the methane emission events mees the 13c values of benthic uvigerina peregrina and cibicidoides spp and planktonic globigerinoides ruber and pulleniatina obliquiloculata foraminifera in these two cores show distinct intervals with negative values as low as 15 85 that indicate the shift of the sulfate methane transition zone smtz and the change in methane release flux in the past the 13c records of u peregrina by a more exhaustive cleaning procedure designed to remove authigenic carbonate coatings show that most 13c values are still lower relative to those in the normal marine environment the mg ca ratios of uvigerina spp by this exhaustive cleaning procedure display higher values and more variable for anomalous tests as high as 13 40 mmol mol than normal ones these observations indicate the 13c depleted signals of foraminifera are mainly originated from diagenetic alteration of authigenic carbonate precipitates with extremely negative 13c values furthermore the mass balance approach is adopted to assess the relative contribution of methane derived authigenic carbonate in the low 13c signals in foraminifera and reveals the maximum contribution can be as high as 30 among the 13c anomalies additionally several major mees with distinct characteristics can be recognized according to remarkable 13c excursions of foraminifera in these two cores

control of sulfate pore water profiles by sedimentary events and the significance of anaerobic oxidation of methane for the burial of sulfur in marine sediments abstract gravity driven mass flow deposits proven by sedimentary and digital echosounder data are indicative for prevailing dynamic sedimentary conditions along the continental margin of the western argentine basin in this study we present geochemical data from a total of 23 gravity cores pore water so 4 is generally depleted within a few meters below the sediment surface by anaerobic oxidation of methane aom the different shapes of so 4 profiles concave kink and s type can be consistently explained by sedimentary slides possibly in combination with changes in the ch 4 flux from below thus mostly representing transient pore water conditions since slides may keep their original sedimentary signature a combined analysis and numerical modeling of geochemical physical properties and hydro acoustic data could be applied in order to reconstruct the sedimentary history we present first order estimates of the dating of sedimentary events for an area where conventional stratigraphic methods failed to this day the results of the investigated sites suggest that present day conditions are the result of events that occurred decades to thousands of years ago and promote a persisting mass transport from the shelf into the deep sea depositing high amounts of reactive compounds the high abundance of reactive iron phases in this region maintains low hydrogen sulfide levels in the sediments by a nearly quantitative precipitation of all reduced sulfate by aom for the total region we estimate a so 4 or ch 4 flux of 6 6 10 10 moles per year into the zone of aom projected to the global continental slope and rise area this may sum up to about 2 6 10 12 moles per year provided that the sulfur is completely fixed in the sediments it is about twice the global value of the recent global sulfur burial in marine sediments of 1 2 10 12 moles per year as previously estimated thus aom obviously contributes very significantly to the regulation of global sulfur reservoirs which is hitherto not sufficiently recognized this finding may have implications for global geochemical models as sulfur burial is an important control factor in the development of atmospheric oxygen levels over time

marine pore water sulfate profiles indicate in situ methane flux from underlying gas hydrate marine pore water sulfate profiles measured in piston cores are used to estimate methane flux toward the sea floor and to detect anomalous methane gradients within sediments overlying a major gas hydrate deposit at the carolina rise and blake ridge u s atlantic continental margin here sulfate gradients are linear implying that sulfate depletion is driven by methane flux from below rather than by the flux of sedimentary organic matter from above thus these linear sulfate gradients can be used to quantify and assess in situ methane flux which is a function of the methane inventory below

multiple sulfur isotope constraints on sulfate driven anaerobic oxidation of methane evidence from authigenic pyrite in seepage areas of the south china sea abstract multiple sulfur isotope signatures and secondary ion mass spectroscopy sims sulfur isotope compositions of pyrite from two seafloor sites dh cl11 and hd109 in seepage areas of the south china sea were measured in order to study isotope effects of sulfate driven anaerobic oxidation of methane so 4 aom the multiple sulfur isotopes of pyrite reveal variable ranges for both sites 34 s between 44 1 and 2 9 for dh cl11 and between 43 8 and 1 6 for hd109 33 s between 0 02 and 0 17 for dh cl11 and between 0 03 and 0 14 for hd109 sims analysis reveals an extreme variability of 34 s values between 50 3 and 2 7 in dh cl11 between 50 1 and 52 4 in hd109 for three types of pyrite 1 framboids 2 zoned aggregates with radial overgrowth surrounding a framboidal core and 3 euhedral pyrite crystals the synchronous changes of geochemical proxies sulfate and methane concentrations 34 s sulfate and 18 o sulfate 34 s pyrite and pyrite content at the sulfate methane transition zone smtz at site dh cl11 are interpreted to be induced by so 4 aom under steady state conditions in contrast pyrite content and 34 s value fluctuations throughout core hd109 suggest that the sediment at this site was affected by multiple pyritization events during diagenesis multiple sulfur isotope signatures of early diagenetic pyrite i e with low and high 34 s values the latter above 315 cmbsf in dh cl11 above 70 cmbsf in hd109 in the upper sediment column suggest that organoclastic sulfate reduction osr and sulfur disproportionation generated the observed isotopic signatures in contrast to the early diagenetic 34 s depleted framboids the higher sims 34 s values of overgrowth and euhedral crystals suggest a late diagenetic 34 s enriched pool of dissolved sulfide derived from so 4 aom at the current and paleo smtzs interestingly pyrite resulting from so 4 aom in the smtz at site dh cl11 reveals a distinct pattern with higher 33 s values different from pyrite resulting from osr and sulfur disproportionation therefore paired 34 s and 33 s values may allow to differentiate osr and so 4 aom although a full understanding of the isotope effects associated with so 4 aom is hampered by uncertainties on the actual electron transfer mechanism in the syntrophic so 4 aom consortium

a kinetic model for the pattern and amounts of hydrate precipitated from a gas steam application to the bush hill vent site green canyon block 185 gulf of mexico 1 we construct a linear kinetic model of hydrate crystallization from a gas stream we use this model to predict the fraction of gas that crystallizes as hydrate in the subsurface of bush hill and the depth profile of subsurface hydrate accumulation this is possible because the bush hill vent is fed by reservoir gas from the nearby jolliet field whose composition is known on the average 9 of the vent gas is precipitated as hydrate in the subsurface although other explanations are possible the observed vent gas compositions and the greater range of hydrate gas compositions are consistent with a single source gas whose venting rate varies by a factor of at least 3 over periods of a few years or less the predicted depth profile of hydrate accumulation and the hydrate content of the bush hill mound suggest that between 1 1 109 and 2 8 109 m3 stp of gas may have accumulated as hydrate between the seafloor and 614 m depth for the radiometrically and geologically suggested system age of 10 000 years the time average venting rate is 106 m3 yr 0 7 106 kg yr if distributed evenly across the 600 m diameter mound as suggested by echo sounder images the methane flux is 3 2 kg m2 yr this is 103 times that inferred for hydrates associated with bottom simulating seismic reflectors the subsurface hydrate accumulation and the cumulative methane venting are related we show how both may be estimated from measurements of vent gas composition bottom water temperature and geothermal gradient

dynamic fluid flow and chemical fluxes associated with a seafloor gas hydrate deposit on the northern gulf of mexico slope abstract gas hydrates outcrop on the seafloor at the bush hill hydrocarbon seep site in the northern gulf of mexico four newly designed fluid flux meters chemical samplers called the mosquito were deployed for 430 days at bush hill to determine how dynamic subsurface fluid flow influences gas hydrate stability and to quantify the associated methane fluxes into the ocean three of the flux meters were deployed adjacent to an outcropping gas hydrate mound while the fourth monitored background conditions the flux meter measurements reveal that the subsurface hydrology in the vicinity of the mound is complex and variable with frequent changes from downward to upward flow ranging from 161 to 273 cm yr and with temporal variations in the horizontal component of flow the continuous record of fluid chemistry indicates that gas hydrate actively formed in the sediments we propose that long periods of downward flow of seawater adjacent to gas vents up to 4 months are driven by local sub pressure resulting from gas ebullition through faults and fractures due to overpressure at depth high frequency variations in flow rates days to weeks are likely due to temporal changes in sediment permeability and the 3 d fluid flow field as a result of active gas hydrate and authigenic carbonate precipitation as well as the presence of free gas gas hydrate formation occurred as a result of long term emanation of ch 4 at focused gas vents followed by a more diffuse intergranular methane flux the estimated ch 4 flux to the water column from focused gas vents across the bush hill seep is 5 10 6 mol yr this significant flux suggests that bush hill and similar hydrocarbon seeps in the northwestern gulf of mexico may be important natural sources of methane to the ocean and possibly the atmosphere

formation of authigenic carbonates at a methane seep site in the middle okinawa trough east china sea abstract the formation of authigenic carbonates at cold seep is closely related to underlying methane seepage and anaerobic oxidation of methane aom and these processes can be reflected in the geochemical characteristics of pore water in the sediments in terms of petrological mineralogical and geochemical analyses of research materials including authigenic carbonates and pore water samples from the middle okinawa trough east china sea these results show that authigenic carbonate minerals are mainly composed of high magnesium calcite low magnesium calcite dolomite and iron dolomite and exhibit extremely negative carbon isotopic compositions 13c 52 30 to 31 10 v pdb and positive oxygen isotopic compositions 18o 3 36 to 7 27 v pdb geochemical analyses of pore water suggest linear sulfate gradients and steep increase of methane and total alkalinity concentrations with depth which indicate sulfate induced aom activities based on the sulfate concentration reduction carbon isotope characteristics of dissolved inorganic carbon dic and increased methane concentration the sulfate methane interface smi is considered to be shallow these data combined with the variations in ion concentrations in pore water suggest that the leakage of biogenic methane and intense aom resulted in the formation of abundant authigenic carbonates which inherited light carbon 12c enriched isotopic signature of methane in the shallow sediments combined geochemical characteristics of pore water and authigenic carbonates intense methane seep activities and associated aom can be inferred both in present day and ancient geological records on the western slope of the middle okinawa trough

methane seepage inferred from the porewater geochemistry of shallow sediments in the beikang basin of the southern south china sea abstract several tens of methane ch4 seepage sites have been discovered in the northern south china sea scs during the investigation and exploration of gas hydrates and cold seeps over the past decade however our knowledge about methane seepage in the southern scs remains scarce especially in the beikang basin on the southern slope of the scs four piston cores bh cl18 bh cl23 bh cl29 and bh cl35 were collected in the beikang basin to understand ch4 cycling in shallow sediments the extrapolated depths of the sulfate methane transition zones smtz of the four cores are between 5 3 and 8 8 mbsf by simulating the porewater profiles of so42 ch4 dic and po43 using the reaction transport model we estimated that the dissolved so42 was predominantly consumed by the anaerobic oxidation of methane aom at rates of 27 5 mmol m 2 yr 1 in bh cl18 28 1 mmol m 2 yr 1 in bh cl23 43 1 mmol m 2 yr 1 in bh cl29 and 39 4 mmol m 2 yr 1 in bh cl35 respectively due to intense aom almost all the methane sourced from subsurface sediments was depleted within the smtz resulting in negligible benthic methane fluxes the kink type depth so42 profile of bh cl23 was apparently attributable to an enhanced methane flux which was not captured by the steady state modelling furthermore highly depleted 13c values of the dissolved inorganic carbon dic in bh cl18 43 6 bh cl23 44 and bh cl29 51 2 also indicated a methane derived dic in contrast relatively higher 13c values in bh cl35 suggested a mixture of 13c enriched dic produced by methanogenesis as shown by the reversal toward more 13c enriched dic below 4 2 mbsf in bh cl29 to our knowledge this is the first quantitative study dealing with methane cycling in the shallow sediments of the southern scs and may have considerable implications for understanding the poorly investigated cold seep and gas hydrate systems on the continental slope of the southern scs

microbial utilization of rare earth elements at cold seeps related to aerobic methane oxidation abstract a major breakthrough in the field of rare earth element ree geochemistry has been the recent discovery of their utility to microbial life as essential metalloenzymes catalyzing the oxidation of methanol to formaldehyde lanthanide dependent bacteria are thought to be ubiquitous in marine and terrestrial environments but direct field evidence of preferential microbial utilization of ree in natural systems is still lacking in this study we report on the ree and trace element composition of the tube of a siboglinid worm collected at a methane seep in the gulf of guinea a tube dwelling annelid that thrives in deep sea chemosynthetic ecosystems high resolution trace element profiles along the chitin tube indicate marked enrichments of lanthanum la and cerium ce in its oxic part resulting in ree distribution patterns that depart significantly from the ambient seawater signature combined with various geochemical and microbiological evidence this observation provides direct support for an active consumption of light ree at cold seeps associated with the aerobic microbial oxidation of methane to further evaluate this hypothesis we also re examine the available set of ree data for modern seep carbonates worldwide while most carbonate concretions at cold seeps generally display ree distribution patterns very similar to those for reduced pore waters in marine sediments we find that seafloor carbonate pavements composed of aragonite commonly exhibit pronounced light ree enrichments as inferred from high shale normalized la gd ratio 0 8 interpreted here as possibly reflecting the signature of lanthanide dependent methanotrophic activity this finding opens new perspectives for revisiting ree systematics in ancient seep carbonates and other microbialites throughout the earth s history in particular the geochemical imprint of aerobic methane oxidation could be possibly traced using ree in archaean stromatolites and other archives of precambrian seawater chemistry potentially providing new insights into the oxygenation of early earth s oceans and associated microbiogeochemical processes

geochemical responses and implications for gas hydrate accumulation case study from site shc in shenhu area within northern south china sea abstract a knowledge of gas hydrates is critical for understanding complex geological geophysical and biogeochemical processes and for explaining the proposed link between gas hydrate destabilization and global sea level changes with the aim of discussing the geochemical characteristics and implications for gas hydrate accumulation this study assesses the changes in a series of organic and inorganic geochemical gas hydrate bearing sediments within a core section obtained at site shc in the shenhu area of the northern south china sea that range from the late miocene to the present high s c ratios 0 53 5 6 and low toc contents

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

chemical and structural characteristics of gas hydrates from the haima cold seeps in the qiongdongnan basin of the south china sea abstract crystalline structure cage occupancy and gas composition are important gas hydrate characteristics which can be used to calculate the gas hydrate s stability and to estimate the energy potential in may and october of 2015 a guangzhou marine geological survey team investigated two sites in the haima cold seeps in the qiongdongnan basin northern south china sea they recovered massive gas hydrate samples by conventional gravity coring after the shenhu sea area and dongsha sea area the qiongdongnan basin became a new area on the northern slope of the south china sea where gas hydrates have been found in order to reveal the structural and geochemical characteristics of the natural gas hydrates samples were analyzed by micro raman spectroscopy and x ray diffraction under ambient pressure and low temperature conditions the results indicate that the gas hydrate samples from the haima cold seeps are typical structure i hydrates with a hydration number of 6 12 6 19 in addition trace amounts of h2s trapped in the hydrate were identified based on its characteristic vibrational signature the gas composition and 13c ch4 of the hydrate bound gas samples were analyzed for gas source correlation all of the gas samples are dominated by methane with small amounts of ethane and propane and had relatively light 13c ch4 indicating that all of the hydrate bound gases are mixtures of biogenic and thermogenic gas the thermogenic gas is inferred to be mainly derived from the coal layers of the late miocene pliocene yacheng formation in the northern lingshui sag

rare earth elements in seep carbonates as tracers of variable redox conditions at ancient hydrocarbon seeps marine hydrocarbon seeps are dynamic systems which are typically reflected in complex lithologies of limestones forming at these sites in order to test if seep limestones with their multiple generations of carbonate phases reflect variable redox conditions the rare earth element ree patterns of limestones from five ancient hydrocarbon seep localities hollard mound middle devonian iberg mississippian beauvoisin oxfordian canyon and satsop river oligocene and marmorito miocene have been investigated the total ree contents vary widely 0 3 43 7 p p m early microcrystalline phases tend to show higher contents than spatially associated later phases microspar sparite and blocky cement the shale normalized ree patterns of the carbonates show both negative and positive ce anomalies even for individual deposits suggesting that the redox conditions varied significantly it is apparent that in addition to anoxic conditions oxic conditions were common in some ancient seep environments intermittent oxygenation is presumed to have resulted from abrupt changes in fluid flow

modeling sulfate reduction in methane hydrate bearing continental margin sediments does a sulfate methane transition require anaerobic oxidation of methane the sulfate methane transition smt a biogeochemical zone where sulfate and methane are metabolized is commonly observed at shallow depths 1 30 mbsf in methane bearing marine sediments two processes consume sulfate at and above the smt anaerobic oxidation of methane aom and organoclastic sulfate reduction osr differentiating the relative contribution of each process is critical to estimate methane flux into the smt which in turn is necessary to predict deeper occurrences of gas hydrates in continental margin sediments to evaluate the relative importance of these two sulfate reduction pathways we developed a diagenetic model to compute the pore water concentrations of sulfate methane and dissolved inorganic carbon dic by separately tracking dic containing 12c and 13c the model also computes 13c dic values the model reproduces common observations from methane rich sediments a well defined smt with no methane above and no sulfate below and a 13c dic minimum at the smt the model also highlights the role of upward diffusing 13c enriched dic in contributing to the carbon isotope mass balance of dic a combination of osr and aom each consuming similar amounts of sulfate matches observations from site u1325 integrated ocean drilling program expedition 311 northern cascadia margin without aom methane diffuses above the smt which contradicts existing field data the modeling results are generalized with a dimensional analysis to the range of smt depths and sedimentation rates typical of continental margins the modeling shows that aom must be active to establish an smt wherein methane is quantitatively consumed and the 13c dic minimum occurs the presence of an smt generally requires active aom

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

significant contribution of authigenic carbonate to marine carbon burial carbon is removed from the earth s surface through the formation and burial of carbon bearing rocks and minerals an analysis of pore water profiles collected from marine sediments around the globe suggests that the precipitation of authigenic calcium carbonate accounts for around 10 of the carbonate that accumulates in marine sediments globally carbon is removed from the earth s surface through the formation and burial of carbon bearing rocks and minerals1 2 the formation of calcium carbonate and its burial in marine sediments accounts for around 80 of the total carbon removed from the earth s surface however the fraction of calcium carbonate that precipitates in the oceans versus that which precipitates authigenically in marine sediments is unclear here we compile measurements of the calcium concentration of pore fluids collected at 672 seafloor sites around the globe to calculate the global flux of calcium within marine sediments we use these data combined with alkalinity measurements of pore fluids to quantify authigenic calcium carbonate precipitation we estimate that the net calcium flux into marine sediments that can be ascribed to authigenic carbonate precipitation amounts to around 1 1012 mol yr 1 as such we estimate that authigenic carbonate precipitation accounts for at least 10 of global carbonate accumulation we show that much of the precipitation occurs along the eastern margins of ocean basins where organic matter delivery to the sea floor is likely to be high we suggest that authigenic calcium carbonate precipitation represents a non negligible component of the global carbon cycle

stresses and plate boundary forces associated with subduction plate margins primary tectonic stress in the lithosphere is predominantly caused by lateral density variations within the earth and associated topographical loading when such a stress system caused by major sublithospheric density anomalies is intersected by a weak zone which cuts across the lithosphere plate boundary forces develop and modify the plate interior stresses in this paper unite element analysis is used to model the stresses and plate boundary forces associated with subduction plate boundaries with dipping and vertical slabs extending to about 270 and 400 km depths having the subduction fault both locked and unlocked in such regions several types of horizontal deviatoric stress may occur including 1 local compression in the trench arc region caused by the dense sinking slab and the associated surface downflexure 2 plate interior tension which occurs when this compression is intersected by a weak subduction fault 3 local tension associated with thickening of the crust at the arc and elsewhere 4 local tension in the back arc region produced by the underlying low density upper mantle and 5 downbending stresses in the subducting slab thermal stresses and transmitted ridge push which are not included in the modelling here a gradient from compression in the forearc to tension in the back arc can be modelled in terms of these stresses when the fault is partially locked it is however the intersection of the local compression 1 by an unlocked or partially locked subduction fault that modifies the plate interior stresses and gives rise to the slab pull and trench suction plate boundary forces the state of stress in the interior of the overriding plate is also crucially influenced by back arc spreading where this occurs plate boundary forces have been evaluated for each of the models it is shown that slab pull and trench suction may be of comparable magnitude

tectono sedimentary controls on the likelihood of gas hydrate occurrence near taiwan abstract marine sediments on the continental slope of the ne south china sea have appropriate thickness methane generating potential and occur in a suitable pressure temperature regime to host gas hydrate evidence for gas hydrate the bottom simulating reflector bsr is observed to the south of taiwan on reflection seismic records and can be used to suggest that gas hydrates are widely distributed the tectono sedimentary framework south of taiwan bears directly upon methane generation and the likelihood of the presence of significant gas hydrate deposits three zones of probable hydrate occurrence have been delineated along the margins of the ne south china sea 1 in a thick accumulation of sediment along the northern passive margin 2 along a more thinly sedimented eastern active collisional margin and especially 3 in a zone of thick originally passive margin sedimentation into which the collisional margin has encroached obliquely

environmental analysis of paleoceanographic systems based on molybdenum uranium covariation abstract modern low oxygen marine systems exhibit patterns of molybdenum uranium covariation that can be linked to specific attributes and processes of the depositional system including 1 variation in benthic redox conditions 2 the operation of particulate shuttles within the water column and 3 the evolution of watermass chemistry the importance of these factors in each depositional system can be assessed both from the degree of enrichment of authigenic molybdenum mo auth and uranium u auth and from the mo u auth ratio of the sediment relative to the seawater mo u molar ratio of 7 5 7 9 in open ocean systems with suboxic bottomwaters u auth enrichment tends to exceed that of mo auth owing to onset of u auth accumulation at the fe ii fe iii redox boundary resulting in sediment mo u auth ratios less than that of seawater as bottomwaters become increasingly reducing and at least occasionally sulfidic the rate of accumulation of mo auth increases relative to that of u auth and sediment mo u auth ratios equal or exceed that of seawater in restricted marine systems with permanently sulfidic deepwaters the relative enrichment of mo auth and u auth depends on additional factors in the cariaco basin which has an aqueous mo u ratio similar to that of seawater the operation of a particulate mn fe oxyhydroxide shuttle serves to accelerate the transfer of mo to the seafloor leading to strong enrichments in mo auth relative to u auth in the black sea the chemistry of the deep watermass has evolved to the point where its aqueous mo u ratio is only 0 04 that of seawater as a consequence of which sediments deposited under deepwater influence are depleted in mo auth relative to u auth these mo u covariation patterns can be used to gain a better understanding of the watermass attributes and processes of ancient low oxygen marine systems analysis of anoxic facies from two north american paleomarine systems the late pennsylvanian midcontinent sea lpms and the late devonian seaway lds reveals authigenic mo u relationships similar to those of the modern marine environments above implying similar redox and hydrographic controls the observed patterns are consistent with laterally unconfined circulation and strong watermass exchange within the lpms and with markedly restricted deepwater circulation in silled basins of the lds patterns of authigenic mo u covariation may prove useful in analysis of other paleoceanographic systems to reveal aspects of watermass composition and environmental dynamics

gas hydrate and associated free gas in the dongsha area of northern south china sea abstract a large ongoing cold seep named jiulong methane reef was discovered in the dongsha area of northern south china sea scs in 2004 to understand links among seafloor methane seepage subsurface gas hydrate origin of gas source and structural styles controlling the formation and accumulation of gas hydrate high resolution multi channel seismic data were interpreted in this region bottom simulating reflectors bsrs known as the base of gas hydrate stability zone were identified and mapped additionally seismic attributes interval velocity model and acoustic p wave impedance were obtained to better delineate the spatial distribution of gas hydrate hydrate bearing sediments are characterized by weak reflection i e blanking zone above bsrs high velocities and relatively small acoustic impedance differences while enhanced reflection low frequency and low velocities beneath bsrs suggest the presence of underlying free gas beneath solid hydrate a number of faults mud diapirs and a possible submarine landslide were observed in seismic profiles both faults and mud diapirs are useful conduits allowing methane gas to migrate upward and linking free gas zones beneath bsrs hydrate zones and seafloor methane seepage a submarine landslide with a length scale of 10 km is observed in a seismic profile the landslide may trigger dissociation of gas hydrate accounting for the occurrence of jiulong methane seepage seismic observations and estimated heat flow coupled with geochemical analyses suggest that gas source of methane in this region probably originates from mixture of biogenic and thermogenic methane this study would not only help to understand gas hydrate system in the dongsha area but also provide insights to geological hazards associated with the dissociation of gas hydrate

sulfate reduction using methane in sediments beneath a bathyal cold seep giant clam community off hatsushima island sagami bay japan two sandy sediment cores cores d227 120 and d380 were collected from inside a deep sea giant clam calyptogena soyoae community off hatsushima island western sagami bay central japan 35 59 9 n 139 13 6 e 1160 m deep and a muddy sediment core core d227 202 was obtained from outside the community by the submersibleshinkai 2000 the chloride concentration of the pore waters is constant vertically and sulfate reduction using sedimentary organic matter occurs in core d227 202 21 cm long the chloride concentrations are lower by 7 at the 7 5 9 cm depth in core d227 120 9 cm long and by 3 at the 11 12 cm depth in core d380 16 cm long than those of the overlying bottom waters in the cores from inside of the community sulfate concentration decreases remarkably and dissolved inorganic carbon alkalinity ammonium n and hydrogen sulfide concentrations increase significantly with increasing depth in core d380 34s values of sulfate ions increase from 20 5 to 35 3 and 13c values of dissolved inorganic carbon decrease drastically from 7 0 to 45 with increasing depth from the top to the bottom of the core although the 13c values of the organic carbon of the sediments are 23 7 0 9 in core d380 these results indicate that sulfate reduction using methane is active within the sediments just beneath the living clams and that the hydrogen sulfide produced can be used by endosymbiotic sulfur oxidizing bacteria living in the gills ofc soyoae in the community

upper plate stressing and seismicity in the subduction earthquake cycle we investigate upper plate stressing during the earthquake cycle in a subduction segment using three dimensional 3 d elastic models to address the effects of strongly heterogeneous coupling along strike of the interplate interface we show how heterogeneity controls the locations and mechanisms of seismicity in the upper plate oblique subduction segments two from the aleutians andreanof islands 1986 and rat islands 1965 and one from indonesia biak 1996 are studied all examples of upper plate seismicity from the aleutians represent events occurring toward the beginning of a new cycle while in biak indonesia the examined events occur both toward the end of one cycle and the beginning of the next in the majority of cases studied the location and mode of the upper plate seismicity are consistent with space and time dependent stressing as predicted by modeling this confirms earlier observations that seismicity in the vicinity of large great subduction earthquakes toward the outer rise at intermediate depth and now in the upper plate depends in an interpretable manner on the stage in the earthquake cycle as well as on distribution of coupling along the interplate interface

pore water dissolved inorganic carbon sources and cycling in the shallow sediments of the haima cold seeps south china sea abstract pore water geochemical compositions provide the insights into seafloor cold seep activities and biogeochemical processes of methane derived carbon in sediments in this study six sediment push cores were collected by manned submersible at the center and edge of the active haima cold seeps and a nearby background site pore water compositions of the so42 ca2 mg2 sr2 nh4 dissolved inorganic carbon dic and 13c dic at each sediments core were measured the very shallow depths of sulfate methane transition zone smtz suggest that high methane flux and significant anaerobic oxidation of methane aom activities occur in the shallow sediments reaction transport modelling results show that the relative contributions of aom and organoclastic sulfate reduction osr consuming sulfate are 84 1 and 15 9 at site hm 2 6 and 94 6 and 5 4 at site hm 3 3 respectively furthermore extremely low 13c dic values at the central sites hm 2 6 45 67 and hm 3 3 53 87 indicate an external methane derived dic based on the 13c mass balance we quantify the proportions of pore water dic derived from organic matter degradation and aom as 24 7 and 65 3 at site hm 2 6 and 9 3 and 80 7 at site hm 3 3 respectively an increase in pore water dic concentration leads to carbonate precipitation plots of sr ca vs mg ca ratios reveal that the carbonate phases in sediments are dominantly high mg calcite in this region our study demonstrates that aom can serve as a dominant biogeochemical process through incorporation of methane derived carbon into dissolved and solid inorganic carbon phases significantly influencing dic cycling within the shallow sediments of the haima cold seep area

geochemical constraints on the origin of the pore fluids and gas hydrate distribution at atwater valley and keathley canyon northern gulf of mexico abstract pore fluids from atwater valley at 13 14 and keathley canyon kc 151 in the northern gulf of mexico are surprisingly similar with respect to ionic concentrations and oxygen and strontium isotope values as well as hydrocarbon geochemistry suggesting that these widely separated localities share common deep subsurface fluid origins seafloor mounds with focused fluid migration pathways and inferred near seafloor gas hydrates characterize the at 13 14 region whereas the kc 151 region has a bottom simulating reflector bsr at 310 mbsf which is rather uncommon in the gulf of mexico gom at these sites seafloor gas hydrates were not observed but the sediment surface in the vicinity and particularly at the mounds is populated with chemosynthetic communities that are commonly associated with seafloor gas emission the geochemical results together with the pressure core data suggest that at the at region methane hydrate mostly occurs in near surface sediments at mounds consistent with focused migration pathways in the kc region methane hydrate mostly occurs deeper in the section in highly fractured silty clayey sediments from 220 to 300 mbsf the pore fluids at the at mounds and kc 151 are characterized by higher than seawater salinity the more saline pore fluids at the at mound and at kc151 sites located 350 km apart are almost chemically indistinct ionic ratios indicate that this distinct high salinity fluid is not from in situ salt dome halite dissolution rather this fluid is a subsurface brine derived from jurassic or cenozoic evaporite formation modified by fluid sediment reactions and migrated to the two sites analyzed despite porewater salinities elevated above that of seawater the sediment temperatures are within the range of methane hydrate stability for each of the sites based on cl dilutions the maximum gas hydrate pore volume occupancy at the at mound sites would be 9 at kc cl concentrations in pressure cores imply that in situ hydrate is unevenly distributed with pore volume occupancy of 1 12 significant variations in sulfate gradients were observed with the sulfate to methane transition zone smtz at or near the seafloor at the at mound sites at at 13 2 the well defined smtz is at 8 mbsf and at kc 151 3 it is at 9 mbsf there is no coincidence between the steepness of the sulfate gradients and the presence or depth of a bsr suggesting that the smtz interfaces are measuring different aspects of the subsurface methane hydrology at both at and kc the 13 c dic values clearly indicate that anaerobic oxidation of methane aom is the dominant reaction responsible for sulfate reduction and the increased alkalinities observed the most negative 13 c dic values obtained are 46 3 and 49 6 at the smtzs at at 13 2 and kc 151 3 respectively

using chemical compositions of sediments to constrain methane seepage dynamics a case study from haima cold seeps of the south china sea abstract cold seeps frequently occur at the seafloor along continental margins the dominant biogeochemical processes at cold seeps are the combined anaerobic oxidation of methane and sulfate reduction which can significantly impact the global carbon and sulfur cycles the circulation of methane rich fluids at margins is highly variable in time and space and assessing past seepage activity requires the use of specific geochemical markers in this study we report multiple sedimentary proxy records for three piston gravity cores qdn 14a qdn 14b and qdn 31 from the haima seep of the south china sea scs by combining total organic carbon toc total inorganic carbon tic total nitrogen tn total sulfur ts acid insoluble carbon and sulfur isotope 13corganic carbon and 34sacid insoluble and 34s values of chromium reducibility sulfur 34scrs as well as carbon isotopes of tic 13ctic in sediments our aim was to provide constraints on methane seepage dynamics in this area we identified three sediment layers at about 260 300 cm 380 420 cm and 480 520 cm sediment depth characterized by particular anomalies of low 13ctic values and high ts content high ts and crs contents and high 34sacid insoluble and 34scrs values respectively on this basis we propose that these sediment horizons correspond to distinct methane release events preserved in the sediment record while the exact mechanisms accounting for the presence or absence of these particular geochemical signals in the sediment are not known we propose that they correspond to variations in methane flux and their duration through time overall our results suggest that sedimentary carbon and sulfur and their isotopes are useful tracers for better understanding of methane seepage dynamics over time

phase transformations in a harzburgite composition to 26 gpa implications for dynamical behaviour of the subducting slab abstract the mineralogy adopted by a depleted harzburgite composition has been studied over the pressure interval 5 26 gpa at temperatures of 1300 1400 c the pyroxene garnet component of the harzburgite composition harzburgite minus 82 wt olivine transforms to majorite garnet by 18 19 gpa and further disproportionates to the assemblage of garnet stishovite mg 2 sio 4 spinel above 20 gpa at still higher pressures first ilmenite 22 24 gpa and then perovskite mgsio 3 24 26 gpa are found to coexist with garnet garnet disappears at 26 gpa and almost complete transition to perovskite is achieved at this pressure the mineral proportions and density profiles in the subducting oceanic lithosphere modelled by a combination of 80 harzburgite 20 primitive morb compositions are calculated as a function of depth under conditions isothermal with surrounding pyrolite mantle and also for a temperature distribution in which the slab is substantially cooler than surrounding mantle to below 700 km under isothermal conditions the slab has a density similar to surrounding mantle to a depth of 600 km however between 600 and 700 km the slab is up to 0 08 g cm 3 denser than surrounding mantle this is caused primarily by the higher alumina content in pyrolite as compared to harzburgite which causes the transition to perovskite in pyrolite to occur at substantially higher pressures than in harzburgite the presence of alumina also smears out the garnet perovskite transition in pyrolite over a depth interval of 50 km whereas this transformation is much sharper in the harzburgite composition calculations based on the observed phase equilibria also show that a subducted cool slab remains much denser by 0 1 0 3 g cm 3 than surrounding mantle to a depth of 700 km but possesses a density similar to surrounding mantle below this depth these results have important implications for the dynamical behaviour of slabs possessing different thermal regimes when they encounter the 670 km discontinuity and also for the nature of this discontinuity

environmental controls on sulfur isotopic compositions of sulfide minerals in seep carbonates from the south china sea abstract authigenic carbonates and pyrite associated with sulfate driven anaerobic oxidation of methane aom at methane seeps provide archives to explore the biogeochemical processes involved and seepage dynamics over time the wide range and extremely high 34s values of pyrite 34spy have been used to trace the aom related processes however the detailed mechanism for this phenomenon is not well understood we propose that the characteristics of 34spy were mainly controlled by the competition between sulfate reduction and sulfate supply as well as the redox condition to test this hypothesis we investigated sr ca and mg ca ratios trace element compositions pyrite contents and sulfur isotopic compositions in seep carbonates from site f and haima in the northern south china sea calcite and aragonite contents were distinguished through the sr ca and mg ca ratios the data show that aragonites are always associated with relatively low 34spy values compared to calcites the mo contents show a good correlation with pyrite contents in calcites and aragonites and the slope in aragonites is larger than that in calcites this relationship indicates that the aragonite precipitated in a relatively open system with higher mo availability thus we conclude that sulfides with low 34s values formed at high supply of sulfate under the relatively open system with respect to diffusive replenishment of sulfate where the carbonate precipitation occurred close to the seafloor due to a strong methane flux under vigorous methane flux simultaneously the high potential of less anoxic conditions which could limit the additional pyrite accumulation and or favor the microbial disproportionation could also be the cause of the low 34spy as supported by samples from the haima sites evidence for this assumption is based on the occurrence of bivalve shells and less enrichment in as and sb conversely the positive 34spy values result from near to complete exhaustion of dissolve sulfate via aom within a deeper sulfate methane transition zone where mo is less available the combination of a detailed elemental study of authigenic carbonates with sulfur isotopic composition of sulfide minerals in carbonates is a promising tool for reconstructing the dynamics of seep intensities at modern and potentially geological seep sites

methane source linked to gas hydrate system at hydrate drilling areas of the south china sea porewater geochemistry and numerical model constraints abstract methane of various origins is widespread in the sediments of continental margins representing a key component of the carbon cycle in the ocean its occurrence in shallow marine sediments can be linked to the gas hydrate reservoir in the deep sedimentary column however this type of methane is difficult to track due to complex reactions occurred during its long distance migration porewater geochemical analyses of two 7 m piston cores d17 15 and w01 16 combined with a reaction transport model were applied to quantify methane related biogeochemical processes and to explore the linkage of shallow methane to gas hydrate reservoir in the hydrate drilling areas of shenhu and dongsha in the south china sea scs the model results revealed that anaerobic oxidation of methane aom is the predominant biogeochemical process in both cores consuming approximately 85 and 74 of sulfate in the modeled domain respectively the crossplot of dissolve inorganic carbon dic accumulation versus sulfate depletion of the system suggests that methane consumed during the aom is mainly originated from external methane sources rather than local methanogenesis using a 13cdic mass balance calculation together with model derived fractions of different dic sources we estimated that the 13c value of the external methane in core d17 15 is 66 6 which is in accordance with the previous reported 13c values of hydrate bound methane in the area these results suggest that methane consumed in shallow sediments in the hydrate drilling areas of the scs is mainly originated from local gas hydrate reservoir the proposed approach has the potential to be used to differentiate the sources of methane which will provide constraints on dynamics of methane in gas hydrate bearing marine sediments

the impact of diffusive transport of methane on pore water and sediment geochemistry constrained by authigenic enrichments of carbon sulfur and trace elements a case study from the shenhu area of the south china sea abstract sulfate driven anaerobic oxidation of methane sd aom controls methane release from marine sediments to the ocean a variety of authigenic precipitates within sediments has been widely used to identify past occurrences of sd aom however the lack of a systematic evaluation of the formation conditions of these precipitates sometimes impedes the recognition of sd aom in past environments characterized by diffusive transport of methane carbon sulfur and trace element geochemistry of pore water and sediments was investigated at a site affected by upward methane diffusion in the shenhu area of the south china sea here the sulfate methane transition zone smtz is located 7 6 m below the seafloor based on sulfate and methane concentrations the slope of 34s vs 18o values of sulfate is consistent with diffusive transport of methane concentration and isotope profiles of pore water species point to diffusive rather than advective transport of solutes enhanced sulfate reduction inferred from 34s values of sulfate agrees with the local occurrence of relatively abundant 34s enriched iron sulfide minerals the time required to produce the observed authigenic iron sulfides around the smtz is estimated to be 1 1 ka based on the amount of iron sulfide minerals and the present diffusive flux of sulfate no enrichment of authigenic carbonate barium ba and molybdenum mo was detected in the studied sediments this observation is consistent with the calculation that the current fluxes of pore water calcium magnesium ba and mo are too low to allow for authigenic enrichments the low fluxes are largely controlled by diffusion which facilitates the formation of 34s enriched iron sulfide minerals the observed enrichment patterns unlike those of sediments affected by advective seepage are expected to be prevalent in modern and ancient continental margin sediments and may contribute to the identification of past methane rich zones and overlying sd aom zones as sinks for methane in the geological record

trace element systematics in cold seep carbonates and associated lipid compounds abstract seeping of methane rich fluids at submarine cold seeps drives intense microbial activity and precipitation of authigenic carbonates some trace elements play an important role in the biogeochemical processes operating at cold seeps especially as specific enzymatic co factors related to methanogenesis and the anaerobic oxidation of methane aom however it is unclear whether microbial trace metal utilization can be traced by the geochemical composition of seep carbonates in this study we analyzed a series of authigenic carbonate samples recovered from various seep settings worldwide and report for the first time trace element concentrations for total lipid fractions combined with biomarker analyses and determination of elemental abundances in associated inorganic mineral phases carbonate phases sulfides organic compounds and detrital fractions our results indicate marked enrichments of co ni cu mo and w in the archaeal and bacterial lipids associated with authigenic carbonates which can all be ascribed to previously identified enzymatic pathways in addition to the microbial communities involved in aom which most likely control specific lipid bound enrichments of co ni mo and w in seep carbonates cu was found to display higher concentrations in the lipid fractions extracted from a few authigenic carbonate samples formed closer to the sediment water interface hence possibly related to the presence of aerobic methane oxidizing bacterial assemblages in the near seafloor environment while the above mentioned trace metals are relatively enriched in all studied inorganic and organic fractions the very low w concentrations measured in carbonate phases combined with their pronounced enrichment in associated lipid fractions and inferred microbial requirement suggest that tungsten depletion in pore waters could possibly act as a limiting factor on aom at cold seeps finally two other trace elements li and ti also displayed particular enrichments in studied lipid fractions which despite no reported evidence could possibly indicate that they are also involved as metalloenzymes in microbial methane oxidation processes at cold seeps

new insights into cerium anomalies and mechanisms of trace metal enrichment in authigenic carbonate from hydrocarbon seeps authigenic carbonates that form at marine hydrocarbon seeps provide a unique geological archive of past local environmental conditions and pore fluid geochemistry recent work on such carbonates revealed variable cerium ce anomalies and anomalous enrichments of certain trace metals however the mechanisms accounting for such anomalies remain poorly constrained here we characterize the rare earth element ree patterns of carbonate phases and the trace metal patterns of bulk carbonate rocks sampled at three hydrocarbon seeps located at congo fan pockmarks cf and the gulf of mexico sites ac645 and gb425 the analyzed cf gb425 and ac645 carbonates yielded different ree patterns displaying positive no as well as negative ce anomalies the covariation of molybdenum mo with uranium u including authigenic mo moauth and u uauth enrichments as well as mo u auth ratios proved useful to obtain new insight into the applicability of ce anomalies to constrain past redox conditions trace element patterns suggest that 1 cf carbonates formed in a restricted sulfidic environment while 2 the ac645 site experienced intermittent oxygenation causing negative ce anomalies and 3 environmental conditions were variable at the gb425 mud volcano site interestingly gb425 carbonates show significant mo arsenic as and antimony sb enrichments with the enrichment factor of as asef correlating well with the authigenic fe fraction these results suggest that iron oxyhydroxides played an important role in the adsorption of mo as and sb in the water column and their transfer to the sediment the combination of trace metal and ree geochemistry of authigenic carbonates used here is a promising tool to better assess past variability of redox conditions and biogeochemical processes at marine hydrocarbon seeps

fluid and chemical cycling at bush hill implications for gas and hydrate rich environments 1 the results of a deployment of aqueous flux meters at the bush hill hydrate mound show that persistent hydrologic instability is a primary feature of the globally abundant gas and hydrate rich cold seep environment seven flux meters were deployed for 14 weeks in the area of the bush hill hydrate mound instruments were deployed on microbial mats bivalves adjacent to the surface hydrate mound and a site without visible fauna flow rates were observed to range from downflow of 0 01 mm day to upflow of 15 mm day temporal variability and a major hydrological event were observed to occur on all instruments there is evidence that this event was related to a gas expulsion episode the two instruments which exhibited downflow during the event were near surface hydrates and bubbling vents nearby instruments recorded a rapid increase in flow rates at the time of the event with a subsequent decrease in rates to the previous background values two instruments showed significant output of seawater like fluids while the others output typical methane seep type pore fluids these results at a passive margin site along with our previously published convergent margin results at gas rich northeast pacific cold seeps hydrate ridge eel river margin illustrate the hydrologic complexity of these environments we now propose that these and our earlier results are characteristic of seafloor environments which have abundant free gas and hydrates these mechanisms have major consequences for the near surface geochemical and microbial environment and for the way we interpret measurements made in these areas

authigenic carbonates from seeps on the northern continental slope of the south china sea new insights into fluid sources and geochronology seep carbonates represent excellent archives of seepage activities near seafloors in order to provide insights on sources of seeping fluids stable carbon and oxygen isotopes and sr 87 sr 86 ratios were applied to seep carbonates from shenhu sw dongsha and ne dongsha on the northern continental slope of the south china sea u th dating was selected to constrain the time involved in carbonate formation and the potential driving force the delta c 13 values of the shenhu and ne dongsha carbonates vary from 52 3 parts per thousand to 32 6 parts per thousand v pdb indicating that biogenic methane is the primary carbon source at both locations the delta c 13 values of the sw dongsha carbonates are much higher ranging from 18 8 parts per thousand to 11 parts per thousand v pdb suggesting that thermogenic methane is the predominant carbon source in this region a relatively deep origin of thermogenic methane for the sw dongsha seeps is also supported by somewhat lower sr 87 sr 86 ratios of the samples from 0 709025 to 0 709097 the delta o 18 values of the shenhu and ne dongsha carbonates vary from 1 4 to 5 1 v pdb while the delta o 18 values of the sw dongsha carbonates range from 2 5 to 3 4 v pdb many samples from shenhu and ne dongsha demonstrate an o 18 enriched fluid source that most likely originated from waters released from the dissociation of locally abundant gas hydrate u th ages of the shenhu carbonates show a large range from 330 ka to 152 ka bp and the ne dongsha carbonates have ages from 77 ka to 63 ka bp interestingly u th ages of most samples are either within times of sea level lowstands or correspond to periods when the sea level was falling taken together our data suggest that in the south china sea gas hydrate dissociation during sea level lowstands or falling stages resulted from reduced hydrastastic pressures which in turn enhanced the seep activities and promoted the formation of authigenic carbonates close to the seafloor c 2013 elsevier ltd all rights reserved

establishing criteria to distinguish oil seep from methane seep carbonates hydrocarbon seeps harbor copious chemosynthesis dependent life the traces of which are preserved in the fossil record within authigenic carbonates these environments are mostly characterized by seepage of methane rich fluids yet numerous crude oil dominated seeps have been discovered in recent years oil seepage has a profound influence on the local fauna but recognizing such seeps in the rock record remains elusive this study presents new geochemical data that will allow for a more confident identification of ancient oil seep deposits geochemical data from modern and ancient seep limestones reveal that oil dominated seep carbonates are enriched in rare earth elements and uranium compared to their methane dominated counterparts these trace element patterns have the potential to serve as a basis for an improved understanding of the adaptation of chemosynthetic life to oil seepage and to better constrain the marine carbon cycle in the geologic past

jiulong methane reef microbial mediation of seep carbonates in the south china sea chemoherm carbonates as well as numerous other types of methane seep carbonates were discovered in 2004 along the passive margin of the northern south china sea lithologically the carbonates are micritic containing peloids clasts and clam fragments some are highly brecciated with aragonite layers of varying thicknesses lining fractures and voids dissolution and replacement is common mineralogically the carbonates are dominated by high magnesium calcites hmc and aragonite some hmcs with mgco3 contents of between 30 38 mol extreme hmc occur in association with minor amounts of dolomite all of the carbonates are strongly depleted in 13c with a range from 35 7 to 57 5 pdb and enriched in 18o 4 0 to 5 3 pdb abundant microbial rods and filaments were recognized within the carbonate matrix as well as aragonite cements likely fossils of chemosynthetic microbes involved in carbonate formation the microbial structures are intimately associated with mineral grains some carbonate mineral grains resemble microbes the isotope characteristics the fabrics the microbial structure and the mineralogies are diagnostic of carbonates derived from anaerobic oxidation of methane mediated by microbes from the succession of hmcs extreme hmc and dolomite in layered tubular carbonates combined with the presence of microbial structure and diagenetic fabric we suggest that extreme hmc may eventually transform into dolomites our results add to the worldwide record of seep carbonates and establish for the first time the exact locations and seafloor morphology where such carbonates formed in the south china sea characteristics of the complex fabric demonstrate how seep carbonates may be used as archives recording multiple fluid regimes dissolution and early transformation events

paleo cold seep activity in the southern south china sea evidence from the geochemical and geophysical records of sediments abstract over the past decade more than 30 cold seep sites have been reported along the continental slope of the south china sea scs these seep sites have all been located in the northern scs and no methane seepage has been discovered in the southern scs here we report on the analysis results of the geochemical and geophysical of the marine core sediments 58s collected from the southern continental slope of the scs we suggest that the sedimentary total sulfur ts total inorganic carbon tic 34scrs values of chromium reducible sulfur crs and carbonate 13ctic values of the bulk sediments coupled with the ratio of nrmaf60mt nrmaf80mt nrm can be used to trace the methane release events in a methane rich environment at least two methane release events were identified at depths of 5 5 6 2 m and 6 8 7 2 m moreover according to the characteristic of the ts toc and nrmaf60mt nrmaf80mt nrm ratio of sediments we suggest the methane flux has changed episodically this inference is supported by the occurrence of the strongly negative carbonate 13ctic values of bulk sediments and the positive 34scrs values which indicate the occurrence of anaerobic oxidation of methane aom and the location of a past or present sulfate methane transition zone smtz respectively ultimately these new data suggest that the combined geochemical and geophysical study of marine sediments provides a valuable tool with which to infer the temporal variations in methane seepage in a marine environment

pore fluid compositions and inferred fluid flow patterns at the haima cold seeps of the south china sea abstract fluid flow patterns at cold seeps provide insights into the mechanism and influence of methane emission into the ocean which is critical in its environmental impact assessment here we report pore fluid compositions of three 8 m long piston cores qdn 14a qdn 14b and r1 collected from the newly discovered active haima cold seeps on the northwestern slope of the south china sea reaction transport models were further applied to quantify related biogeochemical processes and to reveal the patterns of fluid flow extremely low 13c values

geochemical record of methane seepage in authigenic carbonates and surrounding host sediments a case study from the south china sea abstract sediments at marine methane seep sites provide potential archives of past fluid flow that serve to explore seepage activities over time three gravity cores d 8 d f and d 7 were collected from seep sites on the northern slope of the south china sea where gas hydrates were drilled in the subsurface various carbon and sulfur contents 13c values of total inorganic carbon 13ctic 34s values of chromium reducible sulfur 34scrs trace element contents grain size and ams 14c dating of planktonic foraminifera in the sediments were determined to explore the availability of related proxies at seeps and to trace past methane seepage activities evidence for the presence of methane seepage and consequently anaerobic oxidation of methane comes from the occurrence of 13c depleted authigenic carbonate nodules 13c values as low as 49 discovered at an interval of 150 200 cm in core d 7 this finding is supported by high s c ratios and molybdenum enrichment in the same interval however low contents of crs and negative 34scrs values are present it is suggested to reflect a transient methane seepage event which continued for about 1 ka based on the 14c ages cores d 8 and d f have 13ctic values close to zero low s c ratios and crs contents negative 34scrs values and no trace element enrichment suggesting a negligible impact of methane seepage on the sediments the negative 34scrs values of the studied seep impacted and background sediments suggest that the application of 34scrs alone as a proxy to identify aom related process may be insufficient sediment carbon sulfur trace element systematics and 14c ages used here have the potential to be a promising tool to recognize transient methane seepages and constrain their timescales

seep carbonates and preserved methane oxidizing archaea and sulfate reducing bacteria fossils suggest recent gas venting on the seafloor in the northeastern south china sea seep carbonates formed by the synergistic metabolism of methane oxidizing archaea moa and sulfate reducing bacteria srb have been found in many places worldwide but have not been found to date in the south china sea mud volcanoes and shale diapirs have been reported in the south china sea however and microscope and geochernical analysis of carbonates dredged where bottom simulating reflectors bsr suggest the presence of gas hydrate on the northeastern continental slope of the south china sea suggest that the carbonates are hydrocarbon seep related carbonates the carbonates we dredged are chimney like and preserve moa srb fossils their very light carbon isotopic compositions 51 25 to 51 76 parts per thousand suggest that their carbon was derived from microbial methane oxidization these seep carbonates and their preserved moa srb fossils imply gas venting has occurred recently on the seafloor in the northeastern continental slopes in the south china sea c 2005 elsevier ltd all rights reserved

impact of anaerobic oxidation of methane on the geochemical cycle of redox sensitive elements at cold seep sites of the northern south china sea abstract cold hydrocarbon seepage is a frequently observed phenomenon along continental margins worldwide however little is known about the impact of seeping fluids on the geochemical cycle of redox sensitive elements pore waters from four gravity cores d 8 d 5 d 7 and d f collected from cold seep sites of the northern south china sea were analyzed for so42 mg2 ca2 sr2 dissolved inorganic carbon dic 13cdic dissolved fe mn and trace elements e g mo u the sulfate concentration depth profiles 13cdic values and dic ca2 mg2 so42 ratios suggest that organoclastic sulfate reduction osr is the dominant process in d 8 core besides osr anaerobic oxidation of methane aom is partially responsible for depletion of sulfate at d 5 and d 7 cores the sulfate consumption at d f core is predominantly caused by aom the depth of sulfate methane interface smi and methane diffusive flux of d f core are calculated to be 7 m and 0 035 mol m 2 yr 1 respectively the relatively shallow smi and high methane flux at d f core suggest the activity of gas seepage in this region the concentrations of dissolved uranium u were inferred to decrease significantly within the iron reduction zone it seems that aom has limited influence on the u geochemical cycling in contrast a good correlation between the consumption of sulfate and the removal of molybdenum mo suggests that aom has a significantly influence on the geochemical cycle of mo at cold seeps accordingly cold seep environments may serve as an important potential sink in the marine geochemical cycle of mo

sr ca and mg ca ratios in niger delta sediments implications for authigenic carbonate genesis in cold seep environments we report on a reconnaissance analysis of the geochemical composition of authigenic carbonates and sediment samples collected from various seepage sites on the niger deep sea fan our aim has been to investigate whether evidence for the presence of authigenic carbonates and gas hydrates within sediments is discernible from solid phase sediment geochemistry we show that sedimentary sr ca and mg ca ratios can be used to infer the presence of authigenic aragonite sr rich and mg rich carbonate phases high mg calcite dolomite in cold seep settings using sr ca and mg ca ratios the proportion wt of authigenic carbonates in niger fan sediments can be calculated from a mixing model between sediment fractions of terrigenous material biogenic calcite aragonite and high mg calcite this approach was applied to high resolution geochemical profiles along sediment cores recovered from various cold seep settings mud volcano diapirs pockmarks gas hydrate bearing sediments our data reveal that authigenic carbonates occur as discrete phases in sediments from gas hydrate bearing areas suggesting that such carbonate rich sediment layers may represent paleo indicators for ancient methane seepage in marine sediments possibly associated to gas hydrate reservoirs

pore water profiles and authigenic mineralization in shallow marine sediments above the methane charged system on umitaka spur japan sea umitaka spur situated on an unusual collisional plate boundary along the eastern margin of the japan sea hosts gas seeps pock marks collapse structures and gas hydrates piston cores were recovered from this ridge to understand carbon cycling pore fluid gradients and authigenic mineralization above a methane charged system we present the chemistry of fluids and solids from three cores adjacent to seep locations high fluxes of ch4 and alkalinity transport carbon from a deep zone of methanogenesis toward the seafloor methane however reacts with so42 across a shallow sulfate methane transition smt which generates additional alkalinity and hs a fraction of these ch4 oxidation products form authigenic carbonate and pyrite these minerals are not readily apparent from visual inspection of split cores because they exist as micritic coatings on microfossils or as framboidal pyrite they are however readily observed in chemical analyses as peaks of labile ca sr ba or s in sediment at or near the smt carbon inputs and outputs nicely balance across the smt in all three cores if one considers four relevant fluxes loss of alkalinity to the seafloor addition of methane from below addition of alkalinity from below and carbonate precipitation importantly in all cores the magnitude of the fluxes decreases in this order although some carbon rising from depth forms authigenic carbonate most 80 escapes to the ocean as alkalinity nonetheless authigenic fronts in sediment on umitaka spur are a significant reservoir of inorganic carbon given calculated pore fluid fluxes for ca and sr the fronts require tens of thousands of years to form suggesting that the present state and loss of carbon represent long lived processes

authigenic molybdenum formation in marine sediments a link to pore water sulfide in the santa barbara basin pore water and sediment mo concentrations were measured in a suite of multicores collected at four sites along the northeastern flank of the santa barbara basin to examine the connection between authigenic mo formation and pore water sulfide concentration only at the deepest site 580 m where pore water sulfide concentrations rise to 0 1 mm right below the sediment water interface was there active authigenic mo formation at shallower sites 550 430 and 340 m where pore water sulfide concentrations were consistently 0 05 mm mo precipitation was not occurring at the time of sampling a sulfide concentration of 0 1 mm appears to be a threshold for the onset of mo fe s co precipitation a second threshold sulfide concentration of 100 mm is required for mo precipitation without fe possibly as mo s or as particle bound mo mass budgets for mo were constructed by combining pore water and sediment results for mo with analyses of sediment trap material from santa barbara basin as well as sediment accumulation rates derived from 210 pb the calculations show that most of the authigenic mo in the sediment at the deepest site is supplied by diffusion from overlying bottom waters there is however a non lithogenic particulate mo associated with sinking particles that contributes 15 to the total authigenic mo accumulation analysis of sediment trap samples and supernant brine solutions indicates the presence of non lithogenic particulate mo a large fraction of which is easily remobilized and perhaps associated with mn oxides our observations show that even with the very high flux of organic carbon reaching the sediment of santa barbara basin active formation of sedimentary authigenic mo requires a bottom water oxygen concentration below 3 mm however small but measurable rates of authigenic mo accumulation were observed at sites where bottom water oxygen ranged between 5 and 23 mm indicating that the formation of authigenic mo occurred in the recent past but not at the time of sampling copyright 2000 elsevier science ltd

in situ raman detection of gas hydrates exposed on the seafloor of the south china sea gas hydrates are usually buried in sediments here we report the first discovery of gas hydrates exposed on the seafloor of the south china sea the in situ chemical compositions and cage structures of these hydrates were measured at the depth of 1 130 m below sea level using a raman insertion probe rip gh that was carried and controlled by a remotely operated vehicle rov faxian this in situ analytical technique can avoid the physical and chemical changes associated with the transport of samples from the deep sea to the surface natural gas hydrate samples were analyzed at two sites the in situ spectra suggest that the newly formed hydrate was structure i but contains a small amount of c3h8 and h2s pure gas spectra of ch4 c3h8 and h2s were also observed at the scs sgh02 site these data represent the first in situ proof that free gas can be trapped within the hydrate fabric during rapid hydrate formation we provide the first in situ confirmation of the hydrate growth model for the early stages of formation of crystalline hydrates in a methane rich seafloor environment our work demonstrates that natural hydrate deposits particularly those in the early stages of formation are not monolithic single structures but instead exhibit significant small scale heterogeneities due to inclusions of free gas and the surrounding seawater there inclusions also serve as indicators of the likely hydrate formation mechanism these data also reinforce the importance of correlating visual and in situ measurements when characterizing a sampling site

cyclic stressing and seismicity at strongly coupled subduction zones we use the finite element method to analyze stress variations in and near a strongly coupled subduction zone during an earthquake cycle deformation is assumed to be uniform along strike plane strain on a cross section normal to the trench axis and periodic earthquake slip is imposed consistent with the long term rate of plate convergence and degree of coupling simulations of stress and displacement rate fields represent periodic fluctuations in time superimposed on an average field the oceanic plate descending slab and continental lithosphere are assumed here to respond elastically to these fluctuations and the remaining mantle under and between plates is assumed to respond as maxwell viscoelastic in the first part of the analysis we find that computed stress fluctuations in space and time are generally consistent with observed earthquake mechanism variations with time since a great thrust event in particular trench normal extensional earthquakes tend to occur early in the earthquake cycle toward the outer rise but occur more abundantly late in the cycle in the subducting slab downdip of the main thrust zone compressional earthquakes when they occur at all have the opposite pattern our results suggest also that the actual timing of extensional outer rise events is controlled by the rheology of the shallow aseismic portion of the thrust interface the second part of the analysis shows the effects of mantle relaxation on the rate of ground surface deformation during the earthquake cycle models without relaxation predict a strong overall compressional strain rate in the continental plate above the main thrust zone with the strain rate constant between mainshocks however with significant relaxation present a localized region of unusually low compressional or even slightly extensional strain rate develops along the surface of the continental plate above and somewhat inland from the downdip edge of the locked main thrust zone the low strain rate starts in the middle or late part of the cycle depending on position this result suggests that the negligible or small contraction measured on the shumagin islands alaska during 1980 to 1991 may not invalidate an interpretation of that region as being a moderately coupled subduction zone in contrast mantle relaxation causes only modest temporal nonuniformity of uplift rates in the overriding plate and of extensional stress rates in the subducting plate even when the maxwell time is an order of magnitude less than the recurrence interval

gravity and seismicity over the guerrero seismic gap mexico four detailed average station interval 5 km gravity transects were recently conducted in the pacific coastal region of mexico a differential gps technique was used to determine the elevation and coordinates of the gravity stations the profiles are oriented northeast southwest and extend from the coast up to 60 km inland the bouguer gravity anomaly is decreasing consistently along every profile from 60 80 mgal at the coast with an approximately constant regional gradient of 2 2 mgal km normal to the trench a plot of the gravity anomaly against the distance from the trench axis demonstrates that the regional slope in the gravity anomaly is shifting gradually 20 25 mgal inland along the coast of guerrero from the southeast atoyac to the northwest petatlan zihuatanejo a model cross section of the mexican subduction zone msz based on the tomography inversion for the guerrero region shows that the gravity anomaly values and the regional anomaly trend can be explained mostly by the effect of the density contrast between the slab and the continental crust the upper surface of the subducted slab uss and the seismogenic contact zone between the upper plate and the slab is traced clearly in several seismicity cross sections based on the data of the regional seismic network in guerrero the depth and shape of the uss revealed from the seismicity and gravity anomaly data for the same profiles are in good agreement this correlation may be fairly useful when applied to gravity profiles in order to estimate the depth of the uss and the seismogenic contact in other parts of the msz which lack reliable seismicity data

how sulfate driven anaerobic oxidation of methane affects the sulfur isotopic composition of pyrite a sims study from the south china sea abstract sulfate driven anaerobic oxidation of methane so4 aom in marine sediments commonly leads to the precipitation of pyrite it is however frequently challenging to unequivocally unravel the entire history of pyritization because of the common coexistence of so4 aom derived pyrite with pyrite resulting from organiclastic sulfate reduction osr to better understand how so4 aom affects pyritization in methane bearing sediments and how this can be identified we applied secondary ion mass spectroscopy sims to analyze the sulfur isotope composition 34s of authigenic pyrite in addition to sulfur isotope measurements of bulk sulfide and hand picked pyrite aggregates from the two seafloor sites hs148 and hs217 in the shenhu seepage area south china sea authigenic mostly tubular pyrite aggregates from these sites consist of three types of pyrite framboids zoned aggregates with radial overgrowths surrounding a framboidal core and euhedral pyrite crystals framboids with low sims 34s values as low as 41 6 at hs148 and 38 8 at hs217 are dispersed throughout the cores but are especially abundant in the shallow part of the sedimentary column i e above 483 cmbsf in hs148 above 670 cmbsf in hs217 these patterns are interpreted to reflect the dominance of osr during early diagenetic processes in the shallow sediments with increasing depth both 34s values of bulk sulfide minerals and hand picked pyrite aggregates increase sharply at 483 cmbsf in core hs148 and at 700 cmbsf in core hs217 respectively radial pyrite overgrowths and euhedral crystals become abundant at depth typified by high 34s values for hand picked pyrite moreover sims analysis reveals an extreme variability of 34s values for the three pyrite types on a small scale in these zones besides some moderately 34s enriched framboids most of the overgrowths and euhedral crystals display extremely high sims 34s values as high as 114 8 at hs148 and 74 3 at hs217 representing the heaviest stable sulfur isotope composition of pyrite ever reported to the best of our knowledge such an abrupt and extreme increase in 34spyrite values with depth is best explained by an enrichment of 34s in the pool of dissolved sulfide caused by so4 aom in the sulfate methane transition zone smtz the increase in 34s values from framboidal cores to overgrowth layers and euhedral crystals indicates continuous and finally near to complete exhaustion of dissolved sulfate at the smtz following a rayleigh distillation process so4 aom allowed for subsequent growth of later stage pyrite over the initial framboids part of which formed earlier and at shallower depth by osr the combination of a detailed petrographic study of authigenic pyrite with sims analysis of stable sulfur isotopes in organic rich strata proves to be a powerful tool for reconstructing the dynamics of sulfur cycling in modern and potentially ancient sedimentary sequences

relating sulfate and methane dynamics to geology accretionary prism offshore sw taiwan geochemical data ch4 so42 i cl particulate organic carbon poc 13c ch4 and 13c co2 are presented from the upper 30 m of marine sediment on a tectonic submarine accretionary wedge offshore southwest taiwan the sampling stations covered three ridges tai nan yung an and good weather each characterized by bottom simulating reflectors acoustic turbidity and different types of faulting and anticlines sulfate and iodide concentrations varied little from seawater like values in the upper 1 3 m of sediment at all stations a feature that is consistent with irrigation of seawater by gas bubbles rising through the soft surface sediments below this depth sulfate was rapidly consumed within 5 10 m by anaerobic oxidation of methane aom at the sulfate methane transition carbon isotopic data imply a mainly biogenic methane source a numerical transport reaction model was used to identify the supply pathways of methane and estimate depth integrated turnover rates at the three ridges methane gas ascending from deep layers facilitated by thrusts and faults was by far the dominant term in the methane budget at all sites differences in the proximity of the sampling sites to the faults and anticlines mainly accounted for the variability in gas fluxes and depth integrated aom rates by comparison methane produced in situ by poc degradation within the modeled sediment column was unimportant this study demonstrates that the geochemical trends in the continental margins offshore sw taiwan are closely related to the different geological settings

uranium diagenesis in sediments underlying bottom waters with high oxygen content we measured u in sediments both pore waters and solid phase from three locations on the middle atlantic bight mab from the eastern margin of the united states a northern location on the continental shelf off massachusetts oc426 75 m water depth and two southern locations off north carolina en433 1 647 m water depth and en433 2 2648 m water depth these sediments underlie high oxygen bottom waters 250 270 m but become reducing below the sediment water interface due to the relatively high organic carbon oxidation rates in sediments en433 1 212 mol c cm2 y oc426 120 10 mol c cm2 y en433 2 33 mol c cm2 y pore water oxygen goes to zero by 1 4 1 5 cm at en433 1 and oc426 and slightly deeper oxygen penetration depths were measured at en433 2 4 cm all of the pore water profiles show removal of u from pore waters calculated pore water fluxes are greatest at en433 1 0 66 0 08 nmol cm2 y and less at en433 2 and oc426 0 24 0 05 and 0 13 0 05 nmol cm2 y respectively solid phase profiles show authigenic u enrichment in sediments from all three locations the average authigenic u concentrations are greater at en433 1 and oc426 5 8 0 7 nmol g and 5 4 0 2 nmol g respectively relative to en433 2 4 1 0 8 nmol g this progression is consistent with their relative ordering of reduction intensity with greatest reducing conditions in sediments from en433 1 less at oc426 and least at en433 2 the authigenic u accumulation rate is largest at en433 1 0 47 0 05 nmol cm2 y but the average among the three sites on the mab is 0 2 nmol cm2 y pore water profiles suggest diffusive fluxes across the sediment water interface that are 1 4 1 7 times greater than authigenic accumulation rates at en433 1 and en433 2 these differences are consistent with oxidation and loss of u from the solid phase via irrigation and or bioturbation which may compromise the sequestration of u in continental margin sediments that underlie bottom waters with high oxygen concentrations previous literature compilations that include data exclusively from locations where o2 bw 150 m suggest compelling correlations between authigenic u accumulation and organic carbon flux to sediments or organic carbon burial rate sediments that underlie waters with high o2 bw have lower authigenic u accumulation rates than would be predicted from relationships developed from results that include locations where o2 bw 150 m

evidence for intense ree scavenging at cold seeps from the niger delta margin for many trace elements continental margins are the location of intense exchange processes between sediment and seawater which control their distribution in the water column but have yet to be fully understood in this study we have investigated the impact of fluid seepage at cold seeps on the marine cycle of neodymium we determined dissolved and total dissolvable td concentrations for ree and well established tracers of fluid seepage ch4 tdfe tdmn and nd isotopic compositions in seawater samples collected above cold seeps and a reference site i e away from any fluid venting area from the niger delta margin we also analyzed cold seep authigenic phases and various core top sediment fractions pore water detrital component easily leachable phases uncleaned foraminifera recovered near the hydrocast stations methane tdfe and tdmn concentrations clearly indicate active fluid venting at the studied seeps with plumes rising up to about 100 m above the seafloor depth profiles show pronounced ree enrichments in the non filtered samples td concentrations within plumes whereas filtered samples dissolved concentrations exhibit slight ree depletion in plumes relative to the overlying water column and display typical seawater ree patterns these results suggest that the net flux of ree emitted into seawater at cold seeps is controlled by the presence of particulate phases most probably fe mn oxyhydroxides associated to resuspended sediments at the reference site however our data reveal significant enrichment for dissolved ree in bottom waters that clearly relates to diffusive benthic fluxes from surface sediments neodymium isotopic ratios measured in the water column range from end 15 7 to 10 4 evidence that the end values for antarctic intermediate waters aaiw differed from those reported for the same water mass at open ocean settings shows that sediment water interactions take place in the gulf of guinea at each site however the bottom water end signature generally differs from that for cold seep minerals easily leachable sediment phases and detrital fractions from local sediments ruling out the possibility that seepage of methane rich fluids and sediment dissolution act as a substantial source of dissolved nd to seawater in the gulf of guinea taken together our data hence suggest that co precipitation of fe mn oxyhydroxide phases in sub surface sediments leads to quantitative scavenging of dissolved ree at cold seeps preventing their emission into bottom waters most probably it is likely that diffusion from suboxic surface sediments dominates the exchange processes affecting the marine nd cycle at the niger delta margin

sources of methane inferred from pore water δ13c of dissolved inorganic carbon in pockmark g11 offshore mid norway abstract pockmark g11 is the most spectacular one among the pockmarks located at the southern border of voring plateau and 1 2 km away from the northern flank of the storegga slide mid norwegian continental margin for the first time detailed pore water geochemical studies were conducted to address methane hydrate occurrence methane seepage and associated geochemical processes and methane characteristics in the pockmark pore waters collected from five sediments cores inside and one sediment core outside the pockmark were analyzed for dissolved cl sulfate so42 total hydrogen sulfide h2s ca2 mg2 dissolved inorganic carbon dic 13cdic and 18o methane hydrates were recovered in all sediments below 0 75 m in a core inside pockmark g11 which is in good accordance with heavy oxygen isotope 1 9 to 2 3 smow and low cl concentrations 84 9 to 16 1 mm in pore waters pore water profiles indicate that upward migrating methane fluids are spatially variable in the pockmark with methane fluxes ranging from below detection in the center and outside to 0 30 0 54 mol m 2 a 1 inside in the cores with active methane fluxes maximum dic concentrations 19 4 to 21 5 mm and corresponding minimum 13cdic values 52 3 to 54 6 pdb occur within sulfate methane transition smt zones from 0 40 to 0 50 m below seafloor mbsf close to the seafloor simple mass balance modeling and 13cdic measurements within the smt zones suggest that methane in shallow sediments within pockmark g11 is microbial in origin pore water geochemistry and seabed observations suggest that methane fluxes inside pockmark g11 are episodic and derived mostly from the recycling of methane hydrate at depth during sediment burial

methane derived carbonates and authigenic pyrite from the northwestern black sea abstract methane seeps in the northwestern black sea are accompanied by carbonate and pyrite precipitates sediments were sampled at locations on the romanian 120 m depth and ukrainian 180 200 m depth shelf and slope layered carbonate crusts are formed of i carbonate cemented siliciclastic sediment containing dreissenoid bivalves ii microcrystalline high mg calcite or aragonite and iii aragonitic cement the dreissena sediment is subrecent and was deposited during the freshwater phase of the black sea it has been affected by seepage of methane rich fluids which induced intergranular precipitation of authigenic carbonates the microcrystalline carbonates exhibit an intense autofluorescence high mg calcite contains 11 14 mol mgco3 the aragonitic cement 8300 9500 ppm sr forms either isopachous layers or botryoids microbial filaments about 10 20 m in diameter and up to 900 m in length are preserved within and on carbonate crusts the carbonates are depleted in 13c microcrystalline carbonate ranges from 27 to 41 pdb and botryoidal aragonite ranges from 26 to 38 pdb the 13c depletion indicates that the carbonates predominantly derive from the microbial oxidation of methane carbonate deposits do not project up into the oxic water column they are restricted to the anoxic water column and to anoxic sediments revealing the crucial role of anaerobic methane oxidation for carbonate precipitation 14c contents give apparent radiocarbon ages of 20 640 180 a bp for a sample of botryoidal aragonite and 19 110 180 a bp for a sample of microcrystalline carbonate compatible with minimum ages of carbon derived from a fossil hydrocarbon source the 18o values of methane derived carbonates show a narrow range from 1 2 to 0 2 pdb 87sr 86sr ratios of microcrystalline carbonate mean 0 70927 and aragonitic cement mean 0 70918 are indistinguishable from ambient seawater mean 0 70917 and thus indicate a shallow sr source higher 87sr 86sr ratios of the dreissena sediment mean 0 71005 are probably caused by sr derived from detrital mica carbonates are accompanied by blocks and crusts composed of pyrite the framboidal sulphide exhibits a palisade like fabric with framboids arranged to parallel pillars sulphur isotopic ratios 34s ranging from 16 8 to 19 7 cdt indicate that the sulphur derives not from the 34s depleted h2s of the water column or the uppermost sediment layers most likely pyrite formed in the lacustrine sediments after the first incursion of mediterranean seawater

methane seepage inferred from pore water geochemistry in shallow sediments in the western slope of the mid okinawa trough abstract widely distributed cold seeps are crucial sources of carbon to the seawater whereas anaerobic oxidation of methane aom and precipitation of authigenic carbonate might change the compositions of the methane rich fluids and thus reduce the outputs of dissolved carbon to seawater in this work we analyze the pore water compositions of four gravity cores with high methane concentration abnormalities in pore water or in overlying seawater in the western slope of the mid okinawa trough for the northern research area active weak methane seepage through the seafloor is identified in the vicinity of mud volcanoes sites c01 and c10 and strong emissions of gas and fluids may occur in the central mud volcanoes in a submarine canyon at the south c23 site exhibits rarely methane seepage while high rate transportation of methane from deep sediments and associated aom are identified at site c25 where small vertical faults and gas bearing layers are developed at the surrounding aom at site c25 is indicated by 1 quasi linearly decrease of sulfate concentrations with depths 2 intense increases of dissolved inorganic carbon dic concentrations and 3 significantly negative 13c values vary from 7 2 to 26 3 of dic organoclastic degradation in the shallow sediments is not obviously recognized in the study area because of the low concentrations of nh4 as well as negligible sulfate consumption at three of the four sites our data reveal that the cold systems are one of the dic sources the fluxes are ca 11 2 mmol m2 yr to the seawater in the okinawa trough which can be tracked by high depletion of 13c 13cdic ranges from 7 2 to 26 3 vpdb

volcanism around k t boundary time its role in an impact scenario for k t extinction events abstract a widespread debate on the cretaceous tertiary k t boundary 65 1 ma extinctions has largely revolved around an impact or a volcanic cause evidence for both events is strengthened by recent work impact triggered deccan volcanism or independent impact and mantle plume events have been suggested this review examines evidence for both events and suggests that they overlapped in time but with globally uneven effects the combined effects produced generally inhospitable k t environments leading to mass extinctions although some life forms flourished afterwards volcanic contributions to the k t boundary environment probably include increased platinum group element pge and acid rain fall out particularly from large scale mantle upwellings e g deccan cameroon and coral sea events a previously proposed post k t greenland starting plume apparently already existed in the cretaceous k t features such as the iridium ir anomaly most likely include longer term volcanic as well as sudden meteoritic impact contributions volcanic contributions were greatest from southern hemisphere sources whereas meteorite impacts featured in the northern hemisphere

tracing the paleo sulfate methane transition zones and h2s seepage events in marine sediments an application of c s mo systematics 1 microbially mediated anaebic oxidation of methane aom coupled with sulfate consumption within the sulfate methane transition zone smtz in marine sediments is a widely recorded biogeochemical reaction and has profound influence on the atmospheric ch4budget marine carbon cycle and composition of sediment pore fluids recognizing the paleo smtzs in the marine sediments rock records can throw light on the variation of paleo methane fluxes and occurrences of cold seep h2s ch4 events through geologic time here we present results from carbonate carbon pyrite sulfur and molybdenum analyses for two sediment cores overlying the methane hydrate deposits in the bay of bengal the results show intimate association of isotopically depleted carbonate carbon and enriched pyrite sulfur constraining the paleo smtz within the sediment column in addition anomalous enrichments of mo concentrations indicate hydrogen sulfide seepage events here we propose a geochemical tool using c s mo sytematics to decipher the paleo smtzs in marine sediments and rocks