sedimentary pyrite formation an update sedimentary pyrite formation during early diagenesis is a major process for controlling the oxygen level of the atmosphere and the sulfate concentration in seawater over geologic time the amount of pyrite that may form in a sediment is limited by the rates of supply of decomposable organic matter dissolved sulfate and reactive detrital iron minerals organic matter appears to be the major control on pyrite formation in normal non euxinic terrigenous marine sediments where dissolved sulfate and iron minerals are abundant by contrast pyrite formation in non marine freshwater sediments is severely limited by low concentrations of sulfate and this characteristic can be used to distinguish ancient organic rich fresh water shales from marine shales under marine euxinic conditions sufficient h2s is produced that the dominant control on pyrite formation is the availability of reactive iron minerals calculations based on a sulfur isotope model indicate that over phanerozoic time the worldwide average organic carbon to pyrite sulfur ratio of sedimentary rocks has varied considerably high cs ratios during permo carboniferous time can be explained by a shift of major organic deposition from the oceans to the land which resulted in the formation of vast coal swamps at that time low cs ratios compared to today during the early paleozoic can be explained in terms of a greater abundance of euxinic basins combined with deposition of a more reactive type of organic matter in the remaining oxygenated portions of the ocean the latter could have been due to lower oceanic oxygen levels and or a lack of transportation of refractory terrestrial organic matter to the marine environment due to the absence of vascular land plants at that time

giant magnetofossils and hyperthermal events abstract magnetotactic bacteria biomineralize magnetic minerals with precisely controlled size morphology and stoichiometry these cosmopolitan bacteria are widely observed in aquatic environments if preserved after burial the inorganic remains of magnetotactic bacteria act as magnetofossils that record ancient geomagnetic field variations they also have potential to provide paleoenvironmental information in contrast to conventional magnetofossils giant magnetofossils most likely produced by eukaryotic organisms have only been reported once before from paleocene eocene thermal maximum petm 55 8 ma sediments on the new jersey coastal plain here using transmission electron microscopic observations we present evidence for abundant giant magnetofossils including previously reported elongated prisms and spindles and new giant bullet shaped magnetite crystals in the southern ocean near antarctica not only during the petm but also shortly before and after the petm moreover we have discovered giant bullet shaped magnetite crystals from the equatorial indian ocean during the mid eocene climatic optimum 40 ma our results indicate a more widespread geographic environmental and temporal distribution of giant magnetofossils in the geological record with a link to hyperthermal events enhanced global weathering during hyperthermals and expanded suboxic diagenetic environments probably provided more bioavailable iron that enabled biomineralization of giant magnetofossils our micromagnetic modelling indicates the presence of magnetic multi domain i e not ideal for navigation and single domain i e ideal for navigation structures in the giant magnetite particles depending on their size morphology and spatial arrangement different giant magnetite crystal morphologies appear to have had different biological functions including magnetotaxis and other non navigational purposes our observations suggest that hyperthermals provided ideal conditions for giant magnetofossils and that these organisms were globally distributed much more work is needed to understand the interplay between magnetofossil morphology climate nutrient availability and environmental variability

rock magnetic signature of gas hydrates in accretionary prism sediments sediments from two ocean drilling program leg 146 sites from the cascadia margin of western north america have magnetic properties indicating diagenesis of magnetic minerals associated with the presence of gas hydrates two indices combining coercivity remanence and susceptibility parameters djh jrs js hcr hc and ds jrs k hcr when combined with thermo magnetic data can be diagnostic of these changes at site 892 ds values are distinctly higher and more scattered above the bottom simulating seismic reflector bsr which marks the base of the hydrate stability zone within the hydrate stability zone at site 892 djh shows two trends an increase from about 50 mbsf to the bsr at 73 mbsf corresponding to an expected increase in hydrate concentration near the bsr and a second increase upwards from 50 mbsf to peak values at less than 21 mbsf associated with hydrate recovered in cores above 19 mbsf at site 889 890 djh increases downhole to about 285 mbsf substantially below the bsr at 225 mbsf high djh sediments within a low cl zone at this site have magnetic mineralogies which are dominated by fine grained magnetic sulfides whereas sediments from above and below this zone are characterised by magnetite magnetic sulfide mixes this trend at site 889 890 is consistent with an interpretation based on pore water geochemistry low cl and bottom water temperature that a fossil gas hydrate zone extended downwards to about 295 mbsf during the last glacial the observed changes in magnetic properties can be attributed to steps in the reduction series from magnetite through sd greigite to pyrite or to overgrowth of sd greigite to md size diagenetic growth of magnetic iron sulfides greigite and or pyrrhotite has been reported in other accretionary wedge sediments thermal demagnetization of multi component isothermal remanent magnetization mirm indicates the presence of a low coercivity magnetic mineral with an unblocking temperature tub between 310 and 350 c high jrs k ratios suggest that the low coercivity low unblocking temperature mineral is predominantly greigite rather than pyrrhotite a low to medium coercivity mineral with tub ca 580 c magnetite is also present in varying amounts hydrate apparently controls the presence of greigite by incorporating h2s shown to be present as a hydrate phase together with methane in hydrate recovered at site 892 release of h2s below the base of the hydrate layer allows overgrowth of greigite grains to md size or the conversion of some of the greigite to pyrite

effect of thermal non equilibrium seafloor topography and fluid advection on bsr derived geothermal gradient abstract the seafloor and bottom simulating reflectors bsrs are interpreted from the 3d seismic data acquired in krishna godavari kg offshore basin in the vicinity of sites drilled cored during national gas hydrate program nghp expedition 01 the shallow structures such as inner toe thrust fault system regional and local linear fault systems and mass transport deposits are inferred from attributes of seafloor time structure as well as from the seismic profiles the geothermal gradient is estimated from the depths and temperatures of the seafloor and the bsr the temperature at the bsr depth is estimated from the methane hydrate and seawater salinity phase boundary assuming that the bsr represents the base of the gas hydrate stability zone the spatial variations in geothermal gradient gtg show a strong correlation with seafloor topography in the kg basin the gtg decreases by 13 30 over the topographic mounds formed due to inner toe thrust faults and recent mass transport deposits the gtg decreases by only 5 10 over the mounds likely due to defocusing of heat flux based on one dimensional topographic modeling hence the gtg perturbation due to topography alone cannot explain the observed gtg anomaly the temperature profile beneath these mounds may not be in equilibrium with the surroundings either due to the recent upliftment of sediments along the inner toe thrust faults or rapid deposition of sediments due to slumping sliding in contrast an increase in gtg by 10 15 is observed in the vicinity of major fault systems we presume that the likely mechanism for the increase in gtg is fluid advection from a deeper part of the basin a detailed thermal modeling involving the effect of surface topography high sedimentation rates fluid advection and sediment thickening due to tectonics is required to understand the thermal profile in kg offshore basin

rise of the base of the gas hydrate zone since the last glacial recorded by rock magnetism gas hydrate a clathrate of methane and water widespread on continental margins has been implicated as a trigger of climate change and submarine slides as a result of methane release when the base of its stability zone moves upward rapidly direct tests of these hypotheses are made difficult by the ephemeral record of gas hydrate in sediment in places a seismic reflector double bottom simulating reflector bsr appears to mark the old base of the gas hydrate layer but the occurrence of this feature is patchy and its interpretation is controversial microbial activity is stimulated in the presence of gas hydrate and results in the production of magnetic iron sulfides the base of the gas hydrate interval is marked by a sharp reduction in the magnetic hysteresis parameter d j h at hydrate ridge on the cascadia margin sampled during ocean drilling program leg 204 this signature occurs between 20 and 65 m below the present day base of the gas hydrate zone at a depth consistent with predictions for the base of gas hydrate stability given water depths and bottom water temperatures appropriate for the last glacial maximum seismic evidence for a double bsr over part of hydrate ridge corroborates the rock magnetic interpretation

pore water anomalies of submarine gas hydrate zones as tool to assess hydrate abundance and distribution in the subsurface what have we learned in the past decade abstract the most significant new contributions to the study of natural gas hydrates in the past decade have come from findings of the ocean drilling program odp notably legs 112 141 146 and 164 and to a lesser extent legs 131 160 170 and 186 leg 164 in particular was dedicated to gas hydrate drilling in the blake ridge gas hydrate field in the west atlantic in an unprecedented multidisciplinary research effort paull et al 1996 proc ocean drill program initial rep 164 623 pp paull c k matsumoto r wallace p j dillon w p eds 2000a proc ocean drill program sci results 164 ocean drilling program college station tx 459 pp most important for the progress of hydrochemical studies related to gas hydrates has been the growing awareness of the significance of diffusion modulated advective processes shaping the chemical and isotopic pore water profiles in hydrate zones this started with qualitative evidence for advective flow from drilling the hydrate bearing peru and hydrate free barbados active margins continued with the nankai trough accretionary prism the japan trench slope and the cascadia and costa rica active margins and culminated in the quantitative advection diffusion model of egeberg and dickens chem geol 153 1999 53 for the passive margin setting of the blake ridge advective flow regimes are different at active and passive margins as there is a tendency for the flow at active margins to be focussed along landward dipping thrust planes and faults in the wedge of imbricated thrust sheets that finds expression in a step pattern of the pore water profiles for passive margins but also for some active margin sites advection is from sources below the drilled section either through subvertical faults passive margins or from the decollement zone active margins we have learned that the well known coupled pore water anomalies that are ascribed to hydrate dissociation downward chlorinity decrease combined with 18 o increase hesse and harrison 1981 earth planet sci lett 55 1981 453 need not occur together in the presence of hydrates because the isotope effect may be overprinted by the effects of other reactions such as volcanic ash alteration or by the advection of low 18 o fluids however if the anomalies show up hydrates are present almost invariably with the exception of advected low cl high 18 o waters coming to terms with the effects of advection and diffusion has allowed successful modeling of the simpler hydrate affected pore water profiles at passive margins instrumental for modeling are chlorine isotopes which provide an effective tool to assess advection rates the egeberg and dickens model allows estimation of hydrate concentration and distribution in the subsurface because it separates the effects of advection diffusion and hydrate dissociation but critically depends on samples taken under in situ pressure and temperature conditions modeling the more complex pore water profiles of active margins is a challenge for the future compared to geophysical methods to estimate hydrate concentration the geochemical method gives minimum amounts

environmental magnetic studies on some quaternary sediments of varied depositional settings in the indian sub continent abstract the efficacy and limitations of routine environmental magnetic approaches to characterize and compare the quaternary sediments of varied depositional settings fluvial fluvio lacustrine and marine from the indian sub continent are discussed the various studied profiles fall under tropical wet and dry subtropical humid cold arid and alpine climatic zones at altitudes varying from 2000 m below sea level to 4500 msl the magnetic mineral assemblages are characterized by magnetic susceptibility high field hysteresis parameters and their ratios and bivariate plots besides applying the conventional models we produced new and multi parametric environmental magnetic models for finer discrimination of the mineral magnetic assemblages mma amongst these deposits in similar hinterland setup and climate zones independent mma characteristics are shown by the late quaternary frontal himalayan basins the ganga basin the intermontane pinjor dun and the folded siwalik sequence such variations are attributed to diverse energy conditions due to varied basin morphology climate and the source to sink proximity contrasting channel to overbank mma relations are discernible during the glacial and interglacial periods within the ganga basin the siwalik sediments are notably restricted to antiferromagnetic region of the bivariate plots indicating formation and stability of iron oxides under warm humid to arid conditions the post siwalik intermontane sediments show steep gradients in mma domain within a short distance from proximal to distal fan regime in the tropical peninsular region the source to sink relation is marked by the deccan trap province source and bengal submarine fan sink at least since the last glacial maxima lgm in the middle fan region the mmas for the lacustrine and fluvio lacustrine deposits in the himalayan foothills as well as the higher himalaya indicate detrital variations governed by the fluvial dynamics in the region this demands a more detail catchment to sink studies in each basin and to explore the climatic tele connections amongst the varied altitudinal regions overall this study indicates a strong detrital control on mma governed by the energy conditions of the transporting media that in turn is controlled by catchment morphology regional tectonics and climatic fluctuations in the absence of suitable unmixing models the new bivariate and diamond plots attempted here are helpful in describing the detrital modes within the basin and its comparison to other basins

magnetic hysteresis parameters and day plot analysis to characterize diagenetic alteration in gas hydrate bearing sediments 1 the j meter coercivity spectrometer is a machine capable of rapid and simple measurement of magnetic hysteresis isothermal remanence acquisition and magnetic viscosity of rocks and sediments the j meter was used to study a suite of samples collected from strata in the gas hydrate bearing japex jnoc gsc mallik 5l 38 well 69 5 n 134 6 w in the mackenzie delta of the northwestern canadian arctic the day plot of magnetic hysteresis ratios for these samples is exotic in that the points do not plot along a hyperbola as is usually observed rather they plot as a scatter which is shown to contour into vertical slices using coercivity field hc or saturation magnetization js and horizontal slices using the relative quantity of superparamagnetism jspm js optical microscopy reveals that the magnetic minerals are detrital magnetite and authigenic greigite greigite is dominant in sands which in situ had 70 gas hydrate saturation and in silts in which gas hydrate growth was blocked by insufficient porosity we infer that the silts were the accumulation sites for solutes which had been excluded from the pore waters in neighboring coarser grained sediments during the course of gas hydrate formation consequently we conclude that magnetic properties are related to gas hydrate related processes and as such may have potential as a method of remote sensing for gas hydrate deposits

anaerobic methane oxidation and a deep h2s sink generate isotopically heavy sulfides in black sea sediments the main terminal processes of organic matter mineralization in anoxic black sea sediments underlying the sulfidic water column are sulfate reduction in the upper 2 4 m and methanogenesis below the sulfate zone the modern marine deposits comprise a ca 1 m deep layer of coccolith ooze and underlying sapropel below which sea water ions penetrate deep down into the limnic pleistocene deposits from 9000 years bp sulfate reduction rates have a subsurface maximum at the so42 ch4 transition where h2s reaches maximum concentration because of an excess of reactive iron in the deep limnic deposits most of the methane derived h2s is drawn downward to a sulfidization front where it reacts with fe iii and with fe2 diffusing up from below the h2s fe2 transition is marked by a black band of amorphous iron sulfide above which distinct horizons of greigite and pyrite formation occur the pore water gradients respond dynamically to environmental changes in the black sea with relatively short time constants of ca 500 yr for so42 and 10 yr for h2s whereas the fes in the black band has taken ca 3000 yr to accumulate the dual diffusion interfaces of so42 ch4 and h2s fe2 cause the trapping of isotopically heavy iron sulfide with 34s 15 to 33 at the sulfidization front a diffusion model for sulfur isotopes shows that the so42 diffusing downward into the so42 ch4 transition has an isotopic composition of 19 close to the 23 of h2s diffusing upward these isotopic compositions are however very different from the porewater so42 43 and h2s 15 at the same depth the model explains how methane driven sulfate reduction combined with a deep h2s sink leads to isotopically heavy pyrite in a sediment open to diffusion these results have general implications for the marine sulfur cycle and for the interpretation of sulfur isotopic data in modern sediments and in sedimentary rocks throughout earth s history

quantitative extraction and analysis of carriers of magnetization in sediments we have applied an integrated procedure for quantitative magnetic mineral extraction based on the separation method of petersen von dobeneck vali 1986 to a range of sediment types to examine the efficiency and representative nature of the extraction process carriers of magnetization have been identified by rock magnetic measurements microscopy and x ray diffraction quantification of the extraction efficiencies is achieved by before and after extraction rock magnetic measurements susceptibility anhysteretic and isothermal remanences these magnetic measurements show that our modified extraction method extracts large proportions of the magnetization carriers in a range of sediment types e g over 75 per cent for magnetite dominated sediments the extraction efficiency is dependent on the sample magnetic mineralogy and whether the magnetic grains occur as discrete grains or as inclusions within host grains susceptibility extraction efficiencies are strongly dependent on whether the susceptibility is of paramagnetic or ferrimagnetic origin the amount of material recovered in the extract shows some inverse correlation with the density of the sediment suspension used during extraction in terms of the mineralogies extracted we identify a diverse and complex range of mineral assemblages all sizes of discrete grains of magnetite are extracted including single domain and superparamagnetic grains and chains of bacterial magnetite other commonly extracted iron and titanium oxides are haematite and ilmenite ferrimagnetic chromites and sulphides were also obtained from some samples considerable amounts of quartz and feldspar are extracted due to the presence of magnetic inclusions within these diamagnetic host grains in the deep sea sediments we examined feldspars constitute a large proportion of the extracts but are significantly less abundant in other sediments where quartz is dominant a wide variety of paramagnetic minerals was identified in the extracts including pyroxenes amphiboles chlorites micas mg cr spinels garnets ti oxides apatites tourmaline and zircon many of which contain ferrimagnetic inclusions possibly less than 0 1 m in grain size dissolution of ultrafine grains of magnetite during pre extraction carbonate dissolution as suggested by sun jackson 1994 does not occur in our samples

geochemical and sedimentological control of the magnetic properties of hemipelagic sediments owing to their high sedimentation rates and wide areal extent hemipelagic sediments along continental borderlands are potentially important high resolution recorders of geomagnetic secular variation to assess this possibility we studied suboxic hemipelagic muds from the oregon continental slope and anoxic laminated diatomaceous oozes from the gulf of california these sediments were rapidly deposited with average sedimentation rates of 121 and 135 cm kyr respectively bulk sediment sedimentological and geochemical analyses indicate that the two areas represent contrasting depositional regimes the remanence in both environments resides in fine grained magnetite particles although minor amounts of hematite were observed in the topmost gulf of california sediments despite first order differences the sediments from both areas show similar downcore patterns of systematic increases in solid sulfur mainly as pyrite dramatic decreases in natural nrm anhysteretic and isothermal remanent magnetization intensities and accompanying shifts in the magnetic stabilities these changes are consistent with reduction and dissolution of the ferrimagnetic iron oxides with depth due to early diagenesis of organic matter the magnetic grain size distribution first appears to rapidly coarsen downcore as the smallest and most abundant grains are removed then slowly grows finer as the remaining particles dissolve no evidence was observed for authigenic formation of magnetite provided sufficient concentration and stability of the detrital magnetic particles paleomagnetic directions can survive this dissolution diagenesis however relative paleointensity determinations based on normalization of nrm by concentration in related parameters will give erroneous estimates of paleofield behavior in such diagenetically altered sediments

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

high resolution analysis of early diagenetic effects on magnetic minerals in post middle holocene continental shelf sediments from the korea strait 1 two studied sediment cores from the korea strait contain mud sequences 14 m and 32 62 m in thickness that were deposited during the last 6 000 years the sediments have uniform lithology and geochemical properties however marked down core changes in magnetic properties suggest that diagenesis has significantly impacted the magnetic properties an expanded view of early diagenetic reactions that affect magnetic mineral assemblages is evident in these rapidly deposited continental shelf sediments compared to deep sea sediments the studied sediments are divided into four descending intervals based on magnetic property variations interval 1 is least affected by diagenesis and has the highest concentrations of detrital magnetite and hematite and the lowest solid phase sulfur contents interval 2 is characterized by the presence of paramagnetic pyrite and sharply decreasing magnetite and hematite concentrations which suggest active reductive dissolution of detrital magnetic minerals interval 3 is marked by a progressive loss of hematite with depth and coincides with the minimum magnetite concentration interval 4 has an increasing down core enhancement of authigenic greigite which apparently formed at depths of 3 30 m below the sediment water interface due to arrested pyritization reactions these results indicate delays of thousands of years for acquisition of magnetizations carried by greigite which suggests that studies of geomagnetic field behavior from greigite bearing continental shelf sediments should be conducted with care also virtually complete destruction of detrital magnetic minerals at depth suggests that magnetic studies of rapidly deposited shelf sediments are unlikely to provide a meaningful signature associated with syn depositional environmental processes

water segregation in the course of gas hydrate formation and accumulation in submarine gas seepage fields abstract gas hydrate accumulations associated with submarine gas seepage fields have been detected in the okhotsk sea and the gulf of mexico it is possible that gas hydrates known from the continental margin offshore northern california oregon and nigeria are also connected with gas seepage fields gas hydrated sediments in the accumulations 1 occur at very low subbottom depths sometimes directly at the sea bottom 2 have specific structures resembling cryogenic structures caused by hydrates subhorizontal lenticular bedded and porphyraceous structures 3 the sediments can be distinguished from overlying non hydrated sediments by a higher water content 4 an association of gas hydrates and calcium carbonate concretions occurs the behaviour of gas and water during gas hydrate formation under bottom gas seepage conditions is considered gas hydrate formation occurs at the upper part of the sedimentary cover within the limits of the diffusion halo around the up going flow of gas which saturates the pore water water migrating to the gas hydrate accumulation sites from adjacent sediments is involved in hydrate formation as well the mechanism of gas transportation to the gas hydrate accumulation sites is diffusion the way of water transport during hydrate accumulation is similar to ice segregation during permafrost formation the water segregation is responsible for the formation of specific structure caused by hydrates and for higher than usual water content of hydrate bearing sediments in submarine gas seepage fields

fossil bacterial magnetite in deep sea sediments from the south atlantic ocean the magnetostratigraphy of deep sea sediment cores has proven to be a powerful and high resolution method of dating and comparing sedimentary records in contrast to this widespread application relatively few attempts have been made to analyse and identify the mineralogy grain size and origin of the carrier of the remanent magnetization1 2 it is widely assumed that dominant ferrimagnetic minerals in deep sea sediments are of lithogenic origin that is that they derive from oceanic or continental detritus volcanic ashfalls micrometeorites hydrothermal precipitation or chemical mineralization processes a very different view was taken by kirschvink and lowenstam3 who suggested that fossil remnants of magnetite mineralizing organisms in particular magnetotactic bacteria might play an important part in the magnetization of sediments deposited in certain detritus poor aquatic systems we have used rock magnetic diagnostic methods to characterize the magnetic phases in deep sea sediments from the south atlantic these phases were then extracted and studied with the electron microscope our results indicate that fossil bacterial magnetite is the main carrier of the natural remanent magnetization in sediments ranging in age from quarternary to eocene different types of bacterial magnetosomes were detected most of them occurring in chains of identical particles and nearly all of them falling into the theoretically determined4 size range of single domain magnetitite

diagenesis in anoxic sediments from the california continental borderland and its influence on iron sulfur and magnetite behavior solid phase and pore water profiles of compounds containing iron and sulfur have been determined by wet chemical magnetic and x ray diffraction techniques in three california continental borderland basins the observed profiles have been fit by simple reaction diffusion models in order to determine reaction rates and constrain budgets for iron and sulfur more than 95 of the solid phase reduced sulfur is pyrite and down core profiles are well fit by a model in which net sulfate reduction rates decrease exponentially with depth net sulfate reduction rates determined from models fit to solid phase reduced sulfur measurements and pore water sulfate profiles yield results that are consistent depth integrated sulfate reduction rates for modern sediments in san pedro santa catalina and san nicolas basins are 11 4 6 3 and 6 3 8 8 mol cm 2yr 1 respectively measurements of solid phase iron species indicate that surficial sediments are enriched in easily reducible ferric oxyhydroxides the enrichment is maintained by a combination of oxidation of fe2 diffusing upward from underlying anoxic sediments as well as input of fresh sediment enriched in ferric oxyhydroxides the three primary sources for iron converted to pyrite and the sequence in which they are utilized are ferric oxyhydroxides magnetite and other crystalline oxides and exchangeable iron in phyllosilicates the majority 50 80 of the iron converted to pyrite is from the silicates and budgetary calculations indicate the amount of iron released from san pedro basin silicates agrees within 35 with the amount of magnesium removed from pore water to solid phases fe2 is enriched in near surface pore waters because rates of dissolved iron production by oxyhydroxide reduction exceed rates of sulfate reduction and pyrite formation at depth pore waters are sulfidic because rates of sulfate reduction exceed rates of iron release from silicates sulfide produced at depth diffuses upward until it reaches sediments with available iron causing a step like increase in solid phase sulfur concentration over 90 of the magnetite present in surficial sediments is dissolved at depth due to reaction with h2s a model is developed to predict the depth at which magnetite dissolution should occur based on sulfate reduction rates and the flux of ferric oxyhydroxides the results of this model predict the onset of dissolution at depths of 5 40 cm in different basins and agree well with the observed depths of magnetite dissolution

magnetotactic bacteria and their magnetofossils in sediments abstract living magnetotactic bacteria from freshwater environments show under natural and laboratory conditions a great variety of morphological appearances their magnetosomes vary in number shape and size one species of bacteria yields up to 1000 magnetosomes per cell individual particles reach a size of up to 200 nm as a rule they form elongated chains in bacteria which are held under laboratory conditions loop shaped arrangements and nearly unordered clusters are also found during the laboratory experiments different species of bacteria periodically dominated freshwater samples always exhibited a great variety of different magnetosomes on the other hand samples from marine environment clearly showed the preponderance of one type of particle these observations may reflect specific properties of the physico chemical milieu in which the bacteria grew at the end of our experiments north and south seeking bacteria co existed in roughly equal quantities quaternary unconsolidated sediments from the ammersee bavaria and the antarctic quaternary to tertiary deep sea sediments from the atlantic and pacific and jurassic limestones from the sonnwendgebirge tyrol austria were analyzed for fossil magnetosomes we found that fossil magnetosomes were morphologically similar to those from living bacteria but seemed to be corroded in some cases the observed differences in quantity size and shape of magnetosomes from various sediments may be influenced by variations in bacterial viability and particle stability at different sites but are not determined by geographical latitude

controls on the formation and stability of gas hydrate related bottom simulating reflectors bsrs a case study from the west svalbard continental slope 1 the growth and stability of the free gas zone fgz beneath gas hydrate related bottom simulating seismic reflectors bsrs is investigated using analytical and numerical analyses to understand the factors controlling the formation and depletion of free gas for a model based on the continental slope west of svalbard a continental margin of north atlantic type we find that the fgz is inherently unstable under a wide range of conditions because upward flow of under saturated liquid depletes free gas faster than it is produced by hydrate recycling in these scenarios the 150 m thick fgz that presently exists there would deplete within 105 106 years we suggest the fgz is in a stable state however that is formed by a diffusion dominated mechanism that produces low concentrations of gas in a fgz of steady state thickness gas forms across a thick zone because the upward fluid flux is relatively low and because the gas water solubility decreases to a minimum several hundred meters below the seabed this newly understood solubility curvature effect is complementary to hydrate recycling but becomes the most important factor controlling the presence and properties of the bsr in environments where the rate of upward fluid flow and the rate of hydrate recycling are both relatively low i e rifted continental margins if the present day fgz is in steady state we estimate that the upward fluid flux in the west svalbard site must be less than 0 15 mm a 1

seafloor spreading magnetic anomalies in the enderby basin east antarctica abstract the timing of the early separation of india from the contiguous antarctica australia is still an unresolved problem although it is well established that antarctica and india formed a single indo antarctic platform prior to the fragmentation of eastern gondwanaland inadequate age information either in the form of magnetic anomaly isochrons or dating of oceanic rocks from the conjugate margins of antarctica and india perhaps led several authors to propose incomplete plate reconstruction models particularly for the early separation of india from antarctica analysis of magnetic and satellite derived gravity data in the enderby basin east antarctica reveals the presence of seafloor spreading type linear magnetic anomalies and eight new fracture zones the observed magnetic anomalies can be interpreted as the younger sequence of mesozoic anomalies m11 m0 half spreading rates range from 6 5 to 2 8 cm yr and are comparable with those measured in the bay of bengal these similarities in the mesozoic magnetic anomaly sequence and in the spreading rates provide evidence that these two basins are conjugate and contemporary a consistent plate reconstruction model can be derived from the identified conjugate patterns of mesozoic magnetic anomalies and fracture zones the occurrence of the oldest magnetic anomaly m11 close to the coasts in these two offshore basins unequivocally suggests that the break up of india from antarctica occurred before 134 ma

evidence of slumping sliding in krishna godavari offshore basin due to gas fluid movements abstract the krishna godavari kg offshore basin is one of the promising petroliferous basins of the eastern continental margin of india drilling in this basin proved the presence of gas hydrate deposits in the shallow marine sediments beyond 750 m water depths and provided lithologic and stratigraphic information we obtained multibeam swath bathymetry covering an area of about 4500 km 2 in water depths of 280 1800 m and about 1260 line km of high resolution seismic hrs records the general lithology of midslope deposits is comprised of nannofossil rich clay nannofossil bearing clay and foraminifera bearing clay the hrs records and bathymetry reveal evidence of slumping and sliding of the upper and midslope sediments which result in mass transport deposits mtd in the northwestern part of the study area these deposits exhibit 3 9 5 km widths and extend 10 13 km offshore the boundaries of the mtds are often demarcated by sharp truncation of finely layered sediments fls and the mtds are characterized by acoustically transparent zones in the hrs data average thickness of recent mtds varies with depth i e in the upper slope the thickness is about 45 m while in the lower slope it is about 60 m and in deeper offshore locations they attain a maximum thickness of about 90 m a direct indication for slumping and mass transportation of deposits is provided by the age reversal in 14 c ams dates observed in a sediment core located in the midslope region seismic profiling signatures provide indications of fluid gas movement we propose that the presence of steep topographic gradients high sedimentation rates a regional fault system diapirism fluid gas movement and neotectonic activity may have facilitated the slumping sliding of the upper slope sediments in the kg offshore basin

sediment transport of the godavari river basin and its controlling factors abstract the mean annual water flow and sediment transport of the godavari river are estimated to be 92 km 3 and 170 10 6 ton t respectively in terms of sediment transport and the rate of physical erosion 555 t km 2 yr 1 the position of the godavari would be ninth and fifth respectively among the world rivers geology of the basin is the main controlling factor of the sediment transport the sedimentary rocks located in the lower part of the basin and constituting 7 of the total basin area are responsible for 33 of the sediment load more than 67 of sediment load is silt and clay annual monthly and daily variations in sediment transport indicate that a few selected days in the hydrological year control the annual sediment budget our latest estimates are nearly twice that of our earlier values the presently available global estimates on sediment transport need to be re evaluated

geochemical control of magnetite dissolution in subarctic lake sediments and the implications for environmental magnetism geochemical and mineral magnetic studies of lake sediments from the karsa valley in north sweden demonstrate that down core variations in magnetic properties have been determined by post depositional magnetite dissolution rather than by sediment source changes a low sediment accumulation rate during times of climatic amelioration combined with a higher organic carbon content produced intense reducing conditions with almost complete magnetite loss an increase in the sedimentation rate and a reduction in the organic carbon content primarily due to glacier reformation allowed magnetite preservation thus the mineral magnetic profiles record lake sediment palaeoredox conditions brought about by climate change this new interpretation of mineral magnetic transformations in freshwater sediments may have far reaching implications for other mineral magnetic studies concerned with the alternation between glacial stadial and interglacial interstadial episodes during the quaternary period

interferometric imaging of a salt flank using walkaway vsp data vsp has long been a tool for investigating the location of salt flanks traditionally a salt proximity survey has been used to estimate the location of salt boundaries relative to a wellbore however this technique depends on accurate knowledge of the shape of the salt and the local sediment velocities in addition this method does not work well with complicated 3d salt structure beyond the traditional salt proximity survey many researchers have developed various methods and strategies for improving the locating and or imaging of salt flanks o brien et al 2002 brandsberg dahl et al 2003 zhao et al 2006 with these methods the quality of the resultant image depends on a prior knowledge of the possibly anisotropic overburden velocity field for very long offsets and also is concerned with multi pathing through salt invalidating a 2d assumption in looking for a simpler approach to this problem a single well imaging concept may be attractive using downhole sources and receivers it is possi

simultaneous formation of iron rich layers at different redox boundaries in sediments of the amazon deep sea fan high resolution examinations of the solid phase have been performed on a gravity core that shows the typical amazon fan stratigraphy of the last 20 000 years the sedimentary record is characterized by two pronounced enrichments of solid phase fe the first fe peak from 0 34 to 0 37 m sediment depth consists of an indurated fe oxyhydroxide crust located close to the pleistocene holocene boundary this peak represents a widespread nonsteady state diagenetic feature on the fan the second fe peak is a pronounced enrichment of authigenic fe sulfides located between 6 15 and 6 45 m in the sediment the formation of this maximum is attributed to sulfate reduction that is mainly driven by anaerobic methane oxidation based on so42 pore water profiles and calculations of the time of formation of the fe sulfide enrichment we conclude that both fe rich layers formed more or less simultaneously the condition that caused both fe enrichments is a period of nonsteady state diagenesis induced by a pronounced decrease in sedimentation and organic carbon accumulation rates between the pleistocene and the holocene during this depositional change the redox boundaries and reaction fronts were fixed at particular sediment levels for a prolonged time thus producing higher concentrations of the authigenic minerals than possible under steady state diagenetic conditions

authigenic or detrital origin of pyrrhotite in sediments resolving a paleomagnetic conundrum monoclinic pyrrhotite fe7s8 is widely claimed to carry magnetizations acquired during early diagenesis in anoxic sedimentary environments in contrast geochemical literature indicates that pyrrhotite formation is extremely slow below 180 c which makes it a highly unlikely carrier of early diagenetic remanences in sediments this view is confirmed by the occurrence of late diagenetic fe7s8 in ancient sediments and the general lack of fe7s8 in modern sediments horng et al c s horng m torii k s shea s j kao inconsistent magnetic polarities between greigite and pyrrhotite magnetite bearing marine sediments from the tsailiao chi section southwestern taiwan earth planet sci lett 164 1998 467 481 documented the presence of fe7s8 that carries a magnetic signal indistinguishable from that of detrital magnetite in pleistocene marine sediments from taiwan we tested the possibility that the fe7s8 could have a detrital origin by conducting a source to sink study and found fe7s8 in metamorphic rocks of the taiwan central range and in material eroded from these rocks in the transportation pathway and in the depositional sink this confirms that the fe7s8 has a detrital origin rapid transportation from source to sink e g by typhoon associated flood events probably assists preservation of the fe7s8 which might otherwise oxidize during transportation the widespread occurrence of exhumed metamorphic rocks in orogenic belts around the world makes them a likely source of fe7s8 in marginal sedimentary basins detrital fe7s8 should therefore be more routinely considered to be responsible for paleomagnetic records when it is present in sediments with partial metamorphic provenance

rates of anaerobic oxidation of methane and authigenic carbonate mineralization in methane rich deep sea sediments inferred from models and geochemical profiles abstract pore water chemical data obtained from a 10 5 m long giant gravity core collected in methane rich sediments from 647 m water depth in the northern gulf of mexico n 28 04 00 w 89 43 15 defines sub bottom gradients in unprecedented detail this core penetrated the sulfate methane interface smi at 300 cm below the seafloor cmbsf at the smi dissolved inorganic carbon dic concentrations reach a maximum 13 5 mm and pore water 13 c dic 63 2 pdb and 13 c methane 89 5 pdb values are most negative below the smi pore water sulfate is nearly depleted methane concentrations rise sharply with simultaneous occurrence of a bubble textured sediment and fine grained methane derived authigenic carbonate nodules and cements are common the sharp peaks in dic concentration and isotope values centered at the smi indicate that dic is being produced by anaerobic oxidation of methane aom within a narrow zone centered at the smi the detailed sulfate and dic concentration profiles and dic 13 c values have enabled geochemical models to be constructed that explore the rate of dic formation by aom and its effect on pore water dic 13 c values model results closely match measured dic concentration and 13 c isotope profiles and indicate that microbiological conversion of methane carbon to dic is rapid in geologic terms and that aom is occurring at the present position of the smi isotope values for authigenic carbonate found immediately below the present day smi 13 c 60 2 0 7 pdb at 440 cmbsf are consistent with derivation of the carbonate carbon from methane via aom at the former location of a smi these observations and model results suggest that aom is occurring at rates that would generate the observed profiles and begin the precipitation of methane derived carbonate occur on time scales of centuries model results also show that the time needed to produce the resulting authigenic cements is an order of magnitude greater than that for aom to produce the observed dic profiles the metabolic rates for dic production by aom inferred from modeling the geochemical profiles compare favorably with available rate data obtained from laboratory microbial incubations and radiolabeled tracer experiments

sedimentation stratigraphy and petroleum potential of krishna godavari basin east coast of india the krishna godavari basin is located in the central part of the eastern passive continental margin of india the structural grain of the basin is northeast southwest exposures of upper cretaceous sedimentary rocks demarcate the basin margin toward the north west where the northwest southeast trending pranahita godavari graben abuts the basin the basin contains thick sequences of sediments with several cycles of deposition ranging in age from late carboniferous to holocene a major delta with a thick argillaceous facies that has prograded seaward since the late cretaceous is a hydrocarbon exploration target magnetic and gravity data predicted the basin architecture which was subsequently confirmed by a multichannel seismic sur vey the basin is divided into subbasins by fault controlled ridges sediments accumulated in subbasins more than 5 km thick above the basement ridges thin sediments are found until the jurassic period sediments were deposited in the rift valley and in topo graphic lows this sequence is completely overlain by a lower cretaceous transgressive sedimentary wedge later continued delta progradation characterized basin sedimentation with an areal extent of approximately 45 000 km2 this proven petroliferous basin has potential reservoirs ranging in age from the permian to the pliocene exploratory drilling of more than 350 wells in more than 160 structures has resulted in the discovery of 42 oil and gas bearing structures good source rocks are known from sequences ranging in age from permian carboniferous to early miocene because the reservoir sand bodies have limited lateral variation understanding the stratigraphy and depositional sub environments in different sequences is essential to decipher the favorable locales for reservoir sands tilted fault blocks growth faults and related rollover anticlines provide the structural traps begin page 1624

authigenesis of magnetic minerals in gas hydrate bearing sediments in the nankai trough offshore japan gas hydrate occurrence is one of the possible mechanisms invoked for iron sulfide formation a high resolution rock magnetic study was conducted in iodp expedition 316 hole c0008c located in the megasplay fault zone of the nankai trough offshore japan in this particular zone no bottom simulating reflectors bsr indicating the base of the gas hydrate stability field have been identified two hundred and eighteen pleistocene samples were collected from 70 to 110 m csf in order to document the changes in the concentration grain size and rock magnetic parameters of magnetic minerals through the gas hydrate bearing horizons two different populations of magnetic grains are recognized in the pseudosingle domain range three types of magnetic mineral assemblages are identified iron oxides magnetite ferrimagnetic iron sulfides greigite and pyrrhotite and their mixture greigite and pyrrhotite are authigenic and constitute six layers called is1 is6 is1 is3 is4 and is6 are associated with pore water anomalies suggesting the occurrence of gas hydrates and anoxic conditions is2 and is5 are probable gas hydrates horizons although there is no independent data to confirm it the remaining intervals are mainly composed of detrital iron oxides and paramagnetic iron sulfides two scenarios based on different diagenetic stages are proposed to explain the variations in the magnetic properties and mineralogy over the studied interval the results suggest that rock magnetism appears useful to better constrain the gas hydrate distribution in hole c0008c and counterbalances the low resolution of pore water analyses and the absence of a bsr

seabed morphology and gas venting features in the continental slope region of krishna godavari basin bay of bengal implications in gas hydrate exploration abstract increased oil and gas exploration activity has led to a detailed investigation of the continental shelf and adjacent slope regions of mahanadi krishna godavari kg and cauvery basins which are promising petroliferous basins along the eastern continental margin of india in this paper we analyze the high resolution sparker subbottom profiler and multibeam data in kg offshore basin to understand the shallow structures and shallow deposits for gas hydrate exploration we identified and mapped prominent positive topographic features in the bathymetry data these mounds show fluid gas migration features such as acoustic voids acoustic chimneys and acoustic turbid layers it is interesting to note that drilling coring onboard joides in the vicinity of the mounds show the presence of thick accumulation of subsurface gas hydrate further geological and geochemical study of long sediment cores collected onboard marion dufresne in the vicinity of the mounds and sedimentary ridges shows the imprints of paleo expulsion of methane and sulfidic fluid from the seafloor to understand the origin of the mounds and their relationship with gas hydrate cold seep formation we analyzed the multi channel seismic reflection mcs data close to the mounds the mcs data show that the subsurface layers beneath the mounds are folded below the folded overburden we observe zones of no coherent reflections probably originating from miocene sequence since the study area is located in shale tectonics regime where miocene sequences are known to be overpressured we interpret the zones of no coherent reflections as miocene shale diapirs the upward movement of shale diapirs has folded the overburden layer and resulted in the formation of numerous faults fractures these faults act as permeable pathways for fluid gas movement facilitating the formation of gas hydrate and cold seeps close to shale diapiric mound

reduced magnetization produced by increased methane flux at a gas hydrate vent abstract magnetic susceptibility measurements on near surface sediment cores from the north cascadia accretionary sedimentary prism show that seismic blanking or wipe out zones in the upper few hundred metres of sediments are associated with a prominent low magnetic susceptibility signature seismic blanking and low magnetization are both attributed to high upward methane flux within a vent zone as evidenced by the presence of massive gas hydrate within the cores sedimentological analysis of these cores also reveals the presence of authigenic pyrite within the areas of magnetic susceptibility lows this phenomenon is suspected to be produced by the reducing environment associated with the high upward methane flux and increased bacterial activity within the topmost sediments resulting in diagenesis of highly magnetic detrital minerals such as magnetite into nearly non magnetic pyrite these low magnetic susceptibility zones may produce magnetic anomalies with a magnitude of 10 35 nt near the seafloor such anomalies might be detected using high resolution near bottom magnetometers to provide a means of mapping zones of methane venting

spatial distribution of gas hydrates from high resolution seismic and core data woolsey mound northern gulf of mexico abstract integration of new jumbo piston cores with high resolution seismic reflection data provides the first documentation on the subsurface distribution of gas hydrates at a deep water cold seep the study area woolsey mound is a carbonate hydrate mound of thermogenic origin in the northern gulf of mexico where salt tectonics dominate the regional structure two typologies of unconventional seismo acoustic data were used in this study 1 surface source deep receiver ssdr data and 2 auv borne chirp sub bottom profiler data correlation of the coring results with seismic interpretations supports the hypothesis that distinctive seismic brightening high frequency scattering present in the ssdr records may indicate the presence of solid hydrates this suggests that such unconventional seismic survey may be prospective for mapping gas hydrates in complex deep water settings coring results revealed the nature of the shallow gas hydrate system to be dominated by fine grained sediments gas hydrates were found exclusively in fracture porosity in the vicinity of a major active fault we present a model for woolsey mound where shallow gas hydrates are systematically distributed along segments of faults intersected by transit of thermogenic hydrocarbons this dual nature of the faults being both gas hydrate reservoirs and gas migration routes suggests a very dynamic hydrate stability field for this site

magnetotactic bacteria and single domain magnetite in hemipelagic sediments remanent magnetism in marine sediments has been used extensively over the past twenty years to calibrate the geological time scale study geomagnetic reversals and secular variation and measure the rates of seafloor spreading although these sediments may contain different magnetic minerals magnetite is the most commonly observed and magnetically stable phase and its size shape and post depositional fate affect the magnetic remanence of the sediments biogenic magnetites are single domain with a high natural magnetic remanence nrm and have been suggested as a significant source of magnetic remanence in marine sediments we have studied surface sediments from the santa barbara basin and report the occurrence of living magnetotactic bacteria and the deposition of biogenic ultra fine grained single domain magnetite using a novel extraction technique transmission electron microscopy and squid magnetometry we show that these bacteria and the magnetite they produce are the major source of stable remanent magnetism in these sediments

diagenetic alteration of magnetic signals by anaerobic oxidation of methane related to a change in sedimentation rate geochemical and rock magnetic investigations of sediments from three sites on the continental margin off argentina and uruguay were carried out to study diagenetic alteration of iron minerals driven by anaerobic oxidation of methane aom the western argentine basin represents a suitable sedimentary environment to study nonsteady state processes because it is characterized by highly dynamic depositional conditions mineralogic and bulk solid phase data document that the sediment mainly consists of terrigenous material with high contents of iron minerals as a typical feature of these deposits distinct minima in magnetic susceptibility are observed pore water data reveal that these minima in susceptibility coincide with the current depth of the sulfate methane transition smt where hs is generated by the process of aom the released hs reacts with the abundant iron oxyhydr oxides resulting in the precipitation of iron sulfides accompanied by a nearly complete loss of magnetic susceptibility modeling of geochemical data suggest that the magnetic record in this area is highly influenced by a drastic change in mean sedimentation rate sr which occurred during the pleistocene holocene transition we assume that the strong decrease in mean sr encountered during this glacial interglacial transition induced a fixation of the smt at a specific depth the stagnation has obviously enhanced diagenetic dissolution of iron oxyhydr oxides within a distinct sediment interval this assumption was further substantiated by numerical modeling in which the mean sr was decreased from 100 cm kyr 1 during glacial times to 5 cm kyr 1 in the holocene and the methane flux from below was fixed to a constant value to obtain the observed geochemical and magnetic patterns the smt must remain at a fixed position for 9000 yrs this calculated value closely correlates to the timing of the pleistocene holocene transition the results of the model show additionally that a constant high mean sr would cause a concave up profile of pore water sulfate under steady state conditions copyright 2005 elsevier ltd

evidence of paleo cold seep activity from the bay of bengal offshore india 1 we report evidence of paleo cold seep associated activities preserved in methane derived carbonates in association with chemosynthetic clams calyptogena sp from a sediment core in the krishna godavari basin bay of bengal visual observations and calculations based on high resolution wet bulk density profile of a core collected on board r v marion dufresne may 2007 show zones of sharp increase in carbonate content 10 55 vol within 16 20 meters below seafloor mbsf the presence of calyptogena clam shells chimneys shell breccias with high mg calcite cement and pyrite within this zone suggest seepage of methane and sulfide bearing fluid to the seafloor in the past highly depleted carbon isotopic values 13c ranges from 41 to 52 vpdb from these carbonates indicate carbon derived via anaerobic oxidation of methane extrapolated mean calendar age 58 7 ka b p of the clastic sediments at a depth of 16 mbsf is close to the upper limit of the u th based depositional age 46 2 3 7 and 53 0 1 6 ka of authigenic carbonates sampled from this level thereby constraining the younger age limit of the carbonate deposition methane expulsion events the observed carbonate deposition might have resulted from the flow of methane enriched fluids through the fracture network formed because of shale diapirism

searching for single domain magnetite in the pseudo single domain sedimentary haystack implications of biogenic magnetite preservation for sediment magnetism and relative paleointensity determinations 1 magnetic hysteresis measurements of sediments have resulted in widespread reporting of pseudo single domain like magnetic properties in contrast the ideal single domain sd properties that would be expected to be responsible for high quality paleomagnetic records are rare determining whether sd particles are rare or common in sediments requires application of techniques that enable discrimination among different magnetic components in a sediment we apply a range of such techniques and find that sd particles are much more common than has been reported in the literature and that magnetite magnetofossils the inorganic remains of magnetotactic bacteria are widely preserved at depth in a range of sediment types including biogenic pelagic carbonates lacustrine and marine clays and possibly even in glaci marine sediments thus instead of being rarely preserved in the geological record we find that magnetofossils are widespread this observation has important implications for our understanding of how sediments become magnetized and highlights the need to develop a more robust basis for understanding how biogenic magnetite contributes to the magnetization of sediments magnetofossils also have grain sizes that are substantially smaller than the 1 15 m size range for which there is reasonable empirical support for relative paleointensity studies the different magnetic response of coexisting fine biogenic and coarser lithogenic particles is likely to complicate relative paleointensity studies this issue needs much closer attention despite the fact that sediments have been subjected to paleomagnetic investigation for over 60 years much remains to be understood about how they become magnetized

composition and origin of authigenic carbonates in the krishna godavari and mahanadi basins eastern continental margin of india abstract the mineralogical and stable isotopic composition of authigenic carbonates from the krishna godavari kg and mahanadi basin provide a deeper insight into the processes inducing carbonate formation in the sediments of the eastern continental margin of india in the bay of bengal authigenic carbonate cements micro nodules bioturbation casts and tubes from 12 core locations drilled during the indian national gas hydrate program nghp expedition 01 were investigated for this study three main processes responsible for authigenic carbonate precipitation are identified organoclastic sulfate reduction anaerobic oxidation of methane and methanogenesis evidence of vigorous methane seepage is indicated in carbonates recovered at sites 7 10 12 kg basin and 19 mahanadi basin these methane derived carbonates display typical paragenetic carbonate mineralogies aragonite high mg calcite with 15 mol mgco 3 ca rich dolomite two separate horizons of methane derived carbonates are correlated between 4 drill holes up to 16 km apart the upper horizon has been dated with 14 c 40 100 51 600 a bp 1950 and clearly indicates that methane seepage has been much more vigorous in the past possibly due to the effect of lower sea level on the gas hydrate stability zone across the margin

seismic imaging of a fractured gas hydrate system in the krishna godavari basin offshore india gas hydrate was discovered in the krishna godavari kg basin during the india national gas hydrate program nghp expedition 1 at site nghp 01 10 within a fractured clay dominated sedimentary system logging while drilling lwd coring and wire line logging confirmed gas hydrate dominantly in fractures at four borehole sites spanning a 500 m transect three dimensional 3d seismic data were subsequently used to image the fractured system and explain the occurrence of gas hydrate associated with the fractures a system of two fault sets was identified part of a typical passive margin tectonic setting the lwd derived fracture network at hole nghp 01 10a is to some extent seen in the seismic data and was mapped using seismic coherency attributes the fractured system around site nghp 01 10 extends over a triangular shaped area of 2 5 km2 defined using seismic attributes of the seafloor reflection as well as seismic sweetness at the base of the gas hydrate occurrence zone the triangular shaped area is also showing a polygonal nearly hexagonal fault pattern distinct from other more rectangular fault patterns observed in the study area the occurrence of gas hydrate at site nghp 01 10 is the result of a specific combination of tectonic fault orientations and the abundance of free gas migration from a deeper gas source the triangular shaped area of enriched gas hydrate occurrence is bound by two faults acting as migration conduits additionally the fault associated sediment deformation provides a possible migration pathway for the free gas from the deeper gas source into the gas hydrate stability zone it is proposed that there are additional locations in the kg basin with possible gas hydrate accumulation of similar tectonic conditions and one such location was identified from the 3d seismic data 6 km nw of site nghp 01 10

fault system and thermal regime in the vicinity of site nghp 01 10 krishna godavari basin bay of bengal abstract drilling coring activities onboard joides resolution for hydrate resource estimation have confirmed gas hydrate in the continental slope of krishna godavari kg basin bay of bengal and the expedition recovered fracture filled gas hydrate at the site nghp 01 10 in this paper we analyze high resolution multi channel seismic mcs high resolution sparker hrs bathymetry and sub bottom profiler data in the vicinity of site nghp 01 10 to understand the fault system and thermal regime we interpreted the large scale fault system 5 km predominantly oriented in nnw sse direction near nghp 01 10 site which plays an important role in gas hydrate formation and its distribution the increase in interval velocity from the baseline velocity of 1600 m s to 1750 1800 m s within the gas hydrate stability zone ghsz is considered as a proxy for the gas hydrate occurrence whereas the drop in interval velocity to 1400 m s suggest the presence of free gas below the ghsz the analysis of interval velocity suggests that the high concentration of gas hydrate occurs close to the large scale fault system we conclude that the gas hydrate concentration near site nghp 01 10 and likely in the entire kg basin is controlled primarily by the faults and therefore has high spatial variability we also estimated the heat flow and geothermal gradient gtg in the vicinity of nghp 01 10 site using depth and temperature of the seafloor and the bsr we observed an abnormal gtg increase from 38 c km to 45 c km at the top of the mound which remarkably agrees with the measured temperature gradient at the mound nghp 01 10 and away from the mound nghp 01 03 we analyze various geological scenarios such as topography salinity thermal non equilibrium of bsr and fluid gas advection along the fault system to explain the observed increase in gtg the geophysical data along with the coring results suggest that the fluid advection along the fault system is the primary mechanism that explains the increase in gtg the approximate advective fluid flux estimated based on the thermal measurement is of the order of few tenths of mm yr 0 37 0 6 mm yr

seismic characterization of hydrates in faulted fine grained sediments of krishna godavari basin unified imaging 1 a combination of diffusion and advection in fine grained sediments can create a patchy bottom simulating reflector bsr which has little to no apparent correlation with the overlying hydrate distribution using 2d seismic data from faulted clay rich sediments in the krishna godavari kg basin we show both the hydrate distribution and the bsr structure are fault controlled our demonstration hinges upon a kinematically accurate p wave velocity vp model which is estimated using a composite traveltime inversion depth migration method in an iterative manner the flexibility of the method allows simultaneous usage of traveltimes from multiple discontinuous reflectors the application begins with a simple vp model from time processing which is reflective of a diffusive continuous hydrate and free gas bearing system the application converges in three iterations yielding a final vpmodel which is suggestive of a patchy distribution of hydrates and free gas possibly developing through a combined diffusive advective system the depth image corresponding to the final vp model can be interpreted for faults that suggest ongoing tectonism the bsr appears to be truncated at active faults zones both the final vp model and the corresponding depth image can be reconciled with the hydrate distribution and bsr depth at logging coring sites located 250 m away from the line by projecting the sites along the strike direction of the regional faults

gas hydrates in the ginsburg and yuma mud volcano sediments moroccan margin results of chemical and isotopic studies of pore water abstract two active mud volcanoes on the atlantic continental margin of morocco were sampled during a 1999 expedition on the r v professor logachev the active mud volcanoes named yuma and ginsburg are each about 4 km in diameter rise 150 250 m above the seafloor and are the first active methane related mud volcanoes identified in this region gravity cores from the crest of the active mud volcanoes recovered methane hydrates clathrates thirty five pore water samples were extracted and analyzed from seven gravity cores taken in and around the active mud volcanoes estimated gas hydrate content in mud volcano sediments is 2 11 by volume and 4 19 by pore space chloride ion concentrations of the original mud volcano water are similar to those of surrounding seawater but are isotopically different the water is very high in oxygen to 35 and light in hydrogen to 70 pore water with these features has not been previously described in the published literature

intense pyrite formation under low sulfate conditions in the achterwasser lagoon sw baltic sea in comparison to similar low sulfate coastal environments with anoxic sulfidic sediments the achterwasser lagoon which is part of the oder estuary in the sw baltic sea reveals unexpectedly high pyrite concentrations of up to 7 5 wt pyrite occurs mainly as framboidal grains variable in size with diameters between 1 and 20 m pyritization is not uniform down to the investigated sediment depth of 50 cm the consumption of reactive fe is most efficient in the upper 20 cm of the sediment column leading to degrees of pyritization dop as high as 80 to 95 sediment accumulation in the achterwasser takes place in high productivity waters the content of organic carbon reaches values of up to 10 wt indicating that pyrite formation is not limited by the availability of organic matter although dissolved sulfate concentration is relatively low 2 mmol l in the achterwasser the presence of h2s in the pore water suggests that sulfate is unlikely to limit pyrite authigenesis the lack of free fe ii in the pore waters combined with the possibility of a very efficient transformation of fe monosulfides to pyrite near the sediment water interface suggests that pyrite formation is rather controlled by i the availability of reactive fe which limits the fes formation and by ii the availability of an oxidant which limits the transformation of fes into pyrite the ultimate source for reactive fe is the river oder which provides a high portion of reactive fe 65 of the total fe in the form of suspended particulate matter the surficial sediments of the achterwasser are reduced but are subject to oxidation from the overlying water by resuspension oxidation of the sediments produces sulfur species with oxidation states intermediate between sulfide and sulfate e g thiosulfate and polysulfides which transform fes to fes2 at a significant rate this process of fes recycling is suggested to be responsible for the formation of pyrite in high concentrations near the sediment surface with dop values between 80 and 95 even under low sulfate conditions a postdepositional sulfidization takes place in the deeper part of the sediment column at 22 cm depth where the downward diffusion of h2s is balanced by the upward migration of fe ii the vertical fluctuation of the diffusion front intensifies the pyritization of sediments we suggest that the processes described may occur preferentially in shallow water lagoons with average net sedimentation rates close to zero such environments are prone to surficial sediment resuspension initiating oxidation of fe sulfides near the sediment water interface subsequent fes2 formation as well as postdepositional sulfidization leads to a major pyrite spike at depth within the sediment profile

estimation of seismic attenuation of gas hydrate bearing sediments from multi channel seismic data a case study from krishna godavari offshore basin seismic attenuation of gas hydrate bearing sediments is estimated from the multi channel seismic data acquired in krishna godavari kg offshore basin in the vicinity of site nghp 01 10 where 128 m of fracture filled hydrate is confirmed by drilling coring to understand the effect of gas hydrate on the seismic attenuation the seismic quality factor q which is the inverse of seismic attenuation is estimated from two methods first the spectral ratio method which utilizes the logarithmic ratio of the seafloor and bottom simulating reflector bsr spectra and second the peak frequency method which depends upon the variations of the peak frequency of the bsr spectrum as a function of traveltime in the study area the complex fault system has perturbed the seafloor resulting in noisy seafloor spectrum as a result the logarithm of the spectral ratio was also noisy and the q values obtained from spectral ratio method were found to be unstable on the other hand the peak frequency method is found to be relatively stable as it depends only on the peak frequency of the bsr which can be reliably obtained by conventional processing this method yields an effective q value of bsr which represents a weighted average of the interval q values of the water column and the gas hydrate bearing sediments since the q value of water column can be treated as a constant the variations in effective q value can be interpreted in terms of the variations of q in the gas hydrate bearing sediments for a constant thickness of water column the estimated effective q values along the inline and crossline seismic profiles depend on several factors such as gas hydrate free gas and the complex fault system the combined interpretation of the quality factor and the interval velocity model helps to understand these effects in different zones of the seismic data the attenuation of the background marine sediments is estimated from the analysis of crossline seismic profile the gas hydrate bearing sediments as inferred from the velocity model show low attenuation as compared to that of the background sediments in contrast active migration of free gas across the fault system leads to increase in seismic attenuation within the gas hydrate stability zone the complex fault zone may also lead to an increase in seismic attenuation the blanked seismic zone shows a marginal decrease in seismic attenuation probably related to loadbearing form of gas hydrate our attenuation results are compared against the results obtained from waveform inversion albeit with a higher resolution the relative variation of q estimated from the proposed method is close to the average q values estimated from full waveform inversion for most of the cdps the proposed methodology can be a useful technique to estimate seismic attenuation from multi channel seismic data

environmental magnetism past present and future environmental magnetism involves the application of rock and mineral magnetic techniques to situations in which the transport deposition or transformation of magnetic grains is influenced by environmental processes in the atmosphere hydrosphere and lithosphere the first explicit description of environmental magnetism as a distinct field was in 1980 since that time environmental magnetism has become a broad field that is finding application in an ever increasing array of scientific disciplines in this review of the present state of environmental magnetic studies we divide the field into three broad but arbitrary categories the first involves the use of mineral magnetic assemblages in the geological record to study physical processes in depositional environments this category includes the correlation of sediment cores using magnetic susceptibility measurements studies of geomagnetic field behavior the analysis of depositional and postdepositional mechanical processes that affect sediments and the examination of magnetic parameters that might represent proxies for paleoclimatic variation the second category encompasses studies of the processes responsible for variations in the magnetic minerals brought into a sedimentary environment these provenance investigations include studies of changes in catchment derived sediment in lakes fluctuations in contributions from terrigenous aeolian and glaciogenic components in deep sea sediments and the origin of atmospheric particulates the final category addresses in situ changes and transformations of magnetic minerals in sedimentary environments including pedogenesis authigenetic diagenetic formation of ferrimagnetic phases dissolution of magnetic minerals due to reductive diagenesis and contributions of biomagnetism to sedimentary magnetism because environmental magnetism can address problems in so many disciplines and because many of these problems may be inaccessible to other techniques it is likely that the scope of environmental magnetism will continue to expand rapidly environmental magnetism is capable of providing important data for studies of global environmental change climatic processes and the impact of humans on the environment all of which are major research initiatives in the international scientific community these factors suggest that environmental magnetism has a bright and diverse future

diagenetic formation of greigite and pyrrhotite in gas hydrate marine sedimentary systems mineral magnetic results and electron microscope observations from gas hydrate bearing marine sediments cored at southern hydrate ridge during ocean drilling program leg 204 sites 1244 to 1252 cascadia margin offshore oregon demonstrate that authigenic greigite and pyrrhotite formed as a byproduct of microbially mediated diagenetic reactions in the sulphate the anaerobic oxidation of methane aom and the methanic gas hydrate zones geochemical conditions favourable for formation and preservation of greigite and pyrrhotite appear to be a limited source of sulphide whether it derives from microbially driven sulphate reduction in the sulphate zone in the aom zone or in deep sediments undergoing aom so that pyritization reactions are not driven to completion our results indicate that rock magnetic identification of greigite and pyrrhotite should be useful for detecting ancient gas hydrate systems in the marine sedimentary record because it can enable rapid screening of ancient sediments for potential horizons where methane and disseminated gas hydrates might have occurred formation of authigenic greigite and pyrrhotite at different depths within the gas hydrate stability zone also implies that the magnetization of the host sediments will have been acquired at variable times which is likely to compromise paleomagnetic results from greigite and pyrrhotite bearing marine sediments

seismic characterization of hydrates in faulted fine grained sediments of krishna godavari basin full waveform inversion in fine grained faulted sediments both stratigraphic and fault induced structural variations can simultaneously determine the gas hydrate distribution insights into hydrate distribution can be obtained from p wave velocity vp and attenuation qp 1 character of the gas hydrate stability zone ghsz in this paper we apply frequency domain full waveform inversion fwi to surface towed 2d multichannel seismic data from the krishna godavari kg basin india to image the fine scale 100 x 30 m vp and qp 1 variations within the ghsz we validate the inverted vp model by reconciling it with a sonic log from a nearby 250 m well the vp model shows a patchy distribution of hydrate away from the faults dominated parts of the profile hydrates demonstrate stratigraphic control which appears to be permeability driven the qp 1 model suggests that attenuation is relatively suppressed in hydratesbearing sediments elevated attenuation in non hydrate bearing sediments could be driven by the apparent pore fluid immiscibility at seismic wavelengths the vp and the qp 1 models also suggest that fault zones within the ghsz can be hydrate or free gas rich depending on the relative supply of free gas and water from below the ghsz

controls on evolution of gas hydrate system in the krishna godavari basin offshore india in this study we integrate environmental magnetic sedimentological and geochemical records of sediment core of hole nghp 01 10d overlying methane hydrate deposits to decipher the controls on the evolution of fracture filled gas hydrate system in the krishna godavari k g basin four distinct sedimentary units have been identified based on the sediment magnetic signatures an anomalous zone of enhanced magnetic susceptibility unit iii 51 9 160 4 mbsf coinciding with the gas hydrate bearing intervals is due to the presence of magnetite rich detrital minerals brought in by the river systems as a result of higher sedimentation events in k g basin and has no influence over hydrate formation a strong to moderate correlation between magnetite concentration and chromium reducible sulfur crs content indicates significant influence of sulfidization on the magnetic record and could be further exploited as a proxy to decipher paleo h2s seepage events analysis of high resolution seismic bathymetry and sub bottom profiler data reveals the existence of a regional fault system in k g basin the opening and closing dynamics of the faults facilitated the migration and trapping of required gas concentrations resulting in accumulation of gas hydrates at the studied site the seismic data provides support to the rock magnetic interpretations the observed variations in magnetic and geochemical properties have resulted from the episodic flow of methane and sulfide enriched fluids through the fracture filled network formed as a result of shale tectonism our study demonstrated the potential of using an enviro magnetic approach in combination with other proxies to constrain the evolution of gas hydrate system in marine environments

formation of methane related authigenic carbonates in a highly dynamic biogeochemical system in the krishna godavari basin bay of bengal abstract we report the abundant occurrence of authigenic fe rich carbonate high mg calcite hmc and low mg calcite from 11 cores recovered from the krishna godavari basin k g basin bay of bengal the cores were collected as part of the indian gas hydrate exploration program on board r v marion dufresne md 161 may 2007 in different environments including mounds mud diapirs mass flows and hemipelagic sediments over a range of water depths from 647 to 2079 m authigenic carbonates range in size from 1 mm to 12 cm and display various morphologies like roundish or platy micro nodules and tube like forms from the cores 173 carbonate samples have been investigated for their depth distribution mineralogy geochemical and stable isotopic composition the stable carbon isotopic composition of 46 out of 88 measured carbonate samples are around 50 which allows the differentiation into methane related carbonates hmc especially at sites 8 and 15 but also in low abundance at sites 1 5 9 and 12 results indicate that the carbonates at site 8 and 15 represent paleo methane seepage locations the fe rich carbonates occur abundantly at many sites in the k g basin their varying carbon isotopic composition indicates that probably not only sulfate reduction through organic matter degradation but also methanogenesis are the responsible processes for their formation

quantifying magnetite magnetofossil contributions to sedimentary magnetizations under suitable conditions magnetofossils the inorganic remains of magnetotactic bacteria can contribute to the natural remanent magnetization nrm of sediments in recent years magnetofossils have been shown to be preserved commonly in marine sediments which makes it essential to quantify their importance in palaeomagnetic recording in this study we examine a deep sea sediment core from offshore of northwestern western australia the magnetic mineral assemblage is dominated by continental detritus and magnetite magnetofossils by separating magnetofossil and detrital components based on their different demagnetization characteristics it is possible to quantify their respective contributions to the sedimentary nrm throughout the brunhes chron in the studied core the contribution of magnetofossils to the nrm is controlled by large scale climate changes with their relative importance increasing during glacial periods when detrital inputs were low our results demonstrate that magnetite magnetofossils can dominate sedimentary nrms in settings where they are preserved in significant abundances

dissolution and pyritization of magnetite in anoxie marine sediments concentrations of magnetite were determined with depth in sediments of varying h2s content taken from long island sound foam nwc and sachem sites and from a site in the rapidly depositing subaqueous portion of the mississippi delta for the three long island sound sites dissolution of magnetite during burial is clearly demonstrated and for all four sites the rate of magnetite dissolution is proportional to the concentration of dissolved pore water sulfide by combining magnetite depth distributions with sedimentation rate data and measurements of dissolved sulfide we find that magnetite dissolution is well described by the following rate law dcmagdt 1 1 10 5c0 5bcmagamag where cmag is the concentration of magnetite grams per gram of sediment cs is the concentration of dissolved sulfide mm and amag is the specific surface area of magnetite in the sediment cm2 per gram of magnetite this equation means that for a typical range of magnetite surface areas and dissolved sulfide concentrations the half life of magnetite in anoxic marine sediments ranges from about 50 to 1000 years sem observations of magnetite grains from the different sediment sites reveal that magnetite dissolution may be accompanied by extensive replacement by pyrite this only occurs if magnetite is in contact with high concentrations of h2s 1 mm for relatively long periods of time several hundred years with low concentrations of h2s 1 mm magnetite dissolution occurs without identifiable pyrite replacement we suggest that this feature of magnetite diagenesis may prove to be a useful paleosulfide indicator

gas hydrate destabilization and methane release events in the krishna godavari basin bay of bengal abstract methane release events have been linked to global warming alteration of the carbon cycle and influence on biota however unequivocal evidence of paleomethane release events are limited we report several negative carbon stable isotope excursions in planktic and benthic foraminifera in a core md161 8 from the krishna godavari k g basin bay of bengal the most negative 13 c spikes are recorded during the marine isotope stages mis 4 and at the transition of mis 5 to 4 occurrence of highly 13 c depleted average 13 c 48 2 4 vpdb authigenic high magnesian calcite are also reported within this time window from the core md161 8 in the present work an unequivocal explanation for the observed 13 c depletion in the marine planktic and benthic foraminifera is difficult to achieve solely from the optical electron microscopy or c o stable isotope ratio analyses due to possible influence of diagenetic alteration we attribute the observed episodic methane expulsion events as inferred from the negative 13 c excursions and earlier reports on the occurrence chemosynthetic bivalves and mo concentration anomaly to the destabilization of the base of gas hydrate stability zone bghsz sea level drop and shale tectonics induced focused fluid flow are the two possible causes of hydrate destabilization discussed here shale tectonics were possibly responsible for creating fault systems which acted as the conduit for gas flow through the sediment column and subsequent seepage shale and salt tectonics in the passive continental margins being a globally observed phenomenon its role as an important driving force for enhanced methane emission needs detailed investigation to understand the climatic perturbations through geologic time additional evidence of methane emission from site md161 15 further supports the link between shale tectonics and methane emission

new seismic reflection results in the central eromanga basin queensland australia the key to understanding its tectonic evolution abstract regional seismic traverses were recorded from 1980 to 1982 by the bureau of mineral resources geology and geophysics as part of a major multidisciplinary program aimed at studying the geological evolution of the central eromanga basin area and its petroleum potential six fold cdp reflection data were obtained to 20 s reflection time on 1400 km of traverses up to 400 km long crossing the main structural features of the area additional seismic reflection information to complement the study was obtained from 2300 km of older mainly single coverage analogue data which was transcribed to digital format and reprocessed the central eromanga basin area has a complex history and contains four phanerozoic potentially hydrocarbon bearing basins each separated by unconformities i e the devonian adavale basin the late carboniferous to triassic cooper and galilee basins and the jurassic cretaceous eromanga basin the seismic results are being interpreted with the aid of synthetic seismograms from key wells to provide information on the extent nature and relationship of these basins the quality of the reflection data from the devonian sequence is generally very good and these data are proving to be most useful in the study thick devonian sediments were deposited over a wide area well beyond the present confines of the adavale basin and its associated troughs intensive folding and faulting accompanied by major basement uplift including the canaway ridge in the centre of the area took place during the mid carboniferous kanimblan orogeny this was followed by a period of erosion which truncated the devonian sequence and resulted in separation of the adavale basin from the other devonian troughs in the area the preliminary seismic results give some indication also of the extent and nature of the cooper and galilee basins sediments these are generally thin in the central area and thicken to the west and east respectively where coal measures produce good lithologic and seismic markers gentle folding and minor faulting apparent in the seismic reflection sections from the overlying eromanga basin sequence have resulted mainly from basement uplift and minor movements along pre existing faults continuing into the late mid tertiary and possibly from some compaction of the thick sedimentary section the seismic information is also assisting in studies of the petroleum potential of the area providing information particularly on structuring timing and depth of burial of the prospective sediments

seismic and heat flow constraints from the gas hydrate system in the krishna godavari basin india abstract high resolution multi channel seismic reflection data acquired in the offshore domain of the krishna godavari kg basin provide seismic indicators of gas hydrate including bottom simulating reflectors bsr and high reflectivity zones below the bsr as an indication of free gas the recent recovery of gas hydrate by drilling across various sites in the kg basin together with the seismic indicators suggests overall widespread favourable gas hydrate stability conditions the bsrs provide indirect estimates of geothermal gradients and heat flow a simple conductive model is used to calculate heat flow from the depth of the bsr bsr derived heat flow values range from about 30 to 102 mw m2 across the kg basin we also modeled the base of gas hydrate stability zone bghsz to analyse the linkage between gas hydrate occurrences in the basin and its relation to the fault systems in the basin the influence of bottom water temperature pore water salinity and geothermal gradient variation on gas hydrate stability zone ghsz thickness is analysed in relation to both geological formations and tectonics our analysis suggests a highly variable ghsz in the kg basin controlled by local variations in the parameters of stability condition and faulting the bsr derived heat flow increases towards deeper water depths across the basin consistent local variations were also observed with low heat flow values over prominent topographic highs and high heat flow values over the flanks of the topographic highs this variation may be due to focusing and defocusing effects of the topography however local bsr depths patterns may also be due to dynamic effects including the displacement of isotherms by thrust faulting additionally recent mass wasting events and normal faulting resulted in the bghsz being too shallow relative to the predicted model which assumed equilibrium conditions

bulk sediment mineralogy of gas hydrate reservoir at the east nankai offshore production test site abstract bulk sediment mineralogy was measured from a gas hydrate bearing middle pleistocene deepwater turbidite interval at the offshore production test site of the eastern nankai trough area we used core samples recovered from a 60 m long section of the borehole for semi quantitative analysis of mineral and organic contents of the gas hydrate reservoir powder x ray diffraction analysis ignition loss test and field emission scanning electronic microscopy revealed the followings i ten bulk minerals vary in concentration and most of them have a good exponential correlation with median grain size ii the upper muddy section is dominated by coccolith rich hemipelagites whereas the middle and lower sections are characterized by relatively coccolith poor probably humic substance rich turbiditic sediments and iii the common occurrence of authigenic gypsum siderite and framboidal pyrite indicates early diagenesis in anoxic and relatively high salinity conditions probably associated with gas hydrate formation in some degree such mineralogical data can provide useful information on evaluation of thermal properties geomechanical characteristics effective permeability and early diagenetic mechanism of the sediments to characterize and exploit gas hydrate reservoirs

ultrafine grained magnetite in deep sea sediments possible bacterial magnetofossils a new extraction technique now permits ultrafine magnetite crystals to be separated from a variety of deep sea sediments morphologic characterization of these particles with transmission electron microscopy reveals the presence of several distinct crystal types some of which closely resemble those formed by the magnetotactic bacteria the apparently biogenic magnetite particles are of single domain size and dominate the population in calcareous deep sea sediments bacterially precipitated magnetite may therefore be a major source of the stable magnetic remanence in some marine sediments these objects possibly constitute the smallest mineral fossils yet recovered from the sedimentary record

dissolution of titanomagnetite and sulphidization in sediments from lake kinneret israel summary magnetic extracts obtained from sediment cores recovered from lake kinneret israel were subjected to comprehensive scanning electron microscope and in line energy dispersive x ray spectroscopy analyses a variety of ferrimagnetic iron oxides and iron sulphide particles were identified along with apparently non magnetic iron sulphides the majority of the iron oxides are titanomagnetites that have undergone pervasive dissolution and in many cases were only preserved as skeletal 3 d lattices of ilmenite lamellae often the spaces between the submicron ilmenite lamellae are empty also sponge like dissolution features are observed in some residual titanomagnetite grains another group of titanomagnetite grains is characterized by superficial shrinkage cracks which indicate low temperature maghemitization in some cases these cracks appear to be where dissolution started most titanomagnetites contain impurities of al mg si and mn on the order of 1 per cent chromium magnetites with minor ti mg and al content on the order of a few percent but without mn were also found chromium magnetites mostly occur as octahedra in a well preserved state which indicates that they are less prone to dissolution magnetite spherules of 40 60 m in size which were mostly hollow were only found in material collected from the outer zone of the core section that was in contact with the steel core barrel these spherules were not found in material collected from the centre of the core they are therefore interpreted as contaminants that were produced during fabrication of the steel core barrel iron sulphides are present as pyrite either as larger aggregates of cubic crystals each of up to 20 m in size or as 5 10 m framboidal aggregates consisting mostly about 0 3 m idiomorphic octahedra the cubic pyrite crystals have heterogeneous compositions pure pyrite should be non magnetic but even aggregates of up to 150 m in size were magnetically separated they probably therefore contain a magnetic core of greigite or overgrown titanomagnetite the latter possibility is supported by observations of residual titanomagnetites with individual pyrite crystals residing directly on their surfaces or filling space between ilmenite lamellae irregular aggregates of flaky submicron iron sulphides were also found which are interpreted to be greigite the greigite has a low degree of visible crystallization pyrite and or greigite grains were found inside the aggregates or in agglomerates with organic remnants together visual observations and elemental analyses provide clear evidence of pervasive dissolution of primary titanomagnetite and concurrent sulphidization of sediments from lake kinneret

geologic implications of gas hydrates in the offshore of india krishna godavari basin mahanadi basin andaman sea kerala konkan basin gas hydrate resource assessments that indicate enormous global volumes of gas present within hydrate accumulations have been one of the primary driving forces behind the growing interest in gas hydrates gas hydrate volumetric estimates in recent years have focused on documenting the geologic parameters in the gas hydrate petroleum system that control the occurrence of gas hydrates in nature the primary goals of this report are to review our present understanding of the geologic controls on the occurrence of gas hydrate in the offshore of india and to document the application of the petroleum system approach to the study of gas hydrates national gas hydrate program of india executed the national gas hydrate program expedition 01 nghp 01 in 2006 in four areas located on the eastern and western margins of the indian peninsula and in the andaman sea these areas have experienced very different tectonic and depositional histories the peninsular margins are passive continental margins resulting from a series of rifting episodes during the breakup and dispersion of gondwanaland to form the present indian ocean the andaman sea is bounded on its western side by a convergent margin where the indian plate lithosphere is being subducted beneath southeast asia nghp 01 drilled logged and or cored 15 sites 31 holes in the krishna godavari basin 4 sites 5 holes in the mahanadi basin 1 site 2 holes in the andaman sea and 1 site 1 hole in the kerala konkan basin holes were drilled using standard drilling methods for the purpose of logging while drilling and dedicated wireline logging as well as through the use of a variety of standard coring systems and specialized pressure coring systems nghp 01 yielded evidence of gas hydrate from downhole log and core data obtained from all the sites in the krishna godavari basin the mahanadi basin and in the andaman sea the site drilled in the kerala konkan basin during nghp 01 did not yield any evidence of gas hydrate most of the downhole log inferred gas hydrate and core recovered gas hydrate were characterized as either fracture filling in clay dominated sediments or as pore filling or grain displacement particles disseminated in both fine and coarse grained sediments geochemical analyses of gases obtained from sediment cores recovered during nghp 01 indicated that the gas in most all of the hydrates in the offshore of india is derived from microbial sources only one site in the andaman sea exhibited limited evidence of a thermogenic gas source the gas hydrate petroleum system concept has been used to effectively characterize the geologic controls on the occurrence of gas hydrates in the offshore of india

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

progressive dissolution of titanomagnetites at odp site 653 tyrrhenian sea abstract magnetic properties microscopy and ferrous ferric iron ratio document increasing down core alteration of titanomagnetite to iron sulfide at odp site 653 leg 107 tyrrhenian sea changes in magnetic properties are interpreted to indicate a grain size increase in titanomagnetite to 40 m below seafloor bsf and a grain size decrease from this level to about 150 m bsf as dissolution affects first the fine grained and then the coarser grained magnetite the availability of dissolved sulfate from underlying messinian evaporites may account for the sulfide supply at depth and hence for the prolonged dissolution process dissolution at leg 107 sites adjacent to site 653 is less pronounced possibly by the availability of organic matter that can be metabolised by sulfate reducing bacteria