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

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

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

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

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

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

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

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

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

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

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

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

cometary origin of atmospheric methane variations on mars unlikely the detection of methane in the atmosphere of mars was first reported in 2004 since then a number of independent observations of methane have been reported all showing temporal variability up until recently the origin of methane was attributed to sources either indigenous to mars or exogenous where methane is a uv degradation byproduct of organics falling on to the surface most recently a new hypothesis has been proposed that argues that the appearance and variation of methane are correlated with specific meteor events at mars indeed extraplanetary material can be brought to a planet when it passes through a meteoroid stream left behind by cometary bodies orbiting the sun this occurs repeatedly at specific times in a planet s year as streams tend to be fairly stable in space in this paper we revisit this latest hypothesis by carrying out a complete analysis of all available data on mars atmospheric methane including the very recent data not previously published together with all published predicted meteor events for mars whether we consider the collection of individual data points and predicted meteor events whether we apply statistical analysis or whether we consider different time spans between high methane measurements and the occurrence of meteor events we find no compelling evidence for any correlation between atmospheric methane and predicted meteor events

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

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

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

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

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

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

p t t development of archaean granulites in varpaisjarvi central finland ii dating of high grade metamorphism with the u pb and sm nd methods exposed blocks of lower crustal archaean granulites separated by palaeoproterozoic faults occur near the western boundary of the karelian craton in central finland the southwesternmost granulite block in the study area the jonsa block differs from other granulites by having younger zircon u pb and sm nd tdm model ages the u pb ages on zircons and monazites from leucosomes and mesosomes of mafic and pelitic granulites in the jonsa block are ca 2 63 ga which is interpreted as the age of granulite metamorphism igneous enderbites which make up a considerable part of the bedrock in granulite blocks are ca 50 ma older outside the jonsa block both conventional and ion probe zircon ages of mesosomes of migmatitic mafic granulites are 3 2 3 1 ga which are the protolith ages sm nd tdm model ages of the mafic granulites are 2 7 2 9 ga in the jonsa block but ca 3 2 ga in the other granulites the age differences are interpreted to represent terrane accretion that juxtaposed 3 2 ga rocks next to younger rocks all granulites were metamorphosed in similar pt conditions at 2 63 ga granulite metamorphism affected the whole area but was not able to reset zircons in older rocks in all blocks the sm nd garnet whole rock ages are younger ranging from 2 48 2 59 ga this reflects relatively low closure temperature of the sm nd system in garnet in these rocks the u pb age of a palaeoproterozoic dolerite which cuts the granulites is 2 3 ga close to the k ar age of amphibole in a retrogressed fracture

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

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

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

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

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

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

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

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