comment on gas hydrate growth methane transport and chloride enrichment at the southern summit of hydrate ridge cascadia margin off oregon by torres et al earth planet sci lett 226 2004 225 241 formation evolution and destruction of natural gas hydrate systems in marine environments have been extensively discussed since the first gas hydrate samples were recovered n30 yr ago 1 the origin and sources of hydrate bound gases 2 and waters 3 the distribution of phases within and below the gas hydrate stability zone ghsz 4 mechanisms of crystal growth 5 and gas hydrate concentration in pore space 6 are among the most debated issues recent field studies facilitated by ocean drilling program odp 7 8 and dedicated government programs 9 10 provided a wealth of geological geophysical and geochemical information from hydrate bearing locations integrated interpretation of various datasets holds a great potential to advance our understanding of gas hydrate systems and to constrain speculations about the role of gas hydrates as a potential energy source 11 and a component of the global carbon cycle 12

distribution and characteristics of natural gas hydrates in the shenhu sea area south china sea abstract structure type gas composition and cage occupancy are important gas hydrate characteristics which play an important role in calculating gas hydrate stability and estimating the amount of global hydrate bound methane china conducted the fourth gas hydrate drilling campaign gmgs4 in the shenhu sea area ssa of the south china sea in 2016 during which comprehensive logging while drilling lwd data and natural gas hydrate samples were obtained the lwd data shows high electrical resistivity and high sound velocity at various depth indicating the occurrence of gas hydrate in this area coexistence of structure i si and structure ii sii gas hydrates in the south china sea was confirmed for the first time by using raman spectra analysis the methane cage occupancies of large cages and small cages in si hydrate are 0 95 and 0 96 respectively the hydration number of si hydrate is 6 03 gas composition analysis shows that methane 98 7 is the dominate gas with small amount of heavier hydrocarbon gases ethane 0 006 0 976 propane 0 033 0 216 isobutane 0 004 0 047 and other non hydrocarbon gases carbon dioxide

dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the paleocene isotopic records across the latest paleocene thermal maximum lptm indicate that bottom water temperature increased by more than 4 c during a brief time interval 104 years of the latest paleocene 55 6 ma there also was a coeval 2 to 3 excursion in the 13c of the ocean atmosphere inorganic carbon reservoir given the large mass of this reservoir a rapid 13c shift of this magnitude is difficult to explain within the context of conventional hypotheses for changing the mean carbon isotope composition of the ocean and atmosphere however a direct consequence of warming bottom water temperature from 11 to 15 c over 104 years would be a significant change in sediment thermal gradients and dissociation of oceanic ch4 hydrate at locations with intermediate water depths in terms of the present day oceanic ch4 hydrate reservoir thermal dissociation of oceanic ch4 hydrate during the lptm could have released greater than 1 1 to 2 1 1018 g of carbon with a 13c of approximately 60 the release and subsequent oxidation of this amount of carbon is sufficient to explain a 2 to 3 excursion in 13c across the lptm fate of ch4 in oceanic hydrates must be considered in developing models of the climatic and paleoceanographic regimes that operated during the lptm

complex gas hydrate system in a gas chimney south china sea abstract gas hydrates and deep sea cold seeps are of interest to industry and academia for several reasons including their implications for offshore geohazards and unconventional resources and for global climate change and carbon cycling guangzhou marine geological survey gmgs implemented its fifth gas hydrate drilling expedition in the qiongdongnan sea area qdn sa of the northern south china sea scs from june to september 2018 multidisciplinary investigations including logging while drilling lwd pressure and non pressure coring gas composition analyses in situ temperature and permeability measurements pore water geochemical analyses and a regional seismic survey reveal a complex distribution of gas hydrate within a vertical chimney structure lwd profiles record high resistivity anomalies and high p wave velocity variations at 9 174 mbsf suggesting the coexistence of free gas and gas hydrate in the sediment column throughout the investigated gas chimney hydrate bound and void gas is an admixture of methane 70 1 98 3 and heavier hydrocarbons c2h6 0 16 17 59 c3h8 0 05 9 88 i c4h10

a revised global estimate of dissolved iron fluxes from marine sediments literature data on benthic dissolved iron dfe fluxes mol m 2 d 1 bottom water oxygen concentrations o2bw m and sedimentary carbon oxidation rates cox mmol m 2 d 1 from water depths ranging from 80 to 3700 m were assembled the data were analyzed with a diagenetic iron model to derive an empirical function for predicting benthic dfe fluxes inline image where 170 mol m 2 d 1 is the maximum flux for sediments at steady state located away from river mouths this simple function unifies previous observations that cox and o2bw are important controls on dfe fluxes upscaling predicts a global dfe flux from continental margin sediments of 109 55 gmol yr 1 of which 72 gmol yr 1 is contributed by the shelf 2000 m of 41 21 gmol yr 1 is unsupported by empirical data previous estimates of benthic dfe fluxes derived using global iron models are far lower approximately 10 30 gmol yr 1 this can be attributed to i inadequate treatment of the role of oxygen on benthic dfe fluxes and ii improper consideration of continental shelf processes due to coarse spatial resolution globally averaged dfe concentrations in surface waters simulated with the intermediate complexity university of victoria earth system climate model were a factor of 2 higher with the new function we conclude that i the dfe flux from marginal sediments has been underestimated in the marine iron cycle and ii iron scavenging in the water column is more intense than currently presumed

pore size controls on the base of the methane hydrate stability zone in the kumano basin offshore japan the base of the methane hydrate stability zone mhsz in the kumano basin offshore japan is marked by a bottom simulating reflection bsr on seismic data at integrated ocean drilling program site c0002 which penetrates this bsr the in situ temperature profile combined with bulk seawater methane equilibrium conditions suggest that the base of the mhsz is 428 m below seafloor bsf which is 28 m deeper than the observed bsr 400 m bsf we found that submicron pore sizes determined by mercury injection capillary pressure are sufficiently small to cause 64 of the observed uplift of the base of the mhsz by the gibbs thomson effect this is the most thorough characterization of pore sizes within the mhsz performed to date and illustrates the extent to which pore size can influence mhsz thickness our results demonstrate the importance of considering lithology and pore structure when assessing methane hydrate stability conditions in marine sediments

ice age paleotopography a gravitationally self consistent theory of postglacial relative sea level change is used to infer the variation of surface ice and water cover since the last glacial maximum lgm the results show that lgm ice volume was approximately 35 percent lower than suggested by the climap reconstruction and the maximum heights of the main laurentian and fennoscandian ice complexes are inferred to have been commensurately lower with respect to sea level use of these ice age boundary conditions in atmospheric general circulation models will yield climates that differ significantly from those previously inferred on the basis of the climap data set

joint interpretation of geophysical field experiments in the danube deep sea fan black sea abstract gas hydrates are naturally occurring solid compounds of gas and water within almost all sediment rich continental margins due to the large amounts of methane stored in submarine gas hydrates they might serve as future reservoirs for offshore marine gas production assessing the reservoir characteristics requires reliable estimates of both the gas and gas hydrate concentration which can be best addressed using geophysical and geological investigations here we demonstrate the power of joint interpretation of interdisciplinary geophysical techniques and geological laboratory experiments regional 2d multichannel seismic data provide the broad overview of a hydrate bearing area high resolution 2d and 3d seismic reflection data provide detailed images of two working areas the buried s1 channel levee system at 1500 m water depth well within the gas hydrate stability zone and a slope failure location located at 665 m water depth top limit of the hydrate formation next to the s2 channel detailed compressional and shear wave vs velocity depth models were derived from four component ocean bottom seismic data the latter from p to s conversion upon reflection due to their steep reflection angles shear wave events result in less resolved vs models nevertheless in case of a change in elasticity of the sediment matrix due to gas hydrate cementation shear wave events can be used as an indicator as such vs can give insight into the nature of hydrate formation throughout the ghsz we present new developments in the application of common reflection surface normal incidence point tomography and full waveform inversion techniques to enhance model resolution for the seismic data sets 2d and 3d controlled source electromagnetic measurements provide volume information of the resistivity depth distribution models electrical resistivity of the sediment formation depends on its porosity and the resistivity of the pore fluid gas hydrate and free gas generally have much higher electrical resistivities than saline pore fluid and can be assessed using empirical relationships if the porosity and pore fluid salinity are known calibration with logging data laboratory experiments on hydrate or ice bearing sediments and resulting velocity and resistivity values guide the joint interpretation into more accurate saturation estimations beyond that a joint inversion framework supporting forward calculation of specialized geophysical methods at distributed locations is under development in this paper we summarize these individual components of a multi parameter study and their joint application to investigate gas hydrate systems their equilibrium conditions and preservation of bottom simulating reflectors we analyze data from two working areas at different locations and depth levels along the slope of the danube fan which are both characterized by multiple bottom simulating reflectors indicating the presence of gas hydrate in the first working area we located two depth windows with indications for moderate 16 24 gas hydrate formation but no vertical gas migration in the second working area we observed fluid migration pathways and active gas seepage limiting gas hydrate formation to less than 10 at the bsr some discrepancies remain between seismic based and electromagnetic based models of gas and gas hydrate distribution and saturation estimates indicating that further in situ investigations are likely required to better understand the gas hydrate systems at our study areas and to calibrate the inversion processes which will be required for a joint inversion framework as well

abrupt deep sea warming palaeoceanographic changes and benthic extinctions at the end of the palaeocene a remarkable oxygen and carbon isotope excursion occurred in antarctic waters near the end of the palaeocene 57 33 myr ago indicating rapid global warming and oceanographic changes that caused one of the largest deep sea benthic extinctions of the past 90 million years in contrast the oceanic plankton were largely unaffected implying a decoupling of the deep and shallow ecosystems the data suggest that for a few thousand years ocean circulation underwent fundamental changes producing a transient state that although brief had long term effects on environmental and biotic evolution

the impact of lithologic heterogeneity and focused fluid flow upon gas hydrate distribution in marine sediments gas hydrate and free gas accumulation in heterogeneous marine sediment is simulated using a two dimensional 2 d numerical model that accounts for mass transfer over geological timescales the model extends a previously documented one dimensional 1 d model such that lateral variations in permeability k become important various simulations quantitatively demonstrate how focused fluid flow through high permeability zones affects local hydrate accumulation and saturation simulations that approximate a vertical fracture network isolated in a lower permeability shale kfracture kshale show that focused fluid flow through the gas hydrate stability zone ghsz produces higher saturations of gas hydrate 25 70 and free gas 30 60 within the fracture network compared to surrounding shale simulations with a dipping high permeability sand layer also result in elevated saturations of gas hydrate 60 and free gas 40 within the sand because of focused fluid flow through the ghsz increased fluid flux a deep methane source or both together increase the effect of flow focusing upon hydrate and free gas distribution and enhance hydrate and free gas concentrations along the high permeability zones permeability anisotropy with a vertical to horizontal permeability ratio on the order of 10 2 enhances transport of methane charged fluid to high permeability conduits as a result gas hydrate concentrations are enhanced within these high permeability zones the dip angle of these high permeability structures affects hydrate distribution because the vertical component of fluid flux dominates focusing effects hydrate and free gas saturations can be characterized by a local peclet number localized vertical focused and advective flux relative to diffusion relative to the methane solubility gradient somewhat analogous to such characterization in 1 d systems even in lithologically complex systems local hydrate and free gas saturations might be characterized by basic parameters local flux and diffusivity

fluid flow methane fluxes carbonate precipitation and biogeochemical turnover in gas hydrate bearing sediments at hydrate ridge cascadia margin numerical modeling and mass balances a numerical model was applied to investigate and to quantify biogeochemical processes and methane turnover in gas hydrate bearing surface sediments from a cold vent site situated at hydrate ridge an accretionary structure located in the cascadia margin subduction zone steady state simulations were carried out to obtain a comprehensive overview on the activity in these sediments which are covered with bacterial mats and are affected by strong fluid flow from below the model results underline the dominance of advective fluid flow that forces a large inflow of methane from below 869 mumol cm 2 a 1 inducing high oxidation rates in the surface layers anaerobic methane oxidation is the major process proceeding at a depth integrated rate of 870 mumol cm 2 a 1 a significant fraction 14 of bicarbonate produced by anaerobic methane oxidation is removed from the fluids by precipitation of authigenic aragonite and calcite the total rate of carbonate precipitation 120 mumol cm 2 a 1 allows for the build up of a massive carbonate layer with a thickness of i m over a period of 20 000 years aragonite is the major carbonate mineral formed by anaerobic methane oxidation if the flow velocity of methane charge fluids is high enough greater than or equal to10 cm a 1 to maintain super saturation with respect to this highly soluble carbonate phase it precipitates much faster within the studied surface sediments than previously observed in abiotic laboratory experiments suggesting microbial catalysis the investigated station is characterized by high carbon and oxygen turnover rates approximate to1000 mumol cm 2 a 1 that are well beyond the rates observed at other continental slope sites not affected by fluid venting this underlines the strong impact of fluid venting on the benthic system even though the flow velocity of 10 cm a 1 derived by the model is relative low compared to fluid flow rates found at other cold vent sites non steady state simulations using measured fluid flow velocities as forcing demonstrate a rapid respond of the sediments within a few days to changes in advective flow moreover they reveal that efficient methane oxidation in these sediments prevents methane outflow into the bottom water over a wide range of fluid flow velocities 80 cm a 1 only at flow rates exceeding approximately 100 cm a 1 does dissolved methane break through the sediment surface to induce large fluxes of up to 5000 mumol ch4 cm 2 a 1 into the overlying bottom water

the diffusive tortuosity of fine grained unlithified sediments abstract use of fick s first law to estimate solute fluxes is arguably the most common and important calculation related to the geochemistry and environmental chemistry of fine grained unlithified sediments in such porous media the diffusion coefficient in this law must be reduced by the square of the tortuosity to account for the longer diffusive paths experimental methods to estimate tortuosity are specialized or time consuming and not routinely done an analysis of available data shows that tortuosity is accurately correlated to readily measured porosity values through the simple relation 2 1 l n 2

a model of suboxic sedimentary diagenesis suitable for automatic tuning and gridded global domains and solid phase mno2 the model misses some of the very high sediment surface mno2 concentrations observed in for example the california borderlands we believe that these sites must receive a higher input of reactive mn than the crustal abundance of the clay rain the model was deployed on every 2 2 grid point below 1000 m of the global seafloor using gridded maps of organic carbon rain to the seafloor and overlying water chemistry the deep sea carbon burial efficiency is 7 with 95 of respiration by oxic metabolism the model was also run against a new depth oxygen hypsometry of the global ocean with sediment area as a function of overlying oxygen concentration and average depth related to carbon fluxes to include the effects of shallow water sediments this analysispredictsthat shallow waters 1000 m account for 88 ofrespiration and oxygen accounts for 66 of respiration overall denitrification is closely coupled to organic nitrogen fluxes to the sediment and the global rate is estimated to be 1 4 10 14 mol yr 1 a factor of 6 8 higher than the corresponding estimate from middelburg et al 1996 throwing the nitrogen cycle in the ocean into imbalance index terms 4863 oceanography biological and chemical sedimentation 4851 oceanography biological and chemical oxidation reduction reactions 4804 oceanography biological and chemical benthic processes benthos 4805 oceanography biological and chemical biogeochemical cycles 1615 keywords diagenesis

methane formation at costa rica continental margin constraints for gas hydrate inventories and cross decollement fluid flow we present a numerical model study in order to quantify the effects of organic carbon poc degradation and fluid migration on methane and gas hydrate formation at odp site 1040 costa rica convergent margin various model runs show that poc degradation in upper plate sediments yields a potential for methane hydrate formation between 0 8 and 2 5 vol of pore space however observed chlorinity anomalies cannot be explained by the amount and the distribution pattern of gas hydrates moreover pore water profiles of ammonia do not match the observations setting up a moderate upward flow 0 03 cm yr 1 of methane enriched low chlorinity fluids induced by dewatering of oceanic plate sediments leads to a good approximation to measured pore water profiles thus enabling a precise estimate of poc degradation kinetics fluid flow has a strong impact on the location of the upper limit of the modeled gas hydrate occurrence zone ghoz and may increase the total amount of gas hydrate by more than 50 our best estimate of the amount of gas hydrate within the ghoz is on average 1 65 vol of pore space which corresponds to about 2 5 tg of methane per km trench within the frontal prism of slope sediments to comply with the fact that subducted pore waters are rich in sulfate and that there is striking evidence for fluid conduits at various depths we performed additional model runs where we simulated fluid flow by using a gauss type rate law allowing us to define distinct fluid sources we can demonstrate that combined methane production in the upper plate sediments and sulfate reduction at the top of the down going slab is sufficient to prevent the upward movement of the zone of anaerobic oxidation of methane aom to above the decollement at given upward advection rates steep pore water gradients along the plate boundary can be explained by lateral backflow within oceanic plate sediments on a long term in the order of at least some 100 000 years fluid flow along conduits is likely to occur at low rates with temporarily increased pulses all modeled runs are constrained by their compatibility to observed pore water profiles

rayleigh taylor type instability in auroral patches 1 based on observations by a high resolution narrow field of view ccd camera we found small scale 5 25 km finger like structures at the western boundary of auroral patches in images obtained at gillam geomagnetic latitude 65 5 n canada in january 2008 since shear motion was not observed along the boundary of the patches we suggest that these structures are formed by macroscopic rayleigh taylor type plasma instability arising in the magnetospheric equatorial plane from the force balancing of the eastward magnetic tension force and the westward pressure gradient force

evolution of fluid expulsion and concentrated hydrate zones across the southern hikurangi subduction margin new zealand an analysis from depth migrated seismic data identification of methane sources controlling hydrate distribution and concentrations in continental margins remains a major challenge in gas hydrate research lack of deep fluid samples and high quality regional scale seismic reflection data may lead to underestimation of the significance of fluid escape from subducting and compacting sediments in the global inventory of methane reaching the hydrate zone the water column and the atmosphere the distribution of concentrated hydrate zones in relation to focused fluid flow across the southern hikurangi subduction margin was investigated using high quality long offset 10 km streamer pre stack depth migrated multichannel seismic data analysis of low p wave velocity zones bright reverse polarity reflections and dim amplitude anomalies reveals pathways for gas escape and zones of gas accumulation the study shows the structural and stratigraphic settings of three main areas of concentrated hydrates 1 the opouawe bank dominated by focused periodic fluid input along thrust faults sustaining dynamic hydrate concentrations and gas chimneys development 2 the frontal anticline with a basal set of protothrusts controlling permeability for fluids from deeply buried and subducted sediments sustaining hydrate concentrations at the crest of the anticline and 3 the hikurangi channel with buried sand dominated channels hosting significant amounts of gas beneath the base of the hydrate zone in sand dominated channels gas injection into the hydrate zone favors highly concentrated hydrate accumulations the evolution of fluid expulsion controlling hydrate formation offshore southern hikurangi is described in stages during which different methane sources in situ buried and thermogenic have been dominant

enhanced chemical weathering of rocks during the last glacial maximum a sink for atmospheric co2 abstract it has been proposed that increased rates of chemical weathering and the related drawdown of atmospheric co2 on the continents may have at least partly contributed to the low co2 concentrations during the last glacial maximum lgm variations in continental erosion could thus be one of the driving forces for the glacial interglacial climate cycles during quaternary times to test such an hypothesis a global carbon erosion model has been applied to a lgm scenario in order to determine the amount of co2 consumed by chemical rock weathering during that time in this model both the part of atmospheric co2 coming from silicate weathering and the part coming from carbonate weathering are distinguished the climatic conditions during lgm were reconstructed on the basis of the output files from a computer simulation with a general circulation model only the predicted changes in precipitation and temperature have been used whereas the changes in continental runoff were determined with an empirical method it is found that during the lgm the overall atmospheric co2 consumption may have been greater than today by about 20 mainly because of greater carbonate outcrop area related to the lower sea level on the shelves this does not however affect the atmospheric co2 consumption by silicate weathering which alone has the potential to alter atmospheric co2 on the long term silicate weathering and the concomitant atmospheric co2 consumption decreased together with a global decrease of continental runoff compared to present day both by about 10 nevertheless some uncertainty remains because the individual lithologies of the continental shelves as well as their behavior with respect to chemical weathering are probably not well enough known the values we present refer to the ice free continental area only but we tested also whether chemical weathering under the huge ice sheets could have been important for the global budget although glacial runoff was considerably increased during lgm weathering under the ice sheets seems to be of minor importance

constraining the age and evolution of the tuaheni landslide complex hikurangi margin new zealand using pore water geochemistry and numerical modeling

estimation of the global amount of submarine gas hydrates formed via microbial methane formation based on numerical reaction transport modeling and a novel parameterization of holocene sedimentation this study provides new estimates for the global offshore methane hydrate inventory formed due to microbial ch4 production under quaternary and holocene boundary conditions a multi 1d model for particular organic carbon poc degradation gas hydrate formation and dissolution is presented the novel reaction transport model contains an open three phase system of two solid compounds organic carbon gas hydrates three dissolved species methane sulfates inorganic carbon and one gaseous phase free methane the model computes time resolved concentration profiles for all compounds by accounting for chemical reactions as well as diffusive and advective transport processes the reaction module builds upon a new kinetic model of poc degradation which considers a down core decrease in reactivity of organic matter various chemical reactions such as organic carbon decay anaerobic oxidation of methane methanogenesis and sulfate reduction are resolved using appropriate kinetic rate laws and constants gas hydrates and free gas form if the concentration of dissolved methane exceeds the pressure temperature and salinity dependent solubility limits of hydrates and or free gas with a rate given by kinetic parameters global input grids have been compiled from a variety of oceanographic geological and geophysical data sets including a new parameterization of sedimentation rates in terms of water depth we find prominent gas hydrate provinces offshore central america where sediments are rich in organic carbon and in the arctic ocean where low bottom water temperatures stabilize methane hydrates the world s total gas hydrate inventory is estimated at 0 82 x 10sup13 m3 2 10 x 10sup15 m3 ch4 at stp conditions or equivalently 4 18 995 gt of methane carbon the first value refers to present day conditions estimated using the relatively low holocene sedimentation rates the second value corresponds to a scenario of higher quaternary sedimentation rates along continental margins our results clearly show that in situ poc degradation is at present not an efficient hydrate forming process significant hydrate deposits in marine settings are more likely to have formed at times of higher sedimentation during the quaternary or as a consequence of upward fluid transport at continental margins

thermal histories and hydrocarbon generation example from utah wyoming thrust belt the thermal history of the utah wyoming thrust belt in the vicinity of the pineview oil and gas field is studied with a detailed thermal model the model incorporates a heterogeneous thermal conductivity structure radioactive heat generation in the crust sediment compaction variable basal heat flow through time and thermal effects of sedimentation erosion and thrust faulting the analysis is constrained by 206 corrected bottom hole temperatures 660 thermal conductivity measurements and 10 new vitrinite reflectance measurements present surface heat flow in this part of the utah wyoming thrust belt is estimated to be 60 8 mw m2 surface heat flow in the late cretaceous about 70 ma is inferred to have been 38 8 mw m2 from an lysis of vitrinite reflectance data about half of the apparent difference between ancient and present surface heat flow may be attributed to transient thermal effects of sedimentation erosion and thrusting the hypothesis of a constant background thermal state in this part of the thrust belt for the last 70 m y may not be ruled out however thermal history simulations in which the background thermal state changes through time better match constraints imposed by estimated present surface heat flow and vitrinite reflectance data application of the thermal history analysis to hydrocarbon generation is illustrated with a kinetic model of organic maturation oil and gas at the pineview field may have been generated in two stages first stage just before and during thrusting and the second stage during cenozoic warming

a data driven model of the global calcite lysocline gridded maps of sediment calcium carbonate calcite concentration and overlying water saturation state archer 1996 are combined with maps of benthic oxygen fluxes and sediment accumulation rates from jahnke 1996 and cweink 1986 to drive a diagenetic model of calcium carbonate preservation in deep sea sediments the only model input for which we cannot draw a detailed map is the rain rate of calcite to the seafloor so i use the model to calculate the calcite rain rate required to simulate the observed distribution of calcite concentration on the seafloor the predictive power of the model can be checked by searching for input parameters to which the model is sensitive and comparing the model requirements with independent data the model is sensitive to the ratio of organic carbon to calcite rain rates the rain ratio and within the constraint of pelagic sediment trap rain ratio data is unable to reproduce the calcite field without respiratory dissolution the promotion of calcium carbonate dissolution by the oxic degradation of organic carbon however relative to variability in sediment trap data the required model rain ratio is insensitive to the extent of anoxic respiration the stoichiometric ratio of o c during respiration the bioturbation rate the dissolution rate constant the effect of borate chemistry and small offsets in the saturation state for caco3 the model is sensitive to the accumulation rate of non caco3 material and predicts an atlantic pacific difference in non caco3 rain rate which is consistent with observations the model predicts that the dissolution flux of caco3 from sediments is 25 40 1012 mol yr 1 roughly half of the total deep sea caco3 dissolution rate estimated from the water column alkalinity distribution broecker and peng 1987 archer and maier reimer 1994 wollast 1994 the model also predicts that only 20 30 of the flux of caco3 to the seafloor globally escapes dissolution

provenances distribution and accumulation of organic matter in the southern mariana trench rim and slope implication for carbon cycle and burial in hadal trenches abstract it is generally accepted that organic matter content and accumulation rate at the seafloor attenuate with increasing water depth however hadal trenches which represent the deepest portion of the hydrosphere on earth do not abide by this universal rule it has been speculated that hadal trenches would serve as organic matter depocenters where bacteria mediated organic matter degradation intensifies here we examine the contents of total organic carbon toc and total nitrogen tn as well as the 13c values of toc in three gravity cores two box cores and three grab samples with water depth from 4900 to 7118 m in order to reveal the provenances of organic matter spatial distribution and accumulation rates of particulate organic carbon poc in the southern mariana trench rim and slope although the deepest area was not sampled trench rim and slope is also of importance in terms of transportation and accumulation processes of organic matter a vast majority of the sediment samples have bulk toc tn molar ratios and 13c values of toc ranging from 4 2 to 11 and from 21 8 to 18 9 respectively implying that the organic matter was primarily sourced from marine algae two exceptions have been found at 101 cm and 201 cm depths of core gc05 with a possible input of terrestrial material suggested by an abrupt increase in toc contents and toc tn molar ratios accompanied by marked decreases in 13c values of toc moreover toc contents in surface sediments basically increased with water depth based on the published excess 210pb data and by assuming a simplified one dimensional sediment loading despite of frequent occurrences of mass wasting transports in hadal trenches the sedimentation rate in the southern challenger deep 6037 m was estimated to be 0 02 cm yr 1 this much higher sedimentation rate compared to the globally averaged values in deep ocean sediments coupled with the trend of increasing in toc contents with water depth may serve as evidence for the lateral transport of sediment particles induced by the funnel like topography and localized current dynamics within the trench the average poc accumulation rate within the southern challenger deep roughly amounts to 1 5 10 5 g cm 2 yr 1 equivalent to about a seventh of the globally averaged poc accumulation rate in deep ocean seafloor to our knowledge poc accumulation in hadal trenches has been quantified for the first time and we highlight that organic matter degradation in deep hadal trenches should not be neglected and may represent a significant component of the global carbon cycle

understanding long term carbon cycle trends the late paleocene through the early eocene 1 the late paleocene to the early eocene 58 52 ma was marked by significant changes in global climate and carbon cycling the evidence for these changes includes stable isotope records that reveal prominent decreases in i 18 oa ndi 13 c suggesting a rise in earth s surface temperature 4 i c and a drop in net carbon output from the ocean and atmosphere concurrently deep sea carbonate records at several sites indicate a deepening of the calcite compensation depth ccd here we investigate possible causes e g increased volcanic degassing or decreased net organic burial for these observations but from a new perspective the basic model employed is a modified version of geocarb iii however we have coupled this well known geochemical model to loscar long term ocean atmosphere sediment carbon cycle reservoir model which enables simulation of seawater carbonate chemistry the ccd and ocean i 13 c we have also added a capacitor in this case represented by gas hydrates that can store and release 13 c depleted carbon to and from the shallow geosphere over millions of years we further consider accurate input data e g i 13 c of carbonate on a currently accepted timescale that spans an interval much longer than the perturbation several different scenarios are investigated with the goal of consistency amongst inferred changes in temperature the ccd and surface ocean and deep ocean i 13 c the results strongly suggest that a decrease in net organic carbon burial drove carbon cycle changes during the late paleocene and early eocene although an increase in volcanic activity might have contributed importantly a drop in net organic carbon burial may represent increased oxidation of previously deposited organic carbon such as stored in peat or gas hydrates the model successfully recreates trends in earth surface warming as inferred from i 18 o records the ccd and i 13 c at the moment however our coupled modeling effort cannot reproduce the magnitude of change in all these records collectively similar problems have arisen in simulations of short term hyperthermal events during the early paleogene paleocene eocene thermal maximum suggesting one or more basic issues with data interpretation or geochemical modeling remain

towards quantifying the reaction network around the sulfate methane transition zone in the ulleung basin east sea with a kinetic modeling approach abstract we present a kinetic model based upon pore water data collected from eight sites drilled during the second ulleung basin gas hydrate drilling expedition ubgh2 in 2010 three sites were drilled at locations where acoustic chimneys were identified in seismic data and the rest were drilled on non chimney i e background environments our model coupled a comprehensive compositional and isotopic data set is used to illustrate the different biogeochemical processes at play in those two environments in terms of reactions around the sulfate methane transition zone smtz organic matter decomposition is an important process for production of methane dissolved inorganic carbon dic and consumption of sulfate in the non chimney sites whereas anaerobic oxidation of methane aom dominates both carbon and sulfur cycles in the chimney environment different sources of methane mediate aom in the two settings internally produced methane through co 2 reduction cr and methanogenesis fuels aom in the non chimney sites whereas aom is sustained by methane from external sources in the chimney sites we also simulate the system evolution from non chimney to chimney conditions by increasing the bottom methane supply to a non chimney setting we show that the higher ch 4 flux leads to a higher microbial activity of aom and more organic matter decomposition through methanogenesis a higher methanogenesis rate and a smaller cr contribution relative to aom in the chimney sites is responsible for the isotopically light dic and heavy methane in this environment relative to the non chimney sites

time dependent response of the global ocean clathrate reservoir to climatic and anthropogenic forcing 1 buffett and archer 2004 showed a model of the steady state global ocean clathrate reservoir that reproduces drill data and predicts a global methane inventory of about 5000 gton c the model inventory is very sensitive to ocean temperature with factor of two changes in methane inventory in response to temperature changes of 1 5 c here we explore the potential time dependent behavior of the clathrate reservoir assuming simple first order rate constants for methane accumulation and meltdown in response to changes in the predicted steady state reservoir for melting we divide the reservoir into two parts one melting catastrophically and the other slowly evolution of clathrates through geologic time is very sensitive to the choice of these rate constants a positive feedback exists during a transient meltdown event stronger if the catastrophic melting fraction is high and its melting timescale is fast a clathrate reservoir governed by such kinetics would melt down periodically on a multimillion year timescale determined by the rate constant for clathrate accumulation ideally we would like a model that predicts runaway melting in the paleocene and stability in the pleistocene however the model is most unstable when climate is cold rather than warm opting for stability in the pleistocene constrains the melting kinetic parameters leaving one degree of freedom the grow in timescale must be 5 myr or longer to get the deglacial 13c right but shorter than 10 myr in order for clathrates to have built up through geologically recent cooling the model thus constrained predicts methane fluxes of 200 gton c or less on deglaciations but eventual releases of 2000 4000 gton c in response to a 2000 gton c anthropogenic carbon release anthropogenic climate change differs from a deglaciation in that it warms the ocean to temperatures not seen in millions of years

modeled estimates of global reef habitat and carbonate production since the last glacial maximum estimated changes in reef area and caco3 production since the last glacial maximum lgm are presented for the first time based on a model reefhab which uses measured environmental data to predict global distribution of reef habitat suitable reef habitat is defined by temperature salinity nutrients and the depth attenuated level of photosynthetically available radiation par caco3 production is calculated as a function of par when minimum par levels were chosen to restrict reef growth to 30 m depth and less modern reef area totaled 584 746 10 km global carbonate production which takes into account topographic relief as a control on carbonate accumulation was 1 00 gt yr 1 these values are close to the most widely accepted estimates of reef area and carbonate production and demonstrate that basic environmental data can be used to define reef habitat and calcification to simulate reef habitat changes since the lgm the model was run at 1 kyr intervals using appropriate sea level and temperature values these runs show that at the lgm reef area was restricted to 20 of that today and carbonate production to 27 due primarily to a reduction in available space at the lower sea level and secondarily to lower sea surface temperatures nonetheless these values suggest that reef growth prior to shelf flooding was more extensive than previously thought a crude estimate of reef released co2 to the atmosphere since the lgm is of the same order of magnitude as the atmospheric co2 change recorded in the vostok ice core which emphasizes the role of neritic carbonates within the global carbon cycle this model currently addresses only the major physical and chemical controls on reef carbonate production but it provides a template for estimating shallow tropical carbonate production both in the present and in the past as such the model highlights several long standing issues regarding reef carbonates particularly in terms of better defining the roles of light temperature aragonite saturation state and topography on reef calcification

the effect of lithologic heterogeneity on diffusive methane migration in hydrate systems abstract short range diffusive gas migration has been proposed as a means of supplying methane to coarse grained hydrate bearing sediments in this scenario methane is generated microbially in fine grained sediments and moves diffusively into adjacent coarse grained sediments due to a dissolved methane concentration gradient caused by pore size contrasts previous work has shown that short range diffusion is sufficient to supply thin isolated sands 1 m thickness with enough methane to achieve high hydrate saturations 80 in systems with a high ratio of clay to sand several notable hydrate reservoirs such as those encountered at daini atsumi knoll in the nankai trough and walker ridge block 313 and green canyon block 955 in the gulf of mexico exhibit tens of meters of alternating sand silt and clay layers that are decimeters in thickness the coarse grained layers in these settings are typically filled with hydrate to saturations exceeding 80 while short range diffusion is capable of filling thin isolated sands with hydrate thicker sands within more heterogeneous systems require mechanisms other than or in addition to short range diffusion using 1 d numerical simulations we demonstrate that short range diffusive migration of microbial methane is insufficient for reaching consistently high hydrate saturations in such systems this work shows that a different mechanism for gas transport such as advective migration is necessary to achieve these observed hydrate saturations in layered systems

a global prediction of seafloor sediment porosity using machine learning porosity void ratio is a critical parameter in models of acoustic propagation bearing strength and many other seafloor phenomena however like many seafloor phenomena direct measurements are expensive and sparse we show here how porosity everywhere at the seafloor can be estimated using a machine learning technique specifically random forests such techniques use sparsely acquired direct samples and dense grids of other parameters to produce a statistically optimal estimate where direct measurements are lacking our porosity estimate is both qualitatively more consistent with geologic principles than the results produced by interpolation and quantitatively more accurate than results produced by interpolation or regression methods we present here a seafloor porosity estimate on a 5 arc minute pixel registered grid produced using widely available densely sampled grids of other seafloor properties these techniques represent the only practical means of estimating seafloor properties in inaccessible regions of the seafloor e g the arctic

role of critical state framework in understanding geomechanical behavior of methane hydrate bearing sediments a proper understanding of geomechanical behavior of methane hydrate bearing sediments is crucial for sustainable future gas production there are a number of triaxial experiments conducted over synthetic and natural methane hydrate mh bearing sediments and several soil constitutive models have been proposed to describe their behavior however the generality of a sophisticated model is questioned if it is tested only for a limited number of cases furthermore it is difficult to experimentally determine the associated parameters if their physical meanings and significance are not elucidated the objective of this paper is to demonstrate that a simple extension of the critical state framework is sufficient to capture the geomechanical behavior of mh bearing soils from various sources around the world while the significance of each parameter is quantified through variance based global sensitivity analyses our results show that the influence of hydrates can be largely represented by one hydrate dependent parameter pcd which controls the expansion of the initial yield surface this is validated through comparisons with shearing and volumetric response of mh bearing soils tested at various institutes under different confining stresses and with varying degrees of hydrate saturation our study suggests that the behavior of mh bearing soils can be reasonably predicted based on pcd and the conventional critical state parameters of the host sediments that can be obtained through typical geotechnical testing procedures

the challenges of quantifying the carbon stored in arctic marine gas hydrate the quantification of the carbon stored in gas hydrate gh bearing marine sediments still remains a challenge despite recent efforts to develop approaches to better estimate the gh inventory globally these estimates are still highly unconstrained due to insufficient field data and poor understanding of the mechanisms fuelling the gh stability zone ghsz here we use geophysically derived gh saturations to constraint estimates of model derived arctic marine gh inventory at present we also estimate the potential carbon released from gh dissociation under a seabed warming of 2 c over 100 yr we estimate an inventory ranging between 0 28 and 541 gt of carbon which upper bound results in average gh saturations of 0 25 our upper bound is mainly controlled by our imposed upwards carbon rich fluid flow of 0 01 cm yr 1 and it is five times greater than the most recent estimate that only considers in situ degradation of particulate organic carbon poc to obtain the seismically inferred gh saturations of 5 10 offshore west of svalbard and in the beaufort sea an upwards advection of carbon rich fluids equivalent to 0 02 0 04 cm yr 1 is required this mechanism may be the most important source of carbon reaching the ghsz in arctic marine sediments a 2 c seabed temperature increase over 100 yr may reduce the gh inventory by about 88 44 0 7 gt c if poc is the only source and by about 5 4 29 7 gt c if the main source of carbon is the upwards advection of carbon rich fluids

3 d basin scale reconstruction of natural gas hydrate system of the green canyon gulf of mexico our study presents a basin scale 3d modeling solution quantifying and exploring gas hydrate accumulations in the marine environment around the green canyon gc955 area gulf of mexico it is the first modeling study that considers the full complexity of gas hydrate formation in a natural geological system overall it comprises a comprehensive basin re construction accounting for depositional and transient thermal history of the basin source rock maturation petroleum components generation expulsion and migration salt tectonics and associated multi stage fault development the resulting 3d gas hydrate distribution in the green canyon area is consistent with independent borehole observations an important mechanism identified in this study and leading to high gas hydrate saturation 80 vol at the base of the gas hydrate stability zone ghsz is the recycling of gas hydrate and free gas enhanced by high neogene sedimentation rates in the region our model predicts the rapid development of secondary intra salt mini basins situated on top of the allochthonous salt deposits which leads to significant sediment subsidence and an ensuing dislocation of the lower ghsz boundary consequently large amounts of gas hydrates located in the deepest parts of the basin dissociate and the released free methane gas migrates upwards to recharge the ghsz in total we have predicted the gas hydrate budget for the green canyon area that amounts to 3 256 mt of gas hydrate which is equivalent to 340 mt of carbon 7 x 1011 m3 of ch4 at stp conditions and consists mostly of biogenic hydrates

coupled pyrite concentration and sulfur isotopic insight into the paleo sulfate methane transition zone smtz in the northern south china sea abstract the sulfate methane transition zone smtz is an important diagenetic redox boundary within marine sediments where the anaerobic oxidation of methane aom coupled with bacterial sulfate reduction can promote sulfur isotopic enrichments in several solid phase minerals including pyrite fes2 authigenic pyrite can form in concentrated abundances within the smtz and as such can be used as a proxy to identify paleo smtzs this study uses enrichments in 34s and anomalously high abundances of authigenic pyrites in 287 samples from the northern south china sea scs to determine the paleo smtz the pyrite samples were collected from sediment cores acquired at three sites each of which are known to be located in natural gas hydrate bearing regions we assess the relative abundances of authigenic pyrites the types of pyrite morphologies recovered in the cored sediments and the sulfur isotopic values of recovered pyrite samples using two methods 1 handpicked sample analysis using a binocular microscope and 2 the chromium reduction method our results show that pyrite concentrations and sulfur isotopic compositions exhibit synchronous fluctuations particularly from 6 8 m below seafloor mbsf to 8 4 mbsf at all three study sites there is a significant increase in the occurrence of rod like pyrite morphology within this key interval we define the position of the paleo smtz by the presence of anomalously high accumulations of pyrites at greater than 5 0 wt using the handpicking method or greater than 0 5 wt via the chromium reduction method along with positive 34s excursions greater than 10 0 vcdt we discovered a regional paleo smtz that is shallower than the modern smtz suggesting a previous period of elevated methane flux from depth possibly related to widespread gas hydrate dissociation

the effects of sedimentation and compaction on oceanic heat flow summary the estimation of environmental effects forms an important part of the interpretation of oceanic heat flow measurements in particular the perturbations associated with sedimentation and surface temperature changes must be taken into account analytical solutions can be obtained only for individual simplified versions of these problems whereas any real example is complicated by the process of sediment compaction which changes the bulk thermal properties with depth a physical model is developed which uses sediment porosity trends to predict the thermal parameters and material advection rates for an evolving sediment basement system these values are then used in a numerical solution to the heat flow equation to give estimates of the perturbed surface heat flux through time in addition to variations in sedimentation rate sediment type radioactive heat production and surface temperature changes are considered heat flow corrections may vary by up to a factor of 2 according to sediment type while radioactive heat production can offset the effects of sedimentation by as much as 40 per cent the results also indicate that alterations determined from simple analytical models tend to over estimate the true perturbation to the flux

toward a parameterization of global scale organic carbon mineralization kinetics in surface marine sediments an empirical function is derived for predicting the rate depth profile of particulate organic carbon poc degradation in surface marine sediments including the bioturbated layer the rate takes the form of a power law analogous to the middelburg function the functional parameters were optimized by simulating measured benthic o2 and no3 fluxes at 185 stations worldwide using a diagenetic model the novelty of this work rests with the finding that the vertically resolved poc degradation rate in the bioturbated zone can be determined using a simple function where the poc rain rate is the governing variable although imperfect the model is able to fit 71 of paired o2 and no3 fluxes to within 50 of measured values it further provides realistic geochemical concentration depth profiles no3 penetration depths and apparent first order poc mineralization rate constants the model performs less well on the continental shelf due to the high heterogeneity there when applied to globally resolved maps of rain rate the model predicts a global denitrification rate of 182 88 tg yr 1 of n and a poc burial rate of 107 52 tg yr 1 of c with a mean carbon burial efficiency of 6 1 these results are in very good agreement with published values our proposed function is conceptually simple requires less parameterization than multi g type models and is suitable for non steady state applications it provides a basis for more accurately simulating benthic nutrient fluxes and carbonate dissolution rates in earth system models

determination of the dissolved anion composition of ancient lakes from fossil ostracodes the mineralogy of evaporite and other precipitated minerals together with geochemical studies has provided traditional sources of information about the major dissolved ion composition solutes of ancient lakes the paleocompositional resolving power of these methods is generally greatest in high salinity lakes that precipitate numerous solute sensitive evaporite minerals as opposed to dilute saline lakes that precipitate only a few minerals ostracodes live in dilute saline lakes where a species occurrence is determined by the relative proportions of the lake s major dissolved anions so that each species describes specific areas on an anion trilinear diagram moreover the upper salinity tolerance of each species appears to depend upon the types of major anions in solution and is therefore anion specific knowledge about both anion and anion salinity tolerances of an ostracode may ultimately provide a means of estimating absolute anion concentrations in paleolakes because ostracodes are common fossils in lake sediments they provide an important new source of original paleocompositional information suitable for many geologic climatic geochemical and paleontologic studies

basin scale estimates on petroleum components generation in the western black sea basin based on 3 d numerical modelling abstract a new numerical model reconstructing the depositional history 98 0 ma of the western black sea sub basin is presented the model accounts for changing boundary conditions i e water depth bottom water temperature heat flow evolution over time and estimates the rates and total amounts of the in situ biogenic methane generation and thermally driven organic matter maturation in the source rocks the overall thermogenic and biogenic gas generation predicted by the model is estimated at 1560 gt and 3100 gt respectively

contributions from glacially derived sediment to the global iron oxyhydr oxide cycle implications for iron delivery to the oceans estimates of glacial sediment delivery to the oceans have been derived from fluxes of meltwater runoff and iceberg calving and their sediment loads the combined total 2900 tg yr 1 of the suspended sediment load in meltwaters 1400 tg yr 1 and the sediment delivered by icebergs 1500 tg yr 1 are within the range of earlier estimates high resolution microscopic observations show that suspended sediments from glacial meltwaters supraglacial and proglacial sediments and sediments in basal ice from arctic alpine and antarctic locations all contain iron oxyhydr oxide nanoparticles which are poorly crystalline typically 5 nm in diameter and which occur as single grains or aggregates that may be isolated or attached to sediment grains nanoparticles with these characteristics are potentially bioavailable a global model comparing the sources and sinks of iron present as oxyhydr oxides indicates that sediment delivered by icebergs is a significant source of iron to the open oceans beyond the continental shelf iceberg delivery of sediment containing iron as oxyhydr oxides during the last glacial maximum may have been sufficient to fertilise the increase in oceanic productivity required to drawdown atmospheric co2 to the levels observed in ice cores