subsurface gas offshore of northern california and its link to submarine geomorphology abstract the northern california continental margin contains evidence of abundant subsurface gas and numerous seafloor features that suggest a causative link between gas expulsion and geomorphology analyses of seismic reflection sidescan sonar and high resolution multibeam bathymetric data show that the occurrence of subbottom gas and the migration processes beneath the shelf differ from those beneath the slope subsurface gas inferred from enhanced reflectors and other geophysical indicators is spatially variable and related more to total depth and stratigraphy than to underlying structure with the exception of one band of gas that follows the regional structural trend shallow depressions on the seafloor pockmarks are used to infer expulsion sites the largest zone of acoustically impenetrable subsurface gas occurs between water depths of 100 m and 300 m where expulsion features are rare the upper slope water depths 400 600 m has a high concentration of pockmarks diameter 10 20 m in contrast to a near absence of pockmarks at water depths shallower than 400 m of nearly 4000 pockmarks observed on sidescan sonar records more than 95 are located in water depths deeper than 400 m bottom simulating reflectors bsrs on some seismic reflection profiles indicate the possible presence of gas hydrate we find that gas and pore fluid migration in the offshore eel river basin is 1 correlated to surface morphology 2 a contributor to seabed roughness 3 a significant mode of sediment redistribution on the upper slope and 4 potentially a factor in large slope failures

spatial zonation limits magnesite dissolution in porous media abstract we investigate how mineral spatial distribution in porous media affects their dissolution rates specifically we measure the dissolution rate of magnesite interspersed in different patterns in packed columns of quartz sand where the magnesite concentration v v was held constant the largest difference was observed between a mixed column containing uniformly distributed magnesite and a one zone column containing magnesite packed into one cylindrical center zone aligned parallel to the main flow of acidic inlet fluid flow parallel one zone column the columns were flushed with acid water at a ph of 4 0 at flow velocities of 3 6 or 0 36 m d breakthrough data show that the rate of magnesite dissolution is 1 6 2 times slower in the one zone column compared to the mixed column this extent of rate limitation is much larger than what was observed in our previous work 14 for a similar one zone column where the magnesite was packed in a layer aligned perpendicular to flow flow transverse one zone column two dimensional reactive transport modeling with crunchflow revealed that ion activity product iap and local dissolution rates at the grid block scale 0 1 cm vary by orders of magnitude much of the central magnesite zone in the one zone flow parallel column is characterized by close or equal to equilibrium conditions with iap k eq 0 1 two important surface areas are defined to understand the observed rates the effective surface area a e reflects the magnesite that effectively dissolves under far from equilibrium conditions iap k eq a i reflects the effective magnesite surface that lies along the quartz magnesite interface modeling results reveal that the transverse dispersivity at the interface of the quartz and magnesite zones controls mass transport and therefore the values of a e and a i under the conditions examined in this work the value of a e varies from 2 to 67 of the total magnesite bet surface area column scale bulk rates r mgco 3 b in units of mol s vary linearly with a e and a i using a e to normalize rates we calculate a rate constant 10 9 56 mol m 2 s that is very close to the value of 10 10 0 mol m 2 s under well mixed conditions at the grid block scale this implies that the laboratory field rate discrepancy can potentially be caused by differences in the effective surface area if we know the effective surface area of dissolution we will be able to use the rate constant measured in laboratory systems to calculate field rates for some systems in this work approximately 60 70 of the a e is at the magnesite quartz interface this implies that in some field systems where the detailed information that we have for our columns is not available the effective mineral surface area may be approximated by the area of grains residing at the interface of reactive mineral zones although it has long been known that spatial heterogeneities play a significant role in determining physical processes such as flow and solute transport our data are the first that systematically and experimentally quantifies the importance of mineral spatial distribution chemical heterogeneity on dissolution

weathering rock to regolith the amount of pore space in most unweathered granite is too small to support a plant based ecosystem but porosity grows as intact rock interacts with surface waters beneath the soil

weathering of rock to regolith the activity of deep roots in bedrock fractures abstract many areas in the world are characterized by shallow soils underlain by weathered bedrock but root rock interactions and their implications for regolith weathering are poorly understood to test the role of tree roots in weathering bedrock we excavated four pits along a catena in a shale dominated catchment at the susquehanna shale hills critical zone observatory sshczo in central pennsylvania we measured a variety of biological physical and chemical properties including 1 root density distribution and respiration 2 soil gas and 3 elemental compositions mineralogy and morphology of soil rock and rock fracture fill at ridge top mid slope toe slope and valley floor sites as expected root density declined rapidly with depth nevertheless fine roots were present in rock fractures even in the deepest least weathered shale sampled 180 cm below the land surface root densities in shale fractures were comparable between the ridge top and mid slope pits however they were significantly lower in the toe slope despite increasing rock fracture densities which is likely due to a shallower water table depth at the downslope site average root respiration per mass of dry root tissue in rock fractures was comparable to rates in the soil thus the total flux of co2 from root respiration tracked root densities decreasing with depth potential microbial respiration estimated as the laboratory c mineralization potential was about an order of magnitude lower than measured root respiration in both the soil and shale fractures roots were only observed in large aperture 50 m shale fractures that were filled with particulate material the fill in these fractures was mineralogically and geochemically similar to the lowest soil horizons with respect to clay composition element mobility extractable dissolved organic c doc inorganic n species and potentially mineralizable c and n while total c and total n values for the fracture fill were similar to the shale bedrock in the bulk soil depletion profiles al fe k mg and si relative to unweathered shale reflected characteristic weathering of illite and vermiculized chlorite to kaolinite and are similar between soils and fracture fill such similarities indicate that the fracture coatings are likely the result of pedogenic processes that occur at depth in the fractures rather than translocation of soil particles downward into the fractures overall our data suggest that roots and fill in shale fractures down to 180 cm are qualitatively similar to those in surface soil horizons thus the deepest manifestation of the chemical depletion profiles observed in the pits consists of the rock fracture fill and this fill is present at low concentrations with similarly low concentrations of fine roots

the control mechanisms of erosion and weathering at basin scale from cosmogenic nuclides in river sediment the study of a sample of river sediment enables the determination of spatially averaged denudation rates that provide an exceptional perspective on erosion and weathering processes that have taken place within a landscape these measurements are done with in situ produced cosmogenic nuclides e g 10be 26al mostly in quartz from alluvial sediment cosmogenic nuclides are produced when secondary cosmic rays interact with the very uppermost layer of the earth s surface they are produced within a characteristic depth scale of about 1 m which means that the measured concentrations record an integrated denudation history while material passed through this depth interval depending on the denudation rate the resulting integration time scales are 103 to 105 years and one obtains a robust long term estimate of natural denudation that is relatively insensitive to short term changes the last 10 years have seen significant research activity using these methods and an array of fascinating tectono geomorphologic and geochemical insights are emerging amongst these is the ability to identify the physical and chemical processes with which a landscape responds to tectonic activity or climate change a compilation of world wide denudation rates in non glaciated areas that however does not yet include some of the world s most active mountain belts has resulted in the following findings some of which have been unexpected 1 no obvious relationship between precipitation or mean annual temperature and total denudation is apparent 2 topographic relief alone does not result in high rates of denudation 3 denudation rates are high in areas of landscape rejuvenation that is triggered and controlled by tectonic activity faulting escarpment formation and retreat rifting surface uplift 4 rates of weathering using a combination of cosmogenic nuclides and zirconium normalised cation loss balances co vary primarily with physical erosion rates and much less with temperature or precipitation 5 in some areas of high land use short term rates from river load gauging exceed those from cosmogenic nuclides by several orders of magnitude which serves to highlight the severity of geomorphic change caused by human action in the future the control mechanisms over denudation will be determined on all spatial scales ranging from the single soil section to entire river basins the same analysis can be done back through time on well dated terraces lake records and marine sediment cores which is possible with 10be for the past 1 2 my the rates obtained will be used to develop a quantitative understanding of tectonic geomorphologic and geochemical landscape processes which in turn is a prerequisite to design and calibrate models of the response of landscapes to tectonic climate and anthropogenic forcing

emergence of striation patterns in acoustic signals reflected from dynamic surface waves a striation pattern can emerge in high frequency acoustic signals interacting with dynamic surface waves the striation pattern is analyzed using a ray tracing algorithm for both a sinusoidal and a rough surface with a source or receiver close to the surface it is found that part of the surface on either side of the specular reflection point can be illuminated by rays resulting in time varying later arrivals in channel impulse response that form the striation pattern in contrast to wave focusing associated with surface wave crests the striation occurs due to reflection off convex sections around troughs simulations with a sinusoidal surface show both an upward advancing and downward retreating striation patterns that depend on the surface wave traveling direction and the location of the illuminated area in addition the striation length is determined mainly by the depth of the source or receiver whichever is closer in range to the illuminated region even with a rough surface the striation emerges in both directions however broadband 7 13 khz simulations in shallow water indicate that the longer striation in one direction is likely pronounced against a quiet noise background as observed from at sea experimental data

elemental weathering fluxes and saprolite production rate in a central african lateritic terrain nsimi south cameroon abstract the comparison between contemporary and long term weathering has been carried out in the small experimental watershed sew of nsimi south cameroon in order to quantify the export fluxes of major and trace elements and the residence time of the lateritic weathering cover we focus on the hillside system composed of a thick lateritic weathering cover topped by a soil layer this study is built on the recent improvements of the hillside hydrological functioning and on the analyses of major and trace elements the mass balance calculation at the weathering horizon scale performed with the parent rock as reference indicates i strong depletion profiles for alkalis na k rb and alkaline earths mg ca sr ba ii moderate depletion profiles for si p cd cu zn ni and co iii depletion enrichment profiles for al ga ge sn pb lree hree y u fe v cr mn it is noteworthy that i mn and ce are not significantly redistributed according to oxidative processes as it is the case for fe v and cr and ii ge is fractionated compared to silica with enrichment in fe rich horizons the calculations performed for the topsoil with iron crust as parent material reference reveal that the degradation of the iron crust is accompanied by the loss of most of the constituting elements among which are those specifically accumulated as the redox sensitive elements fe v cr and iron oxide related elements like th the overall current elemental fluxes from the hillside system at the springs and the seepage zones are extremely low due to the inert lateritic mineralogy ninety four percent of the whole na flux generated from the hillside corrected from atmospheric deposits 77 mol ha yr represents the current weathering rates of plagioclase oligoclase in the system the other remaining 6 may be attributed to the dissolution of hornblende the silica hillside flux is 300 mol ha yr and can be mostly attributed to the plagioclase and kaolinite dissolution al and ga are exported from the lateritic regolith and maybe due to the dissolution of kaolinite crystals compared to the other immobile elements zr hf nb and th ti is significantly exported among redox sensitive elements fe v cr mn ce only ce and mn are exported out of the hillside system the other elements fe v cr are likely able to be mobilized but over a short distance only rb sr ba ni cu zn are affected by export processes lree and y are exported but in very low amounts in the range from mol ha yr to mmol ha yr while hree and u are exported in negligible quantities a first attempt is carried out to compare the mature ridge top profile from nsimi sew with the immature ridge top weathering profile from the mule hole sew south india developed on similar granodioritic basement in order to get deeper insight into i the contemporary saprolite production rates and ii the combined effect of precipitation in terms of mean annual rainfall mar and evapotranspiration on the aggressiveness of the draining solutions considering i the contemporary na flux as representative of the dissolution of plagioclase crystals and conservative during saprolitization processes and ii steady state of the inter annual recharge r over a 10 years period the current saprolite production rates r are of 22 mm kyr for mule hole sew and 2 mm kyr for nsimi sew respectively even with a very low r mar ratio 0 04 compared to nsimi the chemical weathering at mule hole is active and related to the groundwater exports at mule hole plagioclase crystals are still present in the saprolite and the soil cover leading to a diffuse weathering front the high nsimi r mar ratio 0 2 allows the solution to be still aggressive with respect to the plagioclase and other weatherable minerals at the bedrock interface resulting in their complete breakdown in a few centimetres sharp weathering front leading to a mature saprolite for the nsimi sew if we consider i the low contemporary saprolite production rate 2 mm kyr ii the miocene age average 15 myr of the south cameron plateau landscape and ii the limited movement of africa continent since eocene the long term saprolite production rate should have remained in its lower range from 2 to 10 m myr this suggests that for thick weathering profiles the migration of the weathering front into the bedrock occurs at a relatively uniform rate regardless of present day climatic conditions climate variation leading to the alternation of setup of savanna or humid forest will have an effect on physical erosion rather than chemical erosion for such deep weathering profiles

slope stability change from late pleistocene to holocene in the ulleung basin east sea japan sea abstract a wide variety of mass movement deposits prevail along the entire margin of ulleung basin overlain by a hemipelagic mud unit 1 2 m thick the boundary between the lower turbidite sequence and the upper hemipelagic mud on the basin plain is distinct and approximately coincides with the well known basin wide ash layer ulleung ash of early holocene age in the absence of significant terrigenous sediment input the lithologic change from the turbidite to hemipelagic mud may reflect physical effects of glacio marine eustatic sea level change sea level lowering during the glacial period may have generated excess pore pressure within the sediments caused by reducing hydrostatic confining pressure and facilitated triggering of the deep water slope failure by earthquakes storm waves may also have affected the uppermost slope area to induce small scale slope instabilities during the lowered sea level stand these glacial factors destabilizing the slope appear to have enhanced large scale slope failures during the late pleistocene in contrast the topmost hemipelagic mud attests to a stable slope phase associated with raised sea level in the holocene consistent with the infinite slope stability analyses calculations of the sedimentation rates based on c 14 dates of several ash layers indicate that the pre holocene turbidites on the basin plain had accumulated at an average rate of 40 cm 1000 years with a minimum recurrence interval of 50 years during the late pleistocene mass flux estimations between the slumps and turbidites suggest a predominance of the nondisintegrative mode of slope failures in the ulleung basin in each event of slope failure most of the failed mass rested on the slope as slide slump blocks and only its meager portions were deposited on the basin plain as turbidite layers

sediment geochemical records of productivity and oxygen depletion along the margin of western north america during the past 60 000 years teleconnections with greenland ice and the cariaco basin abstract many sediment records from the margins of the californias alta and baja collected in water depths between 60 and 1200 m contain anoxic intervals laminated sediments that can be correlated with interstadial intervals as defined by the oxygen isotope composition of greenland ice dansgaard oeschger d o cycles these intervals include all or parts of oxygen isotope stage 3 ois3 60 24 cal ka the bolling allerod warm interval b a 15 13 cal ka and the holocene this study uses organic carbon corg and trace element proxies for anoxia and productivity namely elevated concentrations and accumulation rates of molybdenum and cadmium in these laminated sediments to suggest that productivity may be more important than ventilation in producing changes in bottom water oxygen bwo conditions on open highly productive continental margins the main conclusion from these proxies is that during the last glacial interval lgi 24 15 cal ka and the younger dryas cold interval yd 13 11 6 cal ka productivity was lower and bwo levels were higher than during ois3 the b a and the holocene on all margins of the californias the corg and trace element profiles in the lgi b a holocene transition in the cariaco basin on the margin of northern venezuela are remarkably similar to those in the transition on the northern california margin correlation between d o cycles in greenland ice with gray scale measurements in varved sediments in the cariaco basin also is well established synchronous climate driven changes as recorded in the sediments on the margins of the californias sediments from the cariaco basin and in the gisp 2 greenland ice core support the hypothesis that changes in atmospheric dynamics played a major role in abrupt climate change during the last 60 ka millennial scale cycles in productivity and oxygen depletion on the margins of the californias demonstrate that the california current system was poised at a threshold whereby perturbations of atmospheric circulation produced rapid changes in circulation in the eastern north pacific ocean it is likely that the pacific and atlantic oceans were linked through the atmosphere warmer air temperatures during interstadials would have strengthened hadley and walker circulations which in turn would have strengthened the subtropical high pressure systems in both the north pacific and the north atlantic producing increased rainfall over the cariaco basin and increased upwelling along the margins of the californias

tracking the relict bases of marine methane hydrates using their intersections with stratigraphic reflections we describe a three dimensional seismic interpretation approach for identifying the past bases of the methane hydrate stability zone in marine settings the bases commonly crosscut stratigraphic reflections and these lines of intersection can be revealed by maps of seismic amplitude maps for four reflections that are crosscut by the base of the present day offshore mauritania methane hydrate reveal extraordinary curvilinear changes of amplitude that are parallel to one another and extend for as much as 20 km they are interpreted as marking the relict intersections of the bases of the hydrate stability zone and formed by its upward resetting we hypothesize that this was caused by pulsed sedimentation during repeated glacial interglacial cycles over the past 1 25 m y localized deflections in the lines of intersection are indicative of local changes in sediment temperature this technique could provide a wealth of information on local and regional changes in ambient conditions and better estimates of the volumes of methane being released through time it therefore should help test for interdependencies between hydrate dissociation and climate change

plant driven fungal weathering early stages of mineral alteration at the nanometer scale plant driven fungal weathering is a major pathway of soil formation yet the precise mechanism by which mycorrhiza alter minerals is poorly understood here we report the first direct in situ observations of the effects of a soil fungus on the surface of a mineral over which it grew in a controlled experiment an ectomycorrhizal fungus was grown in symbiosis with a tree seedling so that individual hyphae expanded across the surface of a biotite flake over a period of three months ultramicroscopic and spectroscopic analysis of the fungus biotite interfaces revealed intimate fungal mineral attachment biomechanical forcing altered interlayer spacings substantial depletion of potassium 50 nm depth oxidation of the biotite fe ii and the formation of vermiculite and clusters of fe iii oxides our study demonstrates the biomechanical chemical alteration interplay at the fungus biotite interface at the nanometer scale specifically the weathering process is initiated by physical distortion of the lattice structure of biotite within 1 m of the attached fungal hypha only subsequently does the distorted volume become chemically altered through dissolution and oxidation reactions that lead to mineral neoformation

benthic foraminifera associated with cold methane seeps on the northern california margin ecology and stable isotopic composition release of methane from large marine reservoirs has been linked to climate change as a causal mechanism and a consequence of temperature changes during the quaternary and the paleocene these inferred linkages are based primarily on variations in benthic foraminiferal stable isotope signatures few modern analog data exist however to assess the influence of methane flux on the geochemistry or faunal characteristics of benthic foraminiferal assemblages here we present analyses of the ecology and stable isotopic compositions of living rose bengal stained and dead fossil foraminifera 150 mm from cold methane seeps on the slope off of the eel river northern california 500 525 m and discuss potential applications for reconstructions of methane release in the past and present calcareous foraminiferal assemblages associated with calyptogena clam bed seeps were comprised of species that are also found in organic rich environments cosmopolitan paleoceanographically important taxa were abundant these included uvigerina bolivina chilostomella globobulimina a ndnonionella we speculate that seep foraminifera are attracted to the availability of food at cold seeps and require no adaptations beyond those needed for life in organic rich reducing environments oxygen isotopic values of the tests of living foraminiferal assemblages from seeps had a high range up to 0 69 as did carbon isotopic values up to 1 02 many living foraminiferal isotope values were within the range exhibited by the same or similar species in non seep environments carbon isotopic values of fossil foraminifera found deeper in the sediments 18 20 cm however were 4 10 u peregrina and 3 60 b subargentea more negative than living d 13 c values these results suggest that d 13 c values of foraminiferal tests reflect methane seepage and species specific differences in isotopic composition and can indicate temporal variations in seep activity a better understanding of foraminiferal ecology and stable isotopic composition will enhance paleo seep recognition and improve interpretations of climatic and paleoceanographic change 2000 elsevier science b v all rights reserved

geophysical evidence for gas hydrates in the deep water of the south caspian basin azerbaijan new 2 d seismic reflection data from the south caspian sea offshore azerbaijan document for the first time in the deep water up to 650 m of this area the presence of gas hydrates geophysical evidence for gas hydrates consists of a shallow 300 500 m below seafloor zone of pronounced high velocity 2 100 m s as compared with the surrounding sediments 1550 1600 m s this zone appears on the seismic data as a depth limited 200 m thick layer extending down the flank of an elongate structural high and displays seismic blanking effects on the sedimentary section a strong positive polarityorc 0 123u reflector marks the top of this velocity anomaly and is interpreted as the top of the gas hydrate layer similarly a high amplitudeorc 0 11u negative polarity reflector coincides with the base of the highvelocity layer and is interpreted as the base of the hydrate zone both the top and bottom of the hydrate layer approximately parallel the seafloor bathymetry and cut discordantly across the stratigraphic section suggesting that the two reflectors are thermobaric and not stratigraphic interfaces decreasing amplitude with offset at the base of the gas hydrate layer may indicate the accumulation of free gas beneath this interface these gas hydrates fall within the hydrate stability field predicted from thermobaric modeling for the south caspian basin but typically in thinner layers than would be expected from theoretical calculations the minimum predicted water depth that allows hydrate formation is 150 m and the maximum predicted thickness of the gas hydrate stability field is 1350 m q 2001 elsevier science ltd all rights reserved

quantitative detection of methane hydrate through high resolution seismic velocity analysis a laterally extensive high resolution travel time velocity analysis and acoustic wave form inversion were used to quantitatively determine methane hydrate content in deep water sediments of the blake ridge off the southeast u s coast the interval acoustic velocity vp analyses were performed in the p domain by interactively picking the p trajectories of prominent reflections in each of 50 plane wave decomposed common midpoint gathers the reflections correspond to seismic stratigraphic boundaries so that lateral vp changes due to lithology changes are mitigated and vp changes due to changing hydrate content are enhanced two separate interval vp analyses were performed one with thick 0 4 km layers which yielded lower uncertainty but also lower resolution and one with thinner layers 0 1 km yielding higher resolution but slightly larger uncertainties results show no correlation between low sediment reflectivity and vp however in the areas exhibiting a bottom simulating reflector bsr a high vp interval 2 0 km s and 0 15 km thick is seen immediately above the bsr where the bsr is strongest a 256 layer least squares acoustic wave form inversion reveals the bsr to be caused by a vp decrease from 2 0 to 1 5 km s with little or no change in density the inversion also reveals a thin 0 025 km layer of anomalously low vp lying immediately below the bsr two models of methane hydrate distribution are tested each indicating that the volume of methane hydrate in the intervals of elevated vp is up to 25 of the total volume

geological geochemical and microbiological heterogeneity of the seafloor around methane vents in the eel river basin offshore california marine methane vents and cold seeps are common features along continental margins worldwide serving as localized sites for methane release and colonization by microbial and chemosynthetic megafaunal communities the eel river basin erb located on the continental slope off northern california contains active methane vents and seep associated chemosynthetic biological communities cbc on the crests of anticlines in 520 m water depth seep related features on the seafloor have a patchy distribution and include active bubbling vents chemosynthetic clam beds and sulfide oxidizing bacterial mats methane sources supplying local seeps are heterogeneous on all spatial scales and support a large and diverse microbial assemblage involved in the anaerobic oxidation of methane aom to develop a comprehensive understanding of the complex biological geochemical and physical processes associated with and influencing seafloor methane seepage a multidisciplinary approach is required here we present an integrative multidisciplinary study that illustrates the diverse processes associated with seafloor methane seepage within the eel river basin and the complex interactions defining the geochemistry mineralogy and microbiology within this environment

modeling broadband ocean acoustic transmissions with time varying sea surfaces solutions to ocean acoustic scattering problems are often formulated in the frequency domain which implies that the surface is frozen in time this may be reasonable for short duration signals but breaks down if the surface changes appreciably over the transmission time frequency domain solutions are also impractical for source receiver ranges and frequency bands typical for applications such as acoustic communications e g hundreds to thousands of meters 1 50khz band in addition a driving factor in the performance of certain acoustic systems is the doppler spread which is often introduced from sea surface movement the time varying nature of the sea surface adds complexity and often leads to a statistical description for the variations in received signals a purely statistical description likely limits the insight that modeling generally provides in this paper time domain modeling approaches to the sea surface scattering problem are described as a benchmark for comparison the helmholtz inte

an irregular feather edge and potential outcrop of marine gas hydrate along the mauritanian margin the dissociation of marine hydrate that surrounds continental margins is thought to be an agent for past and future climate change as the water depth decreases landwards the base of the hydrate stability zone progressively shallows until hydrate can occur at or immediately below the seabed where an increase in bottom water temperature can cause dissociation but the true extent of these most vulnerable hydrate deposits is unknown here we use exceptional quality three dimensional 3 d seismic reflection imagery offshore of mauritania that reveals a rare example of a bottom simulating reflection bsr that intersects the seabed and delineates the feather edge of hydrate the bsr intersects the seabed at the 636 m isobath but along the 32 km of the margin analysed the intersection is highly irregular intersections and seismic evidence for hydrate less than 4 3 m below the seabed occur in seven small localised areas that are 0 02 0 45 km2 in extent we propose gas flux below the dipping base of the hydrate to these places has been particularly effective the intersections are separated by recessions in the bsr where it terminates below the seabed seawards of the 636 m isobath recessions are areas where the concentration of hydrate is very low or hydrate is absent they are regions that have been bypassed by gas that has migrated landwards along the base of the hydrate or via hydraulic fractures that pass vertically through the hydrate stability zone and terminate at pockmarks at the seabed an irregular feather edge of marine hydrate may be typical of other margins

reconstructing a high paleolatitude mesozoic paleoenvironment from a truncated and deeply buried regolith norwegian north sea abstract the paleoregolith in this study was unconformably overlain by lower cretaceous valanginian marine deposits of the asgard formation and buried below two kilometers of sediments on the utsira high norwegian north sea the upwards intensification of chemical alteration manifested in the replacement of feldspars by clays leaching of base cations and reduced mechanical rock strength in addition to the presence of terrestrial biotitic features suggested weathering during subaerial exposure of the utsira high during the mesozoic the paleoregolith can be divided into three weathering facies altered coherent rock saprock and saprolite whereas the absence of true soil horizons suggested erosional truncation of the regolith before burial possible remnants of the eroded soil were identified as pedogenic mud aggregates in the overlying asgard formation bearing similar compositional characteristics as the saprolite clay mineralogy bulk geochemical composition micromorphology and depth of biotic features were used to reconstruct the paleoenvironmental conditions that prevailed during the formation of the regolith in the mesozoic north sea our results point to a regolith formation under periodically water restricted conditions promoting the co existence of interstratified illite smectite beidellite and kaolinite deep presence of endomycorrhizae the fungal extension of the root structure and precipitation of iron oxide nodules application of recently developed geochemical climofunctions indicated that the regolith formed in a mesic 10 12 c and subhumid 1157 1406 mm yr environment and under strongly acidic soil conditions ph 5 2 5 6

a reactive transport model for marcellus shale weathering abstract shale formations account for 25 of the land surface globally and contribute a large proportion of the natural gas used in the united states one of the most productive shale gas formations is the marcellus a black shale that is rich in organic matter and pyrite as a first step toward understanding how marcellus shale interacts with water in the surface or deep subsurface we developed a reactive transport model to simulate shale weathering under ambient temperature and pressure conditions constrained by soil and water chemistry data the simulation was carried out for 10 000 years since deglaciation assuming bedrock weathering and soil genesis began after the last glacial maximum results indicate weathering was initiated by pyrite dissolution for the first 1000 years leading to low ph and enhanced dissolution of chlorite and precipitation of iron hydroxides after pyrite depletion chlorite dissolved slowly primarily facilitated by the presence of co2 and organic acids forming vermiculite as a secondary mineral a sensitivity analysis indicated that the most important controls on weathering include the presence of reactive gases co2 and o2 specific surface area and flow velocity of infiltrating meteoric water the soil chemistry and mineralogy data could not be reproduced without including the reactive gases for example pyrite remained in the soil even after 10 000 years if o2 was not continuously present in the soil column likewise chlorite remained abundant and porosity remained small if co2 was not present in the soil gas the field observations were only simulated successfully when the modeled specific surface areas of the reactive minerals were 1 3 orders of magnitude smaller than surface area values measured for powdered minerals small surface areas could be consistent with the lack of accessibility of some fluids to mineral surfaces due to surface coatings in addition some mineral surface is likely interacting only with equilibrated pore fluids an increase in the water infiltration rate enhanced weathering by removing dissolution products and maintaining far from equilibrium conditions we conclude from these observations that availability of reactive surface area and transport of h2o and gases are the most important factors affecting rates of marcellus shale weathering of the in the shallow subsurface this weathering study documents the utility of reactive transport modeling for complex subsurface processes such modelling could be extended to understand interactions between injected fluids and marcellus shale gas reservoirs at higher temperature pressure and salinity conditions

the role of disseminated calcite in the chemical weathering of granitoid rocks accessory calcite present at concentrations between 300 and 3000 mg kg 1 occurs in fresh granitoid rocks sampled from the merced watershed in yosemite national park ca usa loch vale in rocky mountain national park co usa the panola watershed ga usa and the rio icacos puerto rico calcite occurs as fillings in microfractures as disseminated grains within the silicate matrix and as replacement of calcic cores in plagioclase flow through column experiments using de ionized water saturated with 0 05 atm co2 produced effluents from the fresh granitoid rocks that were dominated by ca and bicarbonate and thermodynamically saturated with calcite during reactions up to 1 7 yr calcite dissolution progressively decreased and was superceded by steady state dissolution of silicates principally biotite mass balance calculations indicate that most calcite had been removed during this time and accounted for 57 98 of the total ca released from these rocks experimental effluents from surfically weathered granitoids from the same watersheds were consistently dominated by silicate dissolution the lack of excess ca and alkalinity indicated that calcite had been previously removed by natural weathering the extent of ca enrichment in watershed discharge fluxes corresponds to the amounts of calcite exposed in granitoid rocks high ca na ratios relative to plagioclase stoichiometries indicate excess ca in the yosemite loch vale and other alpine watersheds in the sierra nevada and rocky mountains of the western united states this ca enrichment correlates with strong preferential weathering of calcite relative to plagioclase in exfoliated granitoids in glaciated terrains in contrast ca na flux ratios are comparable to or less than the ca na ratios for plagioclase in the subtropical panola and tropical rio icacos watersheds in which deeply weathered regoliths exhibit concurrent losses of calcite and much larger masses of plagioclase during transport limited weathering these results indicate that the weathering of accessory calcite may strongly influence ca and alkalinity fluxes from silicate rocks during and following periods of glaciation and tectonism but is much less important for older stable geomorphic surfaces

combined vertical incidence and wide angle seismic study of a gas hydrate zone blake ridge wide angle ocean bottom seismic data and single channel seismic reflection data were collected in june 1992 over an area where gas hydrates are thought to be extensive on blake ridge offshore south carolina wide angle reflections were observed on four woods hole oceanographic institution ocean bottom hydrophones at offsets up to 15 km results from traveltime inversion show that the bottom simulating reflector bsr that marks the base of the hydrate stability field lies 400 500 m below the seafloor and is overlain by a 200 to 300 m thick layer of average velocity 1 9 km s 1 there is no evidence for significant lateral velocity variation associated with lateral changes in bsr character ray tracing calculations show that the observation of the bsr out to large offsets 7 km constrains the maximum vertical velocity gradient to about 0 5 s 1 amplitude versus offset avo analysis was performed for ranges up to about 6 km and incidence angles of more than 70 the high amplitude of the bsr in the normal incidence data can be explained by two models both of which include a strong negative impedance contrast the hydrate wedge model and the free gas layer model synthetic seismogram calculation for the wedge model using the reflectivity method shows a strong increase in the bsr amplitudes at offsets between 3 and 5 km because of postcritical reflections and diving waves within the wedge this amplitude increase is clearly not observed in the data which shows only a relatively modest increase in amplitude with offset this observation agrees better with the calculated avo for the gas layer model the lack of a distinct reflection from the base of the gas layer implies that the gas layer is not thicker than 25 m and or that the base of the gas layer is a gradational boundary

the impact of humans on continental erosion and sedimentation rock uplift and erosional denudation of orogenic belts have long been the most impor tant geologic processes that serve to shape continental surfaces but the rate of geomor phic change resulting from these natural phe nomena has now been outstripped by human activities associated with agriculture con struction and mining although humans are now the most important geomorphic agent on the planet s surface natural and anthro pogenic processes serve to modify quite dif ferent parts of earth s landscape in order to better understand the impact of humans on continental erosion we have examined both long term and short term data on rates of sediment transfer in response to glacio fl u vial and anthropogenic processes phanerozoic rates of subaerial denuda tion inferred from preserved volumes of sedimentary rock require a mean conti nental erosion rate on the order of 16 m per million years m m y resulting in the accumulation of 5 gigatons of sediment per year gt yr erosion irregularly increased over the 542 m y span of phanerozoic time to a pliocene value of 53 m m y 16 gt yr current estimates of large river sediment loads are similar to this late neogene value and require net denudation of ice free land surfaces at a rate of 62 m m y 21 gt yr consideration of the variation in large river sediment loads and the geomorphology of respective river basin catchments suggests that natural erosion is primarily confi ned to drainage headwaters 83 of the global river sediment fl ux is derived from the high est 10 of earth s surface subaerial erosion as a result of human activity primarily through agricultural practices has resulted in a sharp increase in net rates of continental denudation although less well constrained than estimates based on surviving rock volumes or current river loads available data suggest that present farmland denudation is proceeding at a rate of 600 m m y 75 gt yr and is largely confi ned to the lower elevations of earth s land surface primarily along passive continental margins 83 of cropland erosion occurs over the lower 65 of earth s surface the conspicuous disparity between natu ral sediment fl uxes suggested by data on rock volumes and river loads 21 gt yr and anthropogenic fl uxes inferred from measured and modeled cropland soil losses 75 gt yr is readily resolved by data on thicknesses and ages of alluvial sediment that has been deposited immediately downslope from erod ing croplands over the history of human agriculture accumulation of postsettlement alluvium on higher order tributary channels and fl oodplains mean rate 12 600 m m y is the most important geomorphic process in terms of the erosion and deposition of sedi ment that is currently shaping the landscape of earth it far exceeds even the impact of pleistocene continental glaciers or the cur rent impact of alpine erosion by glacial and or fl uvial processes conversely available data suggest that since 1961 global cropland area has increased by 11 while the global population has approximately doubled the net effect of both changes is that per capita cropland area has decreased by 44 over this same time interval 1 per year this is 25 times the rate of soil area loss antici pated from human denudation of cropland surfaces in a context of per capita food pro duction soil loss through cropland erosion is largely insignifi cant when compared to the impact of population growth

basalt weathering across scales abstract weathering of silicate minerals impacts many geological and ecological processes for example the weathering of basalt contributes significantly to consumption of atmospheric carbon dioxide co2 and must be included in global calculations of such consumption over geological timeframes here we compare weathering advance rates for basalt wd where d and indicate the scale at which the rate is determined and surface area measured respectively from the laboratory to the watershed scales data collected at the laboratory weathering rind soil profile and watershed scales show that weathering advance rate of basalt is a fractal property that can be described by a fractal dimension dr 2 3 by combining the fractal description of rates with an arrhenius relationship for basalt weathering we derive the following equation to predict weathering advance rates at any spatial scale from weathering advance rates measured at the bet scale w d k 0 a d r 2 e e a r t here k0 is the pre exponential factor 1 29 107 mm3 mm 2 yr 1 ea is the activation energy 70 kj mol 1 and a is a spatial constant related to the scale of measurement of bet surface area 10 7 mm the term a d r 2 is the roughness the roughness fractal dimension can be conceptualized as a factor related to both the thickness of the reaction front and the specific surface area within the reaction front however the above equation can also be written in terms of a surface fractal dimension and the hypothetical average grain radius these fractal dimensions provide insight into reaction front geometry and should vary with lithology once the surface area discrepancy has been accounted for using this method we find a one to two order of magnitude range in weathering advance rates measured at any scale or temperature that can be attributed to factors such as changes in erosional regime parent lithology mechanism climate composition of reacting fluid and biological activity our scaled equation when used to predict global basalt co2 consumption based upon global lithologic maps yields an uptake flux 1 75 1013 mol co2 yr 1 within the predicted error of fluxes estimated based upon riverine measurements

observations of gas hydrates in marine sediments offshore northern california biogenic gas hydrates were recovered in shallow cores 6 m deep from the eel river basin in offshore northern california between 40 38 and 40 56 n the gas hydrates contained primarily methane 13c 57 6 to 69 1 and occurred as dispersed crystals small 2 20 mm nodules and layered bands within the sediment these hydrates recovered in sediment at water depths between 510 and 642 m coincide nearly but not exactly with areas showing bottom simulating reflectors bsrs on seismic reflection records this study confirms indirect geophysical and geologic observations that gas hydrates are present north of the mendocino fracture zone in sediment of the eel river basin but probably are absent to the south in the point arena basin this discovery extends the confirmed sites of gas hydrates in the eastern pacific region beyond the peruvian and central american margins to the northern california margin

simulated tomographic reconstruction of ocean current profiles in a bottom limited sound channel tomographic reconstruction of the vertical current profile in a bottom limited sound channel requires solving a difficult ray identification problem an approach to deal with this problem is a ray group method in which received arrival pulses are divided into several ray groups according to the characteristics of the arrival patterns the method is validated using numerically simulated reciprocal acoustic transmission in a synthetic ocean in the luzon strait where the kuroshio current has speeds as high as 1 2 m s for both narrowband and broadband signals four ray groups are found for the synthetic data these are chosen based on arrival time the differential travel time is determined by pairing up the reciprocal arrival peaks and then averaging the differential travel times within the selected time windows compared with the narrowband case the estimated broadband differential travel time is more consistent with that computed from the current magnitude in the synthetic ocean the vertical current profile is reconstructed from the broadband differential travel times by a generalized tikhonov regularization approach the data weighting matrix includes observation error in picking and pairing travel times and model parameter error due to path length uncertainty the time series of the reconstructed current agrees with the synthetic ocean current the fractional residual variance is 0 013 for the surface layer and 0 01 for the entire water column the ray group method mitigates the ray identification problem in the bottom limited environment and could offer valuable data regarding the range integrated current velocity

unraveling weathering episodes in tertiary regoliths by kaolinite dating western ghats india abstract secondary minerals in soils can record climatic changes affecting continental surfaces over geological times their dating should refine our present knowledge about their potential periods of formation as well as their relations with the ongoing change of climate and erosion weathering regimes in the present study twenty kaolinite samples from two lateritic profiles of the karnataka plateau an intensively studied area in the southern india have been dated using electron paramagnetic resonance epr spectroscopy kaolinite ages vary between 0 229 0 24 ma to 40 73 15 37 ma four different groups of age can be identified with ages clustered around 1 0 3 5 9 0 and 39 0 ma these groups of age indicate local preferential weathering periods that coincide with distinct indian climatic events described in independent studies such as monsoon strengthening thus regional or subcontinental factors likely prevailed over global forcing in the imprint of climatic events in the regolith profiles these results confirm that despite their simple mineralogy laterites can contain several relictual and coexisting generations of secondary minerals and that epr dating of kaolinite contributes to unraveling the complex history of continental surfaces over geological periods

a mechanism for the formation of methane hydrate and seafloor bottom simulating reflectors by vertical fluid expulsion bottom simulating reflectors bsr are observed commonly at a depth of several hundred meters below the seafloor in continental margin sedimentary sections that have undergone recent tectonic consolidation or rapid accumulation they are believed to correspond to the deepest level at which methane hydrate clathrate is stable we present a model in which bsr hydrate layers are formed through the removal of methane from upward moving pore fluids as they pass into the hydrate stability field in this model most of the methane is generated below the level of hydrate stability but not at depths sufficient for significant thermogenic production the methane is primarily biogenic in origin the model requires either a mechanism to remove dissolved methane from the pore fluids or disseminated free gas carried upward with the pore fluid the model accounts for the evidence that the hydrate is concentrated in a layer at the base of the stability field for the source of the large amount of methane contained in the hydrate and for bsrs being common only in special environments strong upward fluid expulsion into the hydrate stability field does not occur in normal sediment depositional regimes so bsrs are uncommon upward fluid expulsion does occur as a result of tectonic thickening and loading in subduction zone accretionary wedges and in areas where rapid deposition results in initial undercconsolidation in these areas hydrate bsrs are common the most poorly quantified aspect of the model is the efficiency with which methane is removed and hydrate is formed as pore fluids pass into the hydrate stability field the critical boundary in the phase diagram between the fluid plus hydrate and fluid only fields is not well constrained however the amount of methane required to form the hydrate and limited data on methane concentrations in pore fluids from deep sea boreholes suggest very efficient removal of methane from rising fluid that may contain less than the amount required for free gas production in most fluid expulsion regimes the quantity of fluid moved upward to the seafloor is great enough to continually remove the excess chloride and the residue of isotope fractionation resulting from hydrate formation thus as observed in borehole data there are no large chloride or isotope anomalies remaining in the local pore fluids the differences in the concentration of methane and probably of co2 in the pore fluid above and below the base of the stability field may have a significant influence on early sediment diagenetic reactions

gas hydrates on the northern california continental margin the inner continental margin of northern california is underlain by a well defined and extensive acoustic reflector that crosses other reflectors and mimics the surface of the sea floor this bottom simulating reflector bsr lies at a typical subsurface depth of about 250 m and has been mapped continuously beneath the klamath plateau and upper slope water depths of 800 to 1200 m for a distance of more than 130 km it covers an area of at least 3000 km 2 limited data show that the bsr extends northward into oregon and seaward at least to the base of the slope 3000 m the water depths subsurface depths and pervasive nature of the bsr all suggest that it represents the base of a natural gas hydrate using standard phase boundary diagrams for hydrate stability we estimate the local geothermal gradient within the gas hydrate to be about 55 c km this value is higher than that of most subduction margins and may be a result of the youthfulness of the subducting oceanic crust this acoustically inferred gas hydrate is the first to be mapped on the western conterminous united states continental margin

effects of time varying sea surface in marine seismic data abstracta method of imaging sea surfaces based on marine seismic measurements has recently been developed the imaging technique is based on extrapolating decomposed wavefields obtained from dual sensor streamers to the sea surface where an adequate imaging condition is applied earlier feasibility tests of the method involved only controlled data associated with frozen sea surfaces here the issue of time varying effects will be in focus we introduced a modeling approach based on the kirchhoff helmholtz integral and computed the scattered wavefield from time varying rough sea surfaces e g pierson moskowitz sea surfaces we generated data for a realistic wind speed and verify the robustness of the proposed sea surface imaging technique by taking into account possible effects of moving receivers as well as streamers with variable shape we investigate the feasibility of estimating the surface wave velocity from the spectra of the imaged sea surfaces and finally present a successful application of the

the high frequency backscattering angular response of gassy sediments model data comparison from the eel river margin california a model for the high frequency backscatter angular response of gassy sediments is proposed for the interface backscatter contribution we adopted the model developed by jackson et al j acoust soc am 79 1410 1422 1986 but added modifications to accommodate gas bubbles the model parameters that are affected by gas content are the density ratio the sound speed ratio and the loss parameter for the volume backscatter contribution we developed a model based on the presence and distribution of gas in the sediment we treat the bubbles as individual discrete scatterers that sum to the total bubble contribution this total bubble contribution is then added to the volume contribution of other scatters the presence of gas affects both the interface and the volume contribution of the backscatter angular response in a complex way that is dependent on both grain size and water depth the backscatter response of fine grained gassy sediments is dominated by the volume contribution while that of coarser grained gassy sediments is affected by both volume and interface contributions in deep water the interface backscatter is only slightly affected by the presence of gas while the volume scattering is strongly affected in shallow water the interface backscatter is severely reduced in the presence of gas while the volume backscatter is only slightly increased multibeam data acquired offshore northern california at 95 khz provides raw measurements for the backscatter as a function of grazing angle these raw backscatter measurements are then reduced to scattering strength for comparison with the results of the proposed model the analysis of core samples at various locations provides local measurements of physical properties and gas content in the sediments that when compared to the model show general agreement

organic matter in the franciscan and cedros subduction complexes the problems of instantaneous maturation and missing petroleum in accretionary prisms abstract most accretionary complexes are submarine mountain belts consisting predominantly of sedimentary rock past studies of accretionary complexes have concluded that they have minimal petroleum potential although they do contain potential source rocks owing to their great bulk of sedimentary rock large amounts of petroleum must be generated in accretionary prisms and these hydrocarbons must be trapped in the complexes or vented into the ocean or on land to understand the nature and abundance of organic material in accretionary complexes organic matter in the shales limestones and melange matrix of the franciscan and cedros subduction complexes was studied mostly using rock eval pyrolysis the largest amounts of total organic carbon toc were found in the calera limestone from the san francisco area 1 6 toc terrigenous sedimentary rocks of probable trench or trench slope origin contain from

deepwater canyons an escape route for methane sealed by methane hydrate abstract three dimensional seismic imaging and modelling of gas hydrates from offshore of west africa mauritania shows that submarine canyons on stable continental slopes can capture and release methane that is sealed by methane hydrate we demonstrate this by focussing on a canyon which is 200 km long 2 4 to 3 1 km wide up to 550 m deep and has canyon walls that dip at 25 30 incision of the canyon causes cooling of the surrounding sediment and deepening of the base of the methane hydrate stability zone the base of the hydrate deepens by up to 550 m and also dips at 25 30 parallel to the canyon margins it forms a continuous or semi continuous wall of lower permeability sediment that can be mapped along the canyon margins on the basis of aligned high amplitude reflection terminations theoretically these methane barriers could extend for 10 s to 100 s of kilometres parallel to both canyon walls several free gas accumulations are sealed laterally in this way in a canyon margin free gas zone large failures of the sides of the canyon remove the lower permeability hydrate allowing free methane to leak globally submarine canyons and marine gas hydrates occur in similar places on continental margins and canyons incise to depths that are comparable with the position of base of the methane hydrate stability zone therefore deepening of the base of the hydrate as a result of the cooling effect of canyons should be common and this mechanism for methane trapping and release could be generally applicable to present and past marine methane hydrates

modeling scattering effects from time varying sea surface based on acoustic reciprocity in marine seismic processing the sea surface is often considered as a flat mirror hence the effects of different weather conditions during the acquisition are largely ignored however studies have shown that rough sea surface ghosts can severely damage the 4d signal if not handled properly in data processing in order to account for realistic sea surface effects in processing the impact of time varying rough sea surfaces need to be studied in this work we derive a method for modelling source and receiver ghosts from time varying rough sea surface and their interaction with subsurface reflections this method is based on acoustic reciprocity and leads to integral equations of non stationary wavefields these modelling equations can also serve as a basis for investigating source and receiver deghosting methods for time varying rough sea surfaces our developed modelling algorithm is validated against a frequency domain approach for a frozen rough sea surface for a moving simple sea surface the d

mechanisms of magnesium isotope fractionation in volcanic soil weathering sequences guadeloupe magnesium mg stable isotopes are increasingly used as a weathering proxy in soils and rivers but the impact of the mineralogy of secondary phases on isotopefractionation remains obscure a better understanding of the behaviour of mg isotopes during weathering processes is a mandatory step toward deployment of this new tracer for understanding chemical fluxes exported from the critical zone here we investigate isotopic variations in 26mg in bulk soils and clay fractions relative to their parent andesite in three soilweatheringsequences from guadeloupe formed under contrasting climatic conditions soils formed in drier conditions low precipitation contain smectite whereas soils formed under wet conditions high rainfall are characterized by halloysite and fe oxides or kaolinite all clay fractions have mg isotopic compositions 26mg 0 41 to 0 10 similar to or heavier than their parent andesite 26mg 0 47 supporting the preferential incorporation of heavy mg isotopes in secondary mg bearing clay minerals with the first direct measurements on clay fractions soils with lighter mg isotope compositions have greater quantities of exchangeable mg the data support a contribution from sea spray to the exchangeable mg pool correlated to the soilweathering degree this study highlights for the first time that the soil 26mg not only depend on 26mg of the parent rock and on any fractionation that might occur but also on the mg retention on the exchange complex which could in turn be controlled by external inputs such as sea spray