statistical methods for describing seafloor topography we present a method for objectively characterizing a swath of digitally sampled seafloor topography our method analyzes the distribution of surface slopes by compiling surface normal vectors into a two dimensional histogram using an equal area projection the direction of maximum variance first principal axis of the histogram is used to determine the azimuth of lineations in the topography and the variance is used as a measure of seafloor roughness we apply the method to short sections of sea beam swath data and find that the histogram parameters are effective in describing the behavior of the topography in particular similar patterns are observed for a sequence of histograms derived from data collected over the mendocino and the surveyor fracture zones in the northeast pacific because the method does not require any data modification and is suitable for irregularly shaped sample regions it lends itself to real time analysis

inverse square root dependence of mid ocean ridge flank roughness on spreading rate the topographic roughness of mid ocean ridge flanks is known to increase with decreasing spreading rate1 4 but the exact form of this relationship has not been established as the topographic features that make up ridge flank roughness abyssal hills are created near the axes of mid ocean ridges by faulting and volcan ism5 7 the relationship between roughness and spreading rate may shed light on the process of crustal accretion at spreading centres here i present measurements of ridge flank roughness on profiles crossing the world mid ocean ridge system which show that roughness is proportional to the inverse square root of the spreading rate this result is consistent with some very simple inferences of how the topographic roughness of mid ocean ridge flanks scales with the lithospheric thickness near ridge axes as determined by thermal models

normal faulting and the topographic roughness of mid ocean ridge flanks the ocean floor on the flanks of mid ocean ridges is covered by abyssal hills topographic features elongated perpendicularly to the direction of relative plate motion these topographic features are interpreted as normal fault blocks and or volcanic constructions that originated near the ridge axis and were later rafted onto the ridge flanks by seafloor spreading the purpose of this paper is to quantify the topographic roughness of a profile perpendicular to the strike of a number of normal faults given a fault population and a mechanical model for the response of the lithosphere to faulting we obtain expressions for the variation in root mean square roughness with profile length and for the power spectral density of a profile given three parameters a fault density number of faults per unit length crossed by the profile an average squared fault scarp height and a characteristic length of flexure to keep matters simple we make a number of assumptions and approximations namely that the lithosphere behaves as an elastic plate that faults have an infinite length and a vertical dip that the response of topography to a number of faults is simply the sum of the responses to each fault and that faults have random locations and scarp heights independently chosen from some statistical distribution the theory predicts that the roughness length relationship power spectral density should follow power laws for scales wavelengths less than a characteristic scale proportional to the length scale of flexure we compare the predictions of the theory with actual measurements of mid ocean ridge flank roughness and find good first order agreement in particular we use independent estimates of fault densities average squared fault scarp heights and flexural length scales to predict the topographic roughness of the east pacific rise and we find that normal faulting can explain all the observed topographic roughness nevertheless there are some differences between predictions and observations these differences are likely to be due to processes other than faulting that create topographic relief e g volcanism and to spatial correlations of fault scarp heights despite these shortcomings the approach presented here provides a first step in understanding the topographic roughness signal by quantifying the contribution of the geological processes that generate surface relief

microplate tectonics along a superfast seafloor spreading system near easter island there is a growing easter microplate between the pacific and nazca plates the eastern and western boundaries of the microplate are actively spreading and propagating north and south respectively the northern and southern boundaries are broad transform zones most of the interior of the microplate contains the sort of oblique sheared structures characteristic of zones of transferred lithosphere between propagating and failing spreading centres

heterogeneity spectrum and scale anisotropy in the upper crust revealed by the german continental deep drilling ktb holes crustal heterogeneities and their statistical characteristics bear important information about the dynamic processes and evolution of the crust the velocity well log data from the german continental deep drilling project ktb offer a rare opportunity to measure directly the properties of crustal heterogeneities in this study we first estimated the power spectrum of crustal heterogeneities from the p velocity well logs of the two holes for the first time a power law spectrum of crustal heterogeneities was directly observed in the spatial wavelength range from a few meters to a few kilometers the slope of the vertical 1d power spectrum of the random velocity perturbations is about 1 1 corresponding to a flicker noise random medium the scale anisotropy ratio aspect ratio and the horizontal spectral slope of crustal heterogeneities were also estimated by cross correlation analysis between these two holes to be about 1 8 and 2 0 respectively

deep crustal reflections from a vibroseis survey in northern switzerland abstract in 1982 a vibroseis survey comprising 180 km of reflection profiles was run in northern switzerland in order to investigate the suitability of the crystalline basement for the deposition of highly radioactive waste a configuration was chosen with 144 channels 25 m of geophone spacing 20 s sweeps ranging from 11 to 61 hz and stacking of 4 or 8 sweeps of 3 simultaneous vibrators at twice the geophone spacing the listening time was generally 4 s and at 4 sites it was extended to 11s for the detection of deeper crustal reflectors this survey unravelled the complicated fault and thrust system beneath the swiss folded jura mountains the stack from 4 s to 11 s reveals clearly a strong sloping reflector between 3 0 and 3 5 s which is strong evidence for a pronounced differentiation in the upper crust a series of reflections is observed between 5 8 and 7 2 s the top of which can be correlated with the conrad discontinuity a strong layered signal between 9 0 and 9 5 s is interpreted as reflections from the m discontinuity the main features are compatible with results from nearby refraction surveys in the southern rhinegraben rift system which show a distinct velocity increase of about 0 5 km s in the lower crust at a depth ranging from 15 to 20 km followed by an inversion zone or a laminated structure before reaching the moho at about 27 km depth the correlation of the field recordings with the first 10 s of the up sweep only shows some loss of resolution in the uppermost 3 s because of the lower frequency content of the signal however the lower parts of the sections are nearly identical the fact that the deeper reflectors in the sections can consistently be traced laterally is a strong argument for using this processing technique thus high coverage vibroseis surveys utilizing up sweep can be processed for deep crustal reflections even if the recording time is restricted to the standard 4 s provided the surface static corrections are carried out with high precision

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

morphology and tectonics of the kane transform from sea beam bathymetry data high resolution sea beam data have been used to construct an accurate regional scale bathymetric map of the kane transform a large offset slowly slipping 30 mm yr central north atlantic transform the kane transform trends about 100 nearly orthogonal to the regional trend of the adjacent mid atlantic ridge mar rift valley the transform valley walls are composed of an alternating series of ridges and troughs which parallel the regional trend of the mar the ridges narrow and deepen toward the transform valley over distances of 10 20 km and the intervening troughs widen into distinct closed contour depressions that appear to be relict nodal basins the morphologic elements found on either side of the transform valley exhibit along strike continuity and represent undisrupted displacement markers the swath of terrain sandwiched between these markers is only several kilometers wide and is made up of elongate basins and ridges that integrate to create a distinctive transform parallel grain this interval the transform tectonized zone ttz marks the time averaged location of strike slip tectonism as parcels of crust move from each intersection along the transform interface the ttz is widest 6 km near the eastern ridge transform intersection rti and narrows to a strand less than 3 km wide along the central and western portions of the transform along the eastern portion of the transform valley a narrow 3 km median ridge several hundred meters high can be traced for more than 40 km along the valley floor and a similar but shorter feature is also present near the western rti at both rtis the neovolcanic zone from the adjacent rift valley can be traced as a series of constructional edifices across the entire intersection area to the opposing transform valley wall at the eastern rti the neovolcanic zone wraps around the eastern edge of the nodal basin while at the western rti it appears to bisect a paired nodal basin located in the intersection area a line drawn connecting the ends of each neovolcanic zone lies within the narrow ttz and has a strike of 098 using the bathymetric map of the kane transform as a record of the time integrated tectonic evolution of this large slowly slipping ridge transform ridge plate boundary we have constructed a simple model for the evolution of rtis our model emphasizes the interplay between temporal variations in crustal production rates i e variations in the volume of basaltic melt production along oceanic spreading centers and extensional tectonism this model can explain the existence of nodal basins at rtis and relict nodal basins along transforms and fracture zones as well as the occurrence of regularly spaced ridge and trough topography on the ridge flanks

controls on tungsten concentrations in groundwater flow systems the role of adsorption aquifer sediment fe iii oxide oxyhydroxide content and thiotungstate formation abstract groundwater samples were collected along flow paths within the carrizo sand aquifer southeastern texas and the aquia aquifer coastal maryland for analysis of dissolved tungsten w concentrations along with the major solutes ph and measures of in situ redox conditions e g dissolved fe ii fe iii and s ii concentrations in addition sediment samples were collected from both aquifers to evaluate the solid phase speciation of w tungsten concentrations in the carrizo sand aquifer range from 3 64 to 1297 pmol kg 1 mean sd 248 440 pmol kg 1 with the lowest concentrations reported from the recharge area tungsten concentrations progressively increase down gradient along the flow path within carrizo sand aquifer groundwaters reaching the highest levels in sulfidic groundwater roughly 50 60 km from the recharge zone tungsten is strongly correlated with ph and dissolved sulfide i e s ii concentrations in carrizo groundwaters r 0 78 and 0 95 respectively in aquia aquifer groundwaters w concentrations range between 14 3 and 184 4 pmol kg 1 mean sd 63 9 59 pmol kg 1 and exhibit no or at best weak relationships with other geochemical parameters measured along the flow path e g r 0 4 and 0 08 for w vs ph and s ii respectively the data suggest that the increase in w concentrations in carrizo groundwaters reflects in part ph related adsorption desorption which has been shown to be substantial for groundwaters and lake waters with ph 8 owing to the similar chemical properties of w and mo which form thiomolybdates in sulfidic waters the formation of thiotungstate complexes may also be important in the sulfidic waters of the carrizo aquifer the substantially lower w concentrations in aquia aquifer groundwaters reflect the fact that the maximum groundwater ph never exceeds 8 4 mean sd ph 7 97 0 23 dissolved low s ii concentrations remain low mean sd s ii 0 29 0 16 mol kg 1 and that aquia aquifer sediments have substantially higher fe iii oxide oxyhydroxide contents compared to the carrizo sand aquifer our data indicate that ph related adsorption desorption reactions and the fe iii oxide oxyhydroxide content of aquifer sediments are key controls affecting w concentrations in oxic suboxic and anoxic groundwaters and further that the formation of thiotungstate species may also be important in some anoxic i e sulfidic waters

rates of trace metal and nutrient diagenesis in an intertidal creek bank abstract carbon mineralization in marine sediments is a key process involved in the cycling of carbon nutrients and trace metals however as marine sediments are usually diffusion dominated the pace of element and nutrient cycling is slow because consumption of oxidants and or nutrients in the pore waters via microbial activity often outpaces resupply adding an advective flow component to such a system should change the biogeochemical dynamics considerably numerical simulations show that shallow coastal aquifers affected by tidal forces can establish ground water velocities of up to 7 cm h 1 driving a circulation of sea water through the sediments with subsequent discharge although known to enhance solute exchange the impact of advection on early diagenesis has not received much attention to address this issue we mapped the interstitial water chemistry down to 2 5 m sediment depth along a transect on an intertidal creek bank that is subject to a periodic advective flow additionally a recently developed hydrogeological simulation of the creek bank was applied to calculate ages of the sampled pore waters sample ages obtained were used to quantify flow path integrated production or depletion rates for trace metals nutrients and sulphate we find young sea water percolating relatively fast through sediments close to the creek showing strong signs of alteration whereas pore waters from diffusion dominated regions are less altered the increase in inorganic nutrients and some trace elements along the flow path requires high rates of turnover sulphate molybdenum and uranium are almost completely depleted after 200 days while dissolved inorganic carbon dic ammonia and manganese increase averaged production rates for dic appear to be three times higher when advection dominated the subsurface flow regime our results demonstrate that sites dominated by advection generally show signs of faster rates of diagenetic reactions

causes and consequences of variations in faulting style at the mid atlantic ridge both volcanism and faulting contribute to the rugged topography that is created at the mid atlantic ridge mar and preserved off axis in atlantic abyssal hill terrain distinguishing volcanic from fault generated topography is essential to understanding the variations in these processes and how these variations are affected by the three dimensional pattern of mantle upwelling ridge segmentation and offsets here we describe a new quantitative method for identifying fault generated topography in swath bathymetry data by measuring topographic curvature the curvature method can distinguish large normal faults from volcanic features whereas slope methods cannot because both faults and volcanic constructs can produce steep slopes the combination of curvature and slope information allows inward and outward facing fault faces to be mapped we apply the method to sea beam data collected along the mar between 28 and 29 30 n the fault styles mapped in this way are strongly correlated with their location within the ridge segmentation framework long linear small throw faults occur toward segment centers while shorter larger throw curved faults occur toward ends these variations reflect those of active faults within the axial valley we investigate two different physical mechanisms that could affect fault interactions and thus underlie variations in abyssal hill topography at the mar in the first model only one fault is active at a time on each side of the rift valley each fault grows while migrating away from the volcanic center due to dike injection extension across the fault causes a flexural rotation of nearby inactive faults the amount of stress necessary to displace the fault increases as the fault grows when reaching a critical size the fault stops growing as fault activity jumps inward as a new fault starts its growth near the rift valley this model yields a realistic terracelike morphology from the rift valley floor into the rift mountains the relief is caused by the net rotation accumulated in the lithosphere from the active faults e g 10 reached 20 km from the active fault fault spacing is controlled by lithospheric thickness fault angle and the ratio of amagmatic to magmatic extension we hypothesize that this mechanism may be dominant toward ridge segment offsets an alternative model considers multiple active faults each fault relieves stresses as it grows and inhibits the growth of nearby faults causing a characteristic fault spacing such fault interactions would occur in a region of necking instability involving deformation over an extended area this mode of extension would drive a feedback mechanism that would act to regulate the size of nearby faults we hypothesize that this mechanism may be active in the relatively weak regions of strong mantle upwelling near segment midpoints causing the homogeneous abyssal hill fabric in these regions

quantifying the distribution and transport of pelagic sediments on young abyssal hills the post depositional transport of pelagic sediments on rough seafloor is approximated as a nonlinear diffusive process the topographic slope distribution of the sediment water interface depends primarily on the apparent sediment diffusivity the average sediment load l and the root mean square height of the abyssal hill basement h these parameters have been determined for the arsrp corridor on the west flank of the mid atlantic ridge by fitting theoretical models to the slope distribution observed from center beam hydrosweep data the average value of the apparent diffusivity is h varies between 188 and 285 m with a mean of 230 7 m l is estimated to be as thin as 10 m on 5 ma crust near the ridge crest thickening to values greater than 50 m at 10 15 ma and then decreasing to less than 30 m on 25 ma crust in the western portion of the corridor the decrease in sediment load with age may be associated with a shallower carbonate compensation depth prior to the late miocene

intraplate extensional tectonics of the eastern basin range inferences on structural style from seismic reflection data regional tectonics and thermal mechanical models of brittle ductile deformation the general lack of a correlation between earthquakes and surface faulting in the interior of the western u s cordillera has motivated our efforts to evaluate the geometry structural style and mechanism of normal faulting characteristic of this region of intraplate extension to address the problem we have interpreted over 1500 km of seismic reflection data and constructed detailed cross sections of the upper crust rheological models of the continental crust were calculated to examine the possibility of shallow quasi plastic flow and its influence on faulting in the eastern basin range the western colorado plateau and the middle rocky mountains our data and interpretations have revealed the following styles of cenozoic deformation 1 steep to low angle dip normal faulting along the wasatch fault 2 low angle dip and listric normal faulting possibly associated with movement on preexisting thrusts 3 the occurrence of asymmetric mostly eastward tilted tertiary basins that are bounded by low to moderate dipping planar and listric faults and 4 at least three vertically stacked en echelon low angle reflections in the mid to upper crust that dip gently westward from 3 km beneath the wasatch plateau to over 15 km at the utah nevada border these reflections are interpreted as normal detachment faults the structural style of the pervasive low to moderate angle dipping faults cannot be easily reconciled with classic brittle failure theory but the interpreted termination of normal faults at or above the deeper low angle reflections suggests the presence of shallow zones of ductile deformation that may have accommodated slip an important observation based on interpretations of seismic reflection profiles is that normal fault zones dip more gently in the subsurface than their associated scarps in unconsolidated surficial deposits segment boundaries of the wasatch fault zone apparently coincide with the positions of east trending displacement transfer structures in thrust sheets and the position of the precambrian uinta aulacogen suggesting that preexisting crustal structure partly controls the geometry of extension to examine the influence of ductile deformation quasi plastic flow was modeled for appropriate geotherms of the basin range and colorado plateau crusts these models provide constraints on the depth to the frictional quasi plastic transition that occurs as shallow as 8 km in the eastern basin range this depth also corresponds to the approximate depth above which most accurately determined earthquake foci for small earthquakes m 5 5 appear to nucleate at 10 15 km in or near the brittle ductile transition the rheological modeling suggests that a thermally deforming continental lithosphere plays an important role in the evolution of this extending intraplate region large scale low angle reflections may represent decollements and normal faults at the top of upper crustal zones of reduced shearing strength and higher ductility

synthetic seismograms for a complex crustal model the algorithm of the original reflectivity method has been vectorized and implemented on a cdc cyber 205 computer calculation times are shortened by a factor of 20 to 30 compared with a general purpose computer with a capacity of several million floating point operations per second mflop the rapid calculation of synthetic seismograms for complex models high frequency sources and all offset ranges is a provision for modeling not only particular phases but the whole observed wavefield as an example we model refraction data of the black forest southwest germany and are able to derive rather tight constraints on the physical properties of the lower crust

east pacific rise from siqueiros to orozco fracture zones along strike continuity of axial neovolcanic zone and structure and evolution of overlapping spreading centers we have used the sea beam multibeam echo sounding system to survey the east pacific rise epr from 8 20 n to 18 30 n obtaining complete coverage of the epr axial neovolcanic zone and all intervening transform faults here we focus on the epr neovolcanic zone and the definition of overlapping spreading centers ocs s between the orozco and siqueiros transform faults the neovolcanic zone is narrow 0 5 2 0 km and continuous along strike and occurs within and near a strike continuous axial summit graben the neovolcanic zone and summit graben reside along the crest of a volcanic axial high which is 2 10 km wide and which continues uninterrupted along strike for 40 140 km within 20 50 km of an intersection with a transform fault the axial neovolcanic zone narrows and deepens and within only 3 6 km of the intersection the neovolcanic zone turns sharply into the transform valley at seven locations between the siqueiros and orozco transform faults the axial neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast with a classic ridge transform ridge plate boundary however offset rise terminations overlap each other by a distance approximately 3 times greater than their offset they curve toward each other and often one merges into the other along strike separating the osc s is an elliptical overlap basin up to 600 m deep whose long axis is subparallel to the adjacent spreading centers the overlap basin is characterized by volcanic constructional edifices a key difference between these offsets and small transform faults in the atlantic is that there are no transform fault structures in the overlap basin which link the offset rise axes a continuous axial depth profile reveals a long wavelength 20 60 km undulation of the rise axis which may be associated with variations in the magmatic budget along strike the lowest points in the axial depth profile occur near transform faults and at osc s suggesting that these features are associated with along strike minima in the magmatic budget we suggest that major phases of volcanic activity propagate episodically along the axis away from centers of magmatic inflation and that osc s develop where magmatic pulses fail to meet due to misalignment of fracture systems sinuous bends in the rise axis or other heterogeneities transform faults fail to develop on fast spreading centers where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate ridge transform fault pattern the osc geometry is unstable and evolves rapidly and the overlapping rises may propagate at rates faster than the local spreading rate one of the two osc s prevails while the other is abandoned while the locations of osc s may not be fixed relative to the rise axis they may tend to recur near the same place creating scars in older lithosphere which resemble fracture zones but whose fine scale structure is quite different there is excellent agreement between the observed structure of osc s and laboratory studies of osc s using wax models photoelastic studies of overlapping cracks and boundary element modeling of overlapping cracks seismic refraction and reflection near two osc s suggests that large offset osc s have separate magma chambers beneath the offset rise axes while small offset osc s may share a single axial magma chamber three dimensional inversion of magnetic anomalies suggests that significant petrologic anomalies may occur near some osc tips waxing and waning of overlapping magma chambers may give rise to the eruption of highly fractionated feti basalts and associated magnetic anomalies

assessing redox zones and seawater intrusion in a coastal aquifer in south korea using hydrogeological chemical and isotopic approaches a shallow 25 m coastal alluvial groundwater system underneath a paddy field in the yangyang area of south korea was investigated to examine the occurrence of redox processes the aquifer is affected by seawater intrusion and is characterized by a highly reducing environment facilitated by high organic matter in the sediments hydrochemical data with 34s and 18o of sulfate were examined for depth specific groundwater from two multilevel samplers that were installed at seaward yy2 and landward yy1 locations shallow groundwater showed distinct patterns of redox zoning evidence of significant bacterial sulfate reduction bsr was observed throughout the nearly entire depths of the two boreholes while at the depths of active seawater intrusion in yy2 conditions suitable for methanogenesis were never reached thus at yy2 the deep zone of intense bsr was overlain by a zone in which methanogenesis occurred in a low sulfate environment in contrast concurrent bsr and methanogenesis in yy1 occurred at depths with high sedimentary organic matter and low dissolved sulfate due to intensive bsr considerable bsr in the groundwater representing trapped seawater in a clay layer had resulted in a very strong increase of 34ssulfate up to 99 9 the inferred sulfur isotopic enrichment factor e for bsr in the lower part of yy2 was 12 3 while e at yy1 was much higher 45 9 in addition the observed trends of 18osulfate at yy1 indicated significant oxygen isotope exchange of sulfate oxygen with ambient water likely because of lower cell specific rates of bsr and higher sulfur isotope fractionation as indicated by the 34s in contrast there was little evidence of oxygen isotope exchange between water and so42 at yy2 this study indicates that in coastal aquifers with sulfate reducing activity 34s and 18o of sulfate can reveal zones of active seawater intrusion and of trapped seawater this study provides an example of the application of sulfur and oxygen isotope data with hydrochemical and hydrogeologic data to interpret complex redox zonation in an organic rich coastal environment

eocene reorganization of the pacific farallon spreading center north of the mendocino fracture zone during a 1984 sea beam and magnetics survey in the north pacific we collected data in the area north of the mendocino fracture zone where a change in the direction of seafloor spreading occurred between magnetic anomalies 24 and 21 and also conducted detailed surveys of the bends in the surveyor and mendocino fracture zones associated with this change in plate motion we have analyzed these data combined with all available magnetics and bathymetric data from previous cruises to determine whether the ridge reorientation and reorganization occurred by way of rift propagation or rift rotation through differential asymmetric spreading the reorganization began just after anomaly 24 to the north of the surveyor fracture zone but did not begin between the mendocino and surveyor fracture zones until the time of anomaly 22 a 2 5 m y delay produced by the old mendocino transform trend constraining the farallon plate lithosphere between the mendocino and surveyor transforms to move in the old spreading direction until the fracturing through of a new mendocino transform fault in the new orientation most of the reorientation of spreading rift segments occurs within 2 3 m y the entire reorganization requires about 7 m y we find direct evidence for seven propagation episodes accounting for the primary reorientation of 75 of the length of the ridge system and nowhere is the data more consistent with rift rotation than with rift propagation as the principal means of reorientation we conclude that the primary mechanism by which the reorganization occurred was rift propagation assuming that rift propagation can account for all of the ridge reorganization we present forward models of magnetic anomaly profiles which succeed in fitting the data well we then present a model for the tectonic evolution of the reorganization which fits the data well using 11 episodes of both northward and southward propagation

seismic structure of gneissic granitic upper crust geological and petrophysical evidence from the strona ceneri zone northern italy and implications for crustal seismic exploration summary we present a stochastic seismic model of gneissic granitic upper crust by estimating the amplitude probability distribution and the autocorrelation function of the inferred seismic velocity field of the strona ceneri zone northern italy unlike previous investigations of the adjacent ivrea zone and the lewisian gneiss complex scotland the lithologies mapped in the strona ceneri zone cannot be directly correlated with laboratory measured seismic velocities the amplitude probability distribution of the extensive petrophysical database and the relatively uniform mineralogical composition of the lithologies indicate that the velocity distribution in the strona ceneri zone can be approximated as being continuous and gaussian based on this assumption and using the spatial autocorrelation function measured from a digitized geological cross section we have modelled the fine scale structure of the strona ceneri zone as a self affine medium with a fractal dimension of 2 50 0 15 and horizontal and vertical characteristic scales of 2 6 0 4 km and 1 4 0 3 km respectively on near offset shot gather seismic data strona ceneri type upper crustal heterogeneity produces a reflected field with the appearance of incoherent scattered noise characteristic of non reflective transparent upper crust found in extended phanerozoic provinces this type of heterogeneity has a considerable effect on seismic imaging of underlying reflective lower crust individual reflection segments are disrupted and shifted in time by up to several tenths of a second and the boundaries of the reflective zone become distorted and blurred at far offsets seismic scattering from heterogeneity of this type reproduces the commonly observed rapid decay and scattered distribution of pg amplitudes comparable amplitude decay patterns result from layered deterministic upper crustal velocity structures with velocity inversions

robust description of statistically heterogeneous seafloor topography through its slope distribution statistical heterogeneity of abyssal hill properties is often evident in seafloor topography even under periods of relatively constant spreading direction and rate in this paper we relate the statistics of topographic slopes computed on finite spatial scales to the autocovariance function and investigate the practicality of using these functions in describing such heterogeneous abyssal hill terrains for a two dimensional homogeneous surface a direct relation exists between the sample autocovariance and the slope distributions at different spatial scales however for a heterogeneous field characterized by large transient signals the computed autocovariance estimate no longer has a clear statistical interpretation and becomes dominated by the transients in contrast the family of slope distributions can still be used to derive stable descriptors of the field slope statistics are thus useful in deriving a more robust estimate of the autocovariance than the usual sample autocovariance moreover slope statistics may also be used to derive stable estimates of quantities not measurable with the autocovariance function or power spectra such as the statistical asymmetry of features examples of the use of slope statistics and a comparison with autocovariance methods are presented we document and quantify evidence of statistical asymmetry in a region of abyssal hills in the northeast pacific and in a second example the presence of multiple lineations in a region where a fracture zone cuts through abyssal hill terrain

segmentation and morphotectonic variations of the central indian ridge 21 10 s 22 25 s bathymetric magnetic and petrographic data have been used to analyze the segmentation and the morphotectonic variations on the central indian ridge cir between 21 10 s and 22 25 s volcanoes abyssal hills and steep slopes corresponding to fault scarps and to volcanic constructional slopes have been obtained by image processing methods applied to sea beam data in the survey area segmentation of the cir is similar to that of a slow spreading ridge the plate boundary morphology is defined by a linear zone composed of an ensemble of median ridges and elongate depressions hourglass shaped valleys are bounded by second and third order discontinuities located at local along axis depth maxima 400 700 m depth variations and marked by oblique extensional basins and en echelon jogs respectively large throw faults are prevalent toward segment ends and suggest greater amagmatic extension in these places along the axial valley the petrographic characteristics of the dredged basalts are correlated with the morphotectonic segmentation and suggest different magma reservoirs the morphology and structure of the rift valley reflect the three dimensional structure of the axial lithosphere and the geometry of mantle upwelling beneath the spreading center abyssal hills are bounded by both inward and outward facing faults in 1 5 ma crust indicating that the formation of abyssal hills is not solely resulting from a combination of volcanic and tectonic processes within the central part of the ridge but that tectonic processes may also occur as far as 20 40 km from the axis by initiating or reactivating some faults at the outward facing slope of the abyssal hills reconstructions of the plate boundary geometry through time using abyssal hills as isochrons provide evidence to show that the nontransform discontinuity located at 21 47 s with an offset as small as 2 3 km is associated with v shaped off axis traces in the form of alignments of disrupted abyssal hill lineations these reconstructions suggest that a small ridge jump is responsible for the initiation of this discontinuity and that this latter migrates away from areas of higher melt delivery three dimensional inversion of magnetic data shows that this discontinuity has been initiated between 0 780 and 0 984 ma and suggests postaccretionary crustal block rotations within the discordant zones

origin of microbiological zoning in groundwater flows reactive transport modeling helps explain the origin of the microbiological zoning observed in pristine freshwater aquifers zoned aquifers have been described previously as either thermodynamic or kinetic phenomena but neither interpretation has proved fully satisfactory drawing on concepts of population dynamics the modeling reported here offers an alternative explanation of how certain microbes exclude others from zones one functional group maintains conditions under which cells in another group die more rapidly than they can reproduce the modeling also lends support to the idea that a group of microbes that appears to dominate a particular zone in an aquifer may in fact coexist with or even be subordinate to another group

synoptic interpretation of seismic reflection and refraction data summary the structure of the upper lithosphere beneath southern germany northern switzerland and west central utah u s a has been investigated in detail by various geophysical methods a synoptic interpretation of travel time and amplitude data obtained in seismic refraction and wide angle reflection surveys combined with near normal incidence reflection observations now permits the elucidation of the fine structure in a more quantitative and unified manner with this scheme it is possible to unambiguously identify low velocity zones and to deduce velocity gradients if reliable amplitude information is included in the inversion process

influence of upwelling saline groundwater on iron and manganese cycling in the rio grande floodplain aquifer salinity contributions from upwelling groundwater significantly degrade water quality in the rio grande a major source of water for the southwestern usa this study considers the influence of this upwelling water on the geochemistry and microbiology of the rio grande floodplain alluvial aquifer the composition of surface water groundwater and floodplain sediment samples collected from three transects in the socorro basin was examined terminal restriction fragment length polymorphism t rflp was also used to examine microbial biomass samples the distribution of salinity in the floodplain groundwater largely reflects the configuration of local groundwater flow and mixing of two major water sources deeply sourced saline groundwater and river water microbial populations in the shallow aquifer consume o2 and no3 and serve to redistribute metal oxides from the saturated zone to locations of groundwater discharge at the surface and possibly near the water table the upwelling saline groundwater affects floodplain microbial processes by transporting reduced metals and organic electron donors to the alluvial aquifer system this enhances metal reduction in the saturated zone and ultimately metal oxidation at or near the surface geochemical modeling suggests that mixing of the saline groundwater with more dilute water in the floodplain creates conditions more favorable for metal oxidation to occur and thereby influences the distribution of metal oxides

stochastic modeling of seafloor morphology a parameterized gaussian model stochastic methods of analysis are useful for quantifying ensemble properties of small scale bathymetric features such as abyssal hills in this paper we model the seafloor as a stationary zero mean gaussian random field completely specified by its autocovariance function we formulate an anisotropic autocovariance function that has five free parameters describing the amplitude anisotropic orientation and aspect ratio characteristic length and hausdorff fractal dimension of seafloor topography parameters estimated from various seafloor regions by an inversion of sea beam data indicate that the seafloor exhibits a wide range of stochastic characteristics within the constraints of the model synthetic topography can be generated at arbitrary scale and resolution from the gaussian model using a fourier method color images of these synthetics are useful for illustrating the stochastic behavior of the model

a global and regional stochastic analysis of near ridge abyssal hill morphology this paper presents the results of a global and regional stochastic analysis of near ridge abyssal hill morphology the analysis includes the use of sea beam data for the estimation of stochastic parameters up to order 4 these parameters provide important quantitative physical information regarding abyssal hills including their rms height azimuthal orientation characteristic width aspect ratio hausdorff dimension skewness tilt and peakiness the global data set consists of 64 sea beam swaths near the rivera cocos and nazca spreading sections of the east pacific rise the mid atlantic ridge and the central indian ridge in one form of analysis the parameters are averaged among spreading rate bins each of the spreading rate subsets can be identified as unique from the others in at least one aspect the slowest spreading rate subset mid atlantic data exhibit the largest scales rms height and characteristic width and length of abyssal hills these parameters generally decrease as spreading rate increases up to the fast spreading rate data pacific cocos but increase going from fast to very fast pacific nazca spreading rate data this indicates some complexity in the relationship between spreading rate and abyssal hill morphology the plan view aspect ratio is nearly twice as large for the fast spreading rate data than for any of the other subsets and is smallest for the very fast spreading rate data the fractal dimension is nearly identical for all spreading rate subsets the vertical skewness is positive for the slow and medium spreading rate data indicating larger peaks than troughs and negative for the fast spreading rate data indicating larger troughs than peaks the kurtosis or peakiness is everywhere larger than the gaussian value of 3 and tends to be larger in the atlantic than the pacific the tilting parameter provides substantial evidence indicating steeper inward facing slopes in the medium and fast spreading rate data but only marginal evidence for it in the slow spreading rate data from an analysis of correlations among parameters it is found that subsets sometimes behave differently from the entire data set in particular while over the global data set the characteristic width exhibits a well resolved positive trend when plotted versus rms height these parameters exhibit a more gradual positive trend in the mid atlantic data and a negative trend in the pacific cocos data in addition the plan view aspect ratio while generally uncorrelated with rms height for the global data set is positively correlated with rms height for the pacific cocos data set these results emphasize a strong uniqueness of the pacific cocos data relative to the rest of the data global set the pacific cocos data consist of 27 swaths concentrated between the siquieros and orozco fracture zones these data provide very good abyssal hill coverage of this well mapped and well studied region and form the basis of a regional analysis of the correlation between ridge morphology and stochastic abyssal hill parameters in this analysis it is found that abyssal hill parameters are correlated with the depth of the adjacent ridge axis indicating that the relative abundance in magma supply which likely controls the ridge axis depth may also be an important influence on the formation of abyssal hills

quantitative methods for analyzing the roughness of the seafloor the bathymetry of the world ocean has been mapped using a variety of acoustic sounding instruments and traditional contouring methods prior to this study relatively few quantitative studies have been focused on the detailed morphology of the seafloor the work of most investigators has generally dealt with the analysis of seafloor relief without regard to spatial frequency or concern for obvious morphologic anisotropies a brief review of previous work is presented and is accompanied by an identification of the specific applications and limitations of those approaches the availability of more precise digital sounding data coupled with the development of higher resolution sonar mapping instruments e g deeptow seamarc now make it possible to investigate quantitatively many important aspects of seafloor relief and the geological processes responsible for its formation typical bathymetric contour maps represent a low frequency deterministic model of the seafloor to describe the higher frequency variability or roughness of the seafloor requires the development of an appropriate statistical method for generating a valid stochastic model new methods are developed herein which allow valid statistical models of the variability of oceanic depths to be derived from existing digital bathymetric soundings the smooth contoured surface often preserved as a geographic grid of depths when supplemented by such a roughness model can provide an essentially complete statistical description of the relief statistical models of seafloor roughness are also valuable tools for predicting acoustic scattering and bottom loss and in addition contain a wealth of information for more comprehensive interpretations of deep sea relief forming geological processes to allow the variability of depths to be described as a function of scale spatial frequency the amplitude spectrum is employed as the fundamental statistic underlying the model since the validity of the amplitude spectrum depends upon the assumption of a statistically stationary sample space a computer algorithm operating in the spatial domain was developed which delineates geographic provinces of limited statistical heterogeneity within each of these provinces a spectral model is derived by fitting the amplitude estimates with one or two two parameter power law functions using specialized regression techniques the distribution of resultant model parameters is examined for a large test area adjacent to the coast of oregon 42 45 n 130 124 w which includes several contrasting geologic environments the distribution of roughness corresponds generally with the various physiographic provinces observed in the region within some provinces additional complexities are apparent in the roughness model which cannot be inferred by simply studying the bathymetry these patterns are related to a variety of geological processes operating in the region such as the convergence of the continental margin and the presence of a propagating rift on the northern gorda rise in contrast a very large area of the continental margin off the east coast of the united states was found to have in common a single distinct amplitude spectrum this amplitude spectrum is almost identical to that found from the tufts abyssal plain region off the west coast of the united states spectra from both of these areas can be clearly separated into two straight line segments of different slope the two segments of these spectra are interpreted as reflecting two dominant relief forming processes the higher frequency band representing a sedimentary regime and the lower frequency band representing an underlying tectonic volcanogenic regime in many cases the calculated roughness statistics are not constant for data collected along different ship track directions this is due to the anisotropic nature of the seafloor relief a simple model is developed which describes the roughness statistics as a function of azimuth the parameters of this model quantify the anistropy of the seafloor allowing insight into the directionality of the corresponding relief forming processes and the physical meaning of the derived model statistical parameters finally the model is used to successfully predict the roughness of a surface at scales much smaller than those resolvable by surface sonar systems the model regression line derived from a hull mounted sonar is compared to data from deep towed sonars and bottom photographs the amplitude of roughness is predicted to within half an order of magnitude over five decades of spatial frequency and this prediction capability can probably be improved even further the stochastic models presented also demonstrate the potential for closely approximating the full two dimensional nature of some areas of the seafloor with only four statistical parameters these parameters can be estimated from random ship track data given a sufficient number of tracks and range of headings

practical geological comparison of some seafloor survey instruments seafloor survey instruments are integral to the study of marine geology because understanding their resolution and limitations is critical we compare how different survey systems represent the seafloor coincident data collected at the galapagos propagator gloria seamarc ii sea beam deep tow camera sled and alvin allow comparisons of how well seafloor features e g faults and volcanoes observed and characterized in high resolution data are represented in lower resolution coarser scale data sets our reported values for the minimum sizes of detected and well represented features show that practical geological resolutions are generally 2 10 times lower than theoretical resolutions care must be taken in evaluating which system to use to address a particular problem

pelagic sedimentation on rough seafloor topography 2 inversion results from the north atlantic acoustic reverberation corridor seafloor slope distributions are sensitive to basement topography the surface shape of sediment distributions and the average thickness of sediment within a region by comparing slope distributions computed from bathymetric data to distributions from realistic models of sedimented topography we determine how characteristic values of average sediment thickness l rms basement variability h and apparent diffusivity vary throughout the office of naval research acoustic reverberation corridor located on the western flank of the mid atlantic ridge north of the kane fracture zone 25 5 27 25 n 44 5 49 w values of l from basin centered subregions show thinner sediments near the axis thickening westward with maximum values found on 10 20 ma seafloor lower values of l are found for older late oligocene early miocene seafloor values of l on young seafloor suggest a sediment accumulation rate of 6 5 2 3 m m y since 10 ma values of h computed from hill centered regions range from 217 to 306 m with an average of 267 27 m hills are generally larger to the north of a low running through the study area values of range from 0 12 to 0 77 m2 yr with an average of 0 32 0 19 m2 yr lower values of kbetter describe the surface shapes of individual high standing ponds while higher values are needed to model the surface shapes of thick deposits in regional lows this difference in with geological setting is probably due to variations in sediment transport mechanisms

geochemical cycling of mercury in a deep confined aquifer insights from biogeochemical reactive transport modeling total dissolved mercury concentrations hgt were measured in groundwaters collected along a flow path within a deep confined aquifer i e the carrizo sand in southeastern texas usa along with major solutes ph and redox sensitive parameters that include dissolved iron species fe ii fe iii and sulfide s ii concentrations mercury concentrations range from below detection 0 43 pmol kg 1 to close to 8 pmol kg 1 and increase 6 fold along the first 13 km of the flow path coincident with the region of the aquifer where dissimilatory fe iii reduction buffers the redox conditions with groundwater flow beyond the fe iii reducing zone and into the region of the aquifer dominated by sulfate reduction the dissolved hgt concentrations decrease by close to a factor of 3 dissolved hgt is positively correlated with dissolved fe ii concentrations r 0 61 and to a lesser extent with dissolved organic carbon doc concentrations r 0 54 and inversely related to both ph r 0 56 and dissolved s ii concentrations r 0 41 although this latter relationship is not statistically significant solution complexation modeling indicates that hg is chiefly complexed with dissolved sulfide and that organic complexes with hg are generally negligible biogeochemical reactive transport modeling demonstrates that the distinct zones of high fe ii followed by high s ii groundwaters observed along the flow path are consistent with microbially mediated fe iii reduction followed by so42 reduction that occurs sequentially along the flow path furthermore the modeling suggests that hg is released to carrizo sand aquifer groundwaters by dissimilatory reduction of the fe iii oxides oxyhydroxides goethite and hematite which are common in the aquifer occurring as coatings on sand grains chiefly quartz subsequently the model suggests that hg is removed from carrizo sand groundwater further down gradient along the flow path by adsorption onto goethite and hematite adsorption onto or co precipitation with fe sulfide minerals like mackinawite sorption onto sedimentary organic matter and or precipitation of hgs s may also play important roles in hg removal from carrizo sand groundwaters

cenozoic and mesozoic structure of the eastern basin and range province utah from cocorp seismic reflection data cocorp seismic reflection data collected from the eastern basin and range in west central utah provide information on cenozoic extensional tectonics mesozoic thrusting and their interrelationships those data show a series of remarkably continuous low angle reflectors that extend more than 120 km perpendicular to strike and can be traced as deep as 15 20 km over that distance none of these events are significantly cut by any high angle normal faults a major detachment beneath the sevier desert can be traced from a surface zone of normal faulting to a depth of 12 15 km with a regional apparent westward dip of 12 tentative correlation of upper and lower plate events suggests 30 60 km of extensional displacement on this detachment whether this structure is a reactivated mesozoic thrust is uncertain west steepening splays off the end of the detachment reach depths of 20 km and may represent a major mesozoic ramp or zones of distributed ductile shearing during extension some events are interpreted to be mesozoic thrusts of which at least one beneath the house range has been reactivated during the cenozoic the snake range decollement dips gently east and has a sense of cenozoic displacement opposite to that of other cenozoic detachments farther east deep events are most numerous beneath the east side of the sevier desert where they occur to depths of 30 km at the top of or perhaps partly within the anomalously low velocity upper mantle

interpretation of subhorizontal crustal reflections by metamorphic and rheologic effects in the eastern part of the pannonian basin summary the geologic origin of subhorizontal reflections often observed in crustal seismic sections was investigated by establishing metamorphic facies and strength of rocks in depth and correlating these properties to seismic reflection sections from eastern hungary estimation of the depths of metamorphic mineral stability zones utilized the principles developed by fyfe et al and known geothermal data of the area the strength versus depth profile was derived by relating local seismic p wave interval velocities to meissner et al s activation energy the results show that the series of subhorizontal reflections observed in the pannonian basin are a consequence of combined metamorphic and rheologic changes in depths the synthesis of the integrated data set suggests that the retrograde alteration of the pre tertiary basement above the percolation threshold was made possible by the softening effect of shear zones and their water conducting capacity the subhorizontal reflections of highest energy of the consolidated crust below the percolation threshold originate in the depths of greenschist amphibolite and granulite metamorphic mineral facies which were formed in geothermal and pressure conditions similar to those existing today these results imply the overprint of earlier variscan metamorphic sequences of the crust by more recent retrograde metamorphic processes

controls on the geochemistry of rare earth elements in sediments and groundwaters of the aquia aquifer maryland usa abstract groundwater samples were collected along two flow paths from the aquia aquifer in maryland usa to investigate the rare earth element ree geochemistry of the flow system as a function of evolving groundwater composition neodymium nd concentrations of aquia aquifer groundwater range from 56 6 to 71 4 pmol kg 1 along the eastern shore flow path i e eastern shore of the chesapeake bay and from 41 5 to 77 9 pmol kg 1 on the western shore flow paths and consequently fall within the range reported for other circumneutral to alkaline ph groundwaters shale normalized ree patterns of eastern and western shore aquia groundwaters are similar i e relatively flat and do not vary substantially along either flow path for example yb nd nasc for aquia aquifer groundwaters exhibit a limited range between 0 7 and 1 1 aquia aquifer sediments were analyzed for rees following a sequential extraction procedure neodymium concentrations for the bulk aquifer sediment range from 26 to 70 mol kg 1 3 7 to 10 1 ppm or a factor of 2 5 to 7 lower than the nd content of the upper continental crust the sequential extractions reveal that the majority of the rees in the aquifer sediments occur associated with the abundant carbonate shell fragments that are ubiquitous within the aquia aquifer sediments furthermore the sequential extraction data indicate that the light rees lree are more readily exchangeable i e outer sphere surface complexed on aquifer mineral surfaces than the heavy rees hree and the hrees and middle rees mree exhibit a greater association with fe mn oxides oxyhydroxides within the sediments analysis of the abundant glauconite pellets within aquia aquifer sediments indicates an overall enrichment in the hrees relative to the lrees for this mineral compared to shale our data suggest that the chief source of rees to aquia aquifer groundwater is dissolution of carbonate biominerals shell fragments that occurs in the recharge zone and before groundwaters become saturated with respect to calcite and aragonite additionally weakly sorbed rees outer sphere surface complexed contribute lrees and mrees to aquia aquifer groundwater as ph increases above 8 leading to the relatively flat shale normalized ree patterns that characterize aquia aquifer groundwaters once released to the solution ree form strong aqueous complexes with carbonate ions lnco 3 and ln co 3 2 with the dicarbonato complexes i e ln co 3 2 predominating the ree are subsequently transported with flowing groundwater in the aquia aquifer as the negatively charged dicarbonato complexes exhibit a low affinity to sorb to negatively charged mineral surfaces in the aquifer sediments

physicochemical gradients diffusive flux and sediment oxygen demand within oil sands tailings materials from alberta canada abstract the athabasca oil sands contain one of the world s largest oil reserves consisting of approximately 168 billion barrels of currently recoverable bitumen with 20 recoverable through open pit mining methods this extraction process produces a considerable amount of fluid fine tailings fft waste material which must be deposited on site in tailings ponds these ponds allow the waste sand clay and residual bitumen to settle out of the water column allowing for the water to be recycled for use again in the extraction process it is vital to gain a better understanding of the processes contributing to the development of physicochemical gradients ph eh oxygen etc that form in these tailings ponds over time with the goal of remediation and subsequent construction of end pit lake systems once oil extraction has ceased to differentiate between the impacts of biotic and abiotic processes in fresh newly processed material and mature fft 38 year old tailings over a 52 week study a specific experimental design was utilized in accordance with novel microsensor profiling techniques the sulfide diffusive fluxes within mature biotic systems measured 37 6 mol m 2 day 1 at the onset of the experiment decreasing over time as fes mineralization progressed in addition do fluxes also showed strong correlation to the physical affects of consolidation and overall biological consumption of o 2 at the fft water interface this holistic study comparing different tailings pond materials provides insight regarding biotransformation and physicochemical controls effecting sediment oxygen demand associated with reclaimed wetlands and end pit lake development

seafloor topography a record of a chaotic dynamical system we suggest that limits on the number of independent variables that play a role in controlling the shape of the seafloor can be obtained directly from bathymetry data to obtain this information seafloor topography is considered as a product of a larger system that produces the oceanic lithosphere we hypothesize that this dynamical system is chaotic and use bathymetric depth values to search for evidence of a strange attractor the dimension of which approximates the number of degrees of freedom in the system an analysis of sea beam multibeam data collected along a flow line of seafloor spreading in the northeast pacific ocean indicates that the dynamical system that forms the topography on the length scale of 100 s of meters has an underlying structure indicative of a high order attractor

fault controlled genesis of the chilung sea valley northern taiwan revealed by topographic lineaments abstract the chilung sea valley lies parallel to the coastline off northern taiwan it connects the coastal area to the chilung shelf through a descending rugged surface this valley is about 65 km in length 7 15 km in width and ends once across the shelf break off the santiaochiao coast in northeastern taiwan the descending gradient of the chilung valley floor is approximately 1 500 being very close to that of the chilung shelf this implies that the valley might have originated from a solely tectonic event that acted on the shelf surface on the basis of a well controlled bathymetric data set collected in the study area some topographic lineaments were extracted tracing these topographic lineaments that connect to a series of the ne sw orientated faults extending from on land taiwan into the ocean we identified three right lateral strike slip faults truncating these lineaments the occurrence of these right lateral strike slip faults is suggested to result from compressive shearing under the orogeny of the convergence between the philippine sea plate and the eurasian plate the shearing upon the fault plane has been inhomogeneous along the path of the chilung valley and has affected the shape of the valley

pelagic sedimentation on rough seafloor topography 1 forward model this is the first paper in a four part sequence exploring the factors that govern the distribution of pelagic sediments in regions of the world mid ocean ridge system where basement outcrops and sediment deposits both contribute to local topography in this paper we present a simple numerical model of pelagic sedimentation in which sediments travel from topographic highs to lows with flux regulated by a diffusivity parameter by applying the forward model to realistic models of basement topography we investigate how various basement structures sediment rain rates and values of affect the morphology of the seafloor through time we quantify the effect of sedimentation as a nonlinear smoothing filter on topography which acts to reduce spectral power at high wavenumber to decrease the root mean square variability of topography to increase the spacing between hills and to lower the relative proportion of high slope seafloor we find that local accumulation rate is highly dependent on local topography and varies greatly with time we also find that sediment compaction does not contribute directly to the surface concavity exhibited by many intrahill sediment ponds

stochastic modeling of seafloor morphology inversion of sea beam data for second order statistics at scale lengths less than 100 km or so statistical descriptions of seafloor morphology can be usefully employed to characterize ridge crest processes off ridge tectonics and vulcanism sedimentation and postdepositional transport we seek to develop methods for the estimation of seafloor statistics that take into account the finite precision resolution and sampling obtained by actual echo sounding systems in this initial paper we restrict our attention to the problem of recovering second order statistics from data sets collected by multibeam devices such as sea beam the seafloor is modeled as a stationary zero mean gaussian random field completely specified by its two point covariance function we introduce an anisotropic two point covariance function that has five free parameters describing the amplitude orientation characteristic wave numbers and hausdorff fractal dimension of seafloor topography we formulate the general forward problem relating this model to the statistics of an ideal multibeam echo sounder in particular the along track autocovariance functions of individual beams and the cross covariance functions between beams of arbitrary separation using these second moments as data functionals we then pose the inverse problem of estimating the seafloor parameters from realistic noisy data sets with finite sampling and beamwidth and we solve this inverse problem by an iterative linearized least squares method the inversion method is applied to sea beam transit data from both the pacific and atlantic oceans sea beam system noise stands out as a sharp spike on the along track autocovariance function and can be modeled as a white noise process whose amplitude generally increases with beam angle the five parameters in our second order model can be estimated from the inversion of data sets comprising 100 200 km of track length in general the cross track wave number is the most poorly determined although uncertainties in the assumed sea beam response may bias the values of the fractal dimension using the assumed beamwidth the measured noise values and the seafloor parameters recovered from the inversion we generate sea beam synthetics whose statistical character can be directly compared with raw sea beam data for most of the track segments we have processed thus far the synthetics are similar to the data in the case of one atlantic profile however the comparison clearly indicates the necessity of incorporating higher order statistics the space domain procedures described in this paper can be extended for this purpose

bacterial sulfate reduction limits natural arsenic contamination in groundwater natural arsenic contamination of groundwater increasingly recognized as a threat to human health worldwide is characterized by arsenic concentrations that vary sharply over short distances variation in arsenic levels in the mahomet aquifer system a regional glacial aquifer in central illinois appears to arise from variable rates of bacterial sulfate reduction in the subsurface not differences in arsenic supply where sulfate reducing bacteria are active the sulfide produced reacts to precipitate arsenic or coprecipitate it with iron leaving little in solution in the absence of sulfate reduction methanogenesis is the dominant type of microbial metabolism and arsenic accumulates to high levels

bathymetric gradients of lineated abyssal hills inferring seafloor spreading vectors and a new model for hills formed at ultra fast rates abstract abyssal hill morphology provides a preliminary measure of the direction and rate of seafloor spreading however additional information e g magnetic anomaly data or a nearby mid ocean ridge is usually required to verify these estimates previous attempts to identify a unique spreading rate proxy from abyssal hill dimensions e g height length width have largely failed due to the relatively large scatter of data or the non linear character of spreading rate trends we present a new stand alone method of determining both spreading rate and spreading direction using the distribution of azimuths for slopes facing toward and away from the ridge axis the spreading rate is determined with the peak height parameter defined as the difference in the height maximum frequency of the two dominant modes observed in the azimuthal histograms this parameter exhibits a clear nearly linear spreading rate trend and allows half spreading rates to be estimated to within 10 20 km myr the spreading direction is determined with the peak width parameter which compares the average width of the two dominant modes in the azimuthal histograms the wider distribution of slope azimuths is oriented away form the paleo ridge axis for all spreading rates and thus spreading direction can be determined the trends in the peak height and width parameters are used to constrain a new model of abyssal hill formation at ultra fast spreading rates which require greater off axis extensional faulting resulting in a few large throw faults on the outward facing hillsides and many smaller throw faults on the inward facing hillsides

a stochastic model for the creation of abyssal hill topography at a slow spreading center the present study is an attempt to model abyssal hill topography i e the randomly varying topography that is superimposed on the subsidence of the ocean floor due to thermal contraction we assume that abyssal hill topography is created by faulting and that it can be represented as the convolution of a series of stochastic fault offsets with a fault impulse response as a constraint the power spectrum of the model generated topography will have to resemble the power spectrum of a profile of abyssal hill topography across a typical slow spreading ridge such as found in the south atlantic using a series of fault offsets positively correlated in space and fault impulse responses limited to a finite width w profiles that statistically resemble abyssal hill topography have been generated if the fault offsets are constrained to be positive i e to cause uplift the synthetic profiles contain a steady state depression at the axis the fit of the synthetically derived topography to the observed topography is very sensitive to the width w of the fault impulse response our results suggest that the effects of a faulting event must be limited to distances of about 3 5 km and that tectonic activity must take place in the immediate vicinity of the inner wall of the axial valley if faulting is indeed the process that creates abyssal hill topography the best fit flexural impulse response to faulting implies a flexural rigidity of 5 1017 n m if instead constructional volcanism contributes significantly to the topography our study suggests that the emplacement of volcanic constructions must also take place within a narrow width w about the axis

quantitative characterization of abyssal hill morphology along flow lines in the atlantic ocean the morphology of the seafloor when sampled along flow lines of relative plate motion represents an indirect geologic time series record of the processes which form it eventually we hope to be able to interpret this information so that we may improve our understanding of the workings of ridge crest and offridge processes through time this study takes an important step toward that goal by quantitatively characterizing through stochastic modeling the abyssal hill morphology along the available sea beam flow line data sets in the atlantic the six sea beam flow line data sets used in this study include two near the kane fracture zone and four near the rio grande and moore fracture zones stochastic parameters estimated from these flow lines such as the mis height lineament azimuth characteristic widths and planview aspect ratio are plotted as a function of crustal age in addition to testing available hypotheses the purpose of this work is to investigate how the abyssal hill morphology changes with age and to correlate this behavior to known changes in ridge characteristics with age these results place important constraints on our understanding of the process of seafloor creation and begin to establish a baseline against which morphology may be interpreted in the absence of other information along the kane fracture zone variations in the estimated abyssal hill lineament azimuth rms height and plan view aspect ratio with crustal age appear to be correlated in various ways with the segmentation history and also possibly with large changes in plate motion direction that have occurred there within the past 80 m y along the rio grande and moore fracture zones variations in the lineament azimuth rms height and other parameters as a function of crustal age are often significant on a 5 10 m y time scale these variations are difficult to interpret as yet to emphasize longer time scale variations the estimated rms heights from the south atlantic flow lines are averaged over common age bins this demonstrates a strong negative correlation of rms height with the spreading rate however when characteristic widths are averaged in this manner no strong correlation with spreading rate is found large variations in this parameter occur on a 25 m y time scale

geochemical and microbiological zonation of the middendorf aquifer south carolina we consider the nature and origin of the chemical and microbiological zonation of the cretaceous middendorf aquifer in the coastal plain of south carolina in light of the chemical evolution of groundwater along a flow path there some types of microbial activity most notably aerobic respiration and iron reduction lead to a net generation of acid as co2 but reaction of groundwater with minerals in the aquifer consumes the acid driving up ph and controlling the evolution of the groundwater s major ion chemistry the thermodynamic energy available to the various functional groups of microbes except the autotrophic hydrogen oxidizing methanogens is everywhere along the flow path sufficient to drive microbial growth the concentrations of acetate formate and lactate do not vary systematically from zone to zone along the flow path as might be expected if each zone were dominated by a single functional group of microbes the overall rate of microbial respiration in the aquifer appears to be limited by the rate of the initial fermentation of organic matter we found no compelling evidence indicating that one functional group of microbes excludes others from any of the aquifer s zones sulfate reduction for example may be non existent in the high iron zone associated with iron reduction or may account for up to 90 of the respiration occurring there zonation of the groundwater into high iron and low iron facies may result from a minor change along the aquifer in the balance between the activities of iron and sulfate reducers