shear induced convective mixing in bottom boundary layers on slopes field data from a stratified lake demonstrate that buoyancy driven convective mixing can be an important mechanism for the formation of well mixed bottom boundary layers bbls on slopes convective turbulence is caused by differential transport of stratified water masses along the slope of a basin because the current velocity usually decreases toward the sediment water at some distance from the bottom is transported faster than is water below during upslope flow this process leads to transport of heavier water on top of lighter water and hence to unstable stratification within the bbls analogously strong bbl stratification occurs during downslope flow high frequency acoustic doppler current profiler adcp and temperature measurements in the bbls of a lake revealed the cyclic occurrence of convective turbulence driven by periodic across slope currents of internal seiching the estimated maximum buoyancy flux within the bbls was in good agreement with the highest observed dissipation rates of turbulent kinetic energy this suggests that in our specific case classical bottom shear production and the mentioned buoyancy driven convective production contributed by similar amounts to the turbulent kinetic energy

active methanogenesis and acetate utilization in powder river basin coals united states coal deposits of the powder river basin prb in wyoming united states contain significant reserves of natural gas proven to be of biogenic origin the timing of gas generation in this basin and many others is either unknown or questionable luca technologies evaluated the prospect that methanogenesis is ongoing in prb coals using a combined laboratory and field approach the anaerobic bioconversion of coal to methane in laboratory experiments was observed in multiple samples indicating that indigenous microorganisms can generate methane from prb coal acetate was found to be a quantitatively important intermediate in methane formation the detection of microbial metabolites including acetate in coalbed methane formation waters and the ability of prb methanogenic consortia to convert the detected metabolites to methane is also consistent with in situ methanogenesis field tests revealed in situ microbial acetate utilization and significant dilution dispersion and migration of injected water and solutes in the coal seam this is believed to be important for contacting significant coal surface area and important for the measurement of in situ microbial activities collectively this information indicates that methanogenesis is active in the coal seams evaluated in the prb and that all of the methane present in the coals need not be a remnant of past methanogenesis

asymmetric tidal mixing due to the horizontal density gradient abstract stratification and turbulent mixing exhibit a flood ebb tidal asymmetry in estuaries and continental shelf regions affected by horizontal density gradients the authors use a large eddy simulation les model to investigate the penetration of a tidally driven bottom boundary layer into stratified water in the presence of a horizontal density gradient turbulence in the bottom boundary layer is driven by bottom stress during flood tides with low gradient ri and flux rf richardson numbers but by localized shear during ebb tides with ri and rf 0 2 in the upper half of the boundary layer if the water column is unstratified initially the les model reproduces periodic stratification associated with tidal straining the model results show that the energetics criterion based on the competition between tidal straining and tidal stirring provides a good prediction for the onset of periodic stratification but the tidally averaged horizontal richardson number rix has a threshold value of abou

internal tides and tidal cycles of vertical mixing in western long island sound in estuaries tidal period variations in the rate of vertical mixing have been observed to result from various causes in liverpool bay and the york river they have been attributed to tidal straining of the along channel density gradient modulating stratification in the hudson river they arise from tidal modulation of the height of the tidal current bottom boundary layer bbl along continental shelves tidal period fluctuations in mixing have been observed to result from the dissipation of internal waves iws western long island sound wlis moored instrument records indicate that large near bottom increases in dissolved oxygen do and heat and a decrease in salt occur during the middle of the flood tide an analysis of water mass signatures indicates that the transport involved is vertical and not horizontal temperature data from a vertical thermistor array deployed in the wlis for 16 days in august 2009 clearly show a tidal cycle of iw activity creating a mean thermocline depression at midflood of approximately 25 of the water depth with individual iw thermocline depressions of as much as 50 of the water depth contemporaneous adcp measurements show increases in shear due to iws during the flood near bottom internal wave activity is maximal at and after midflood and is correlated with near bottom temperature and do tendencies at both tidal and subtidal scales we conclude that internal tides are an important vertical mixing mechanism in the wlis through both increased shear from iws and displacement of the pycnocline into the region of high shear in the bbl

the formation of turbidity maximum zones by minor axis tidal straining in regions of freshwater influence abstractthis study investigates the influence of tidal straining in the generation of turbidity maximum zones tmz which are observed to extend for tens of kilometers along some shallow open coa

high calcification rates and inferred metabolic trade offs in the largest turritellid gastropod turritella abrupta neogene abstract turritella abrupta miocene pliocene is the largest species known from the diverse jurassic recent gastropod family turritellidae in addition to having achieved long length and substantial width the species produces shells of exceptional thickness even before secondary shell deposition we investigated the paleoecology of this species through analysis of drilling and peeling frequencies with comparisons to co occurring turritellid species we used oxygen isotopic sclerochronology to infer the growth rate and lifespan of t abrupta and to make comparisons with other modern and fossil tropical species including the living species turritella terebra with the first sclerochronologies for both of these species presented herein we find that t abrupta was fast growing long lived and comparatively effective at resisting predation t abrupta was notably widespread geographically and temporally but was a relatively rare component of faunas when compared with co occurring turritellids high rates of shell carbonate construction in excess of 35 g in the first year of life likely depended on habitats conducive to calcification late miocene cooling and eastern pacific carbonate limitation are implicated in the range contraction and eventual extinction of this species

wintertime mixed layer measurements at maud rise weddell sea 1 sea ice plays a crucial role in the exchange of heat between the ocean and the atmosphere and areas of intense air sea ice interaction are important sites for water mass modification the weddell sea is one of these sites where a relatively thin first year ice cover is constantly being changed by mixing of heat from below and stress exerted from the rapidly changing and intense winds this study presents mixed layer turbulence measurements obtained during two wintertime drift stations in august 2005 in the eastern weddell sea close to the maud rise seamount turbulence in the boundary layer is found to be controlled by the drifting ice directly measured heat fluxes compare well with previous studies and are well estimated from the mixed layer temperatures and mixing heat fluxes are also found to roughly balance the conductive heat flux in the ice hence little freezing melting was observed the under ice topography is estimated to be hydraulically very smooth comparison with a steady 1 d model shows that these estimates are made too close to the ice ocean interface to be representative for the entire ice floe the main source and sink of turbulent kinetic energy are shear production and dissipation observations indicate that the dynamics of the under ice boundary layer are influenced by a horizontal variability in mixed layer density and an increasing amount of open leads in the area

storm surge induced flux through multiple tidal passes of lake pontchartrain estuary during hurricanes gustav and ike abstract in september 2008 hurricanes gustav and ike generated major storm surges which impacted the lake pontchartrain estuary in louisiana this paper presents analyses of in situ measurements acquired during these storm events the main data used in the analyses were from three bottom mounted moorings equipped with conductivity temperature and depth sensors acoustic doppler current profilers adcps and a semi permanent laterally mounted horizontal acoustic doppler profiler adp these moorings were deployed in the three major tidal channels that connect lake pontchartrain with the coastal ocean a process similar to tidal straining was observed the vertical shear of the horizontal velocity was negligible during the inundation stage but a shear of 0 8 m s over a less than 5 m water column was recorded during the receding stage 2 3 times the normal tidal oscillations the surge reached its peak in the industrial canal 1 4 2 1 h before those in the other two channels the inward flux of water lasted for a shorter time period than that of the outward flux the inward flux was also observed to have much smaller magnitude than the outward flux 960 1200 vs 2100 3100 million m 3 the imbalance was believed to have been caused by the additional water into lake pontchartrain through some small rivers and inundation over the land plus rainfall from the hurricanes the flux through the industrial canal was 8 12 while the flux through the other two tidal passes ranged between 17 and 70 of the total but mostly split roughly half half of the remaining 88 92 of the total

bioassay for estimating the biogenic methane generating potential of coal samples abstract generation of secondary biogenic methane in coal beds is likely controlled by a combination of factors such as the bioavailability of coal carbon the presence of a microbial community to convert coal carbon to methane and an environment supporting microbial growth and methanogenesis a set of treatments and controls was developed to bioassay the bioavailability of coal for conversion to methane under defined laboratory conditions treatments included adding a well characterized consortium of bacteria and methanogens enriched from modern wetland sediments and providing conditions to support endemic microbial activity the contribution of desorbed methane in the bioassays was determined in treatments with bromoethane sulfonic acid an inhibitor of microbial methanogenesis the bioassay compared 16 subbituminous coal samples collected from beds in texas tx wyoming wy and alaska ak and two bituminous coal samples from pennsylvania pa new biogenic methane was observed in several samples of subbituminous coal with the microbial consortium added but endemic activity was less commonly observed the highest methane generation 80 mol methane g coal 56 scf ton or 1 75 cm 3 g was from a south tx coal sample that was collected from a non gas producing well subbituminous coals from the powder river basin wy and north slope borough ak contained more sorbed original methane than the tx coal sample and generated 0 23 mol g up to 16 scf ton or 0 5 cm 3 g new biogenic methane in the bioassay standard indicators of thermal maturity such as burial depth nitrogen content and calorific value did not explain differences in biogenic methane among subbituminous coal samples no original methane was observed in two bituminous samples from pa nor was any new methane generated in bioassays of these samples the bioassay offers a new tool for assessing the potential of coal for biogenic methane generation and provides a platform for studying the mechanisms involved in this economically important activity

sediment pumping by tidal asymmetry in a partially mixed estuary 1 observations collected at two laterally adjacent locations are used to examine the processes driving sediment transport in the partially mixed york river estuary estimates of sediment flux are decomposed into advective and pumping components to evaluate the importance of tidal asymmetries in turbulent mixing at the instrumented location in the estuarine channel a strong asymmetry in internal mixing due to tidal straining is documented with higher values of eddy viscosity occurring during the less stratified flood tide as a result of this asymmetry more sediment is resuspended during the flood phase of the tide resulting in up estuary pumping of sediment despite a net down estuary advective flux at the instrumented location on the adjacent shoal where no pronounced tidal asymmetry in internal mixing was found both the pumping flux and advective flux were directed down estuary the down estuary pumping of sediment on the shoal appears to be driven by asymmetries in bed stress the impact of tidal asymmetries in bed stress at the channel location was negated because the amount of sediment available for resuspension was limited as a result the pumping flux was dominated by the overlying asymmetries in internal mixing the asymmetries in stratification appear to exert an important control on the vertical distribution of sediment by both impacting the eddy diffusivity as well as the fall velocity during the more turbulent flood tide the fall velocities are smaller suggesting the kolmogorov microscale is setting the upper bound on floc diameter

observations of stratified turbulent mixing in an estuary neap to spring variations during high river flow abstract observations of turbulent mixing stratification and currents were made with a free falling microstructure profiler and a 600 khz adcp in the hudson river off manhattan in may 1994 the ship was anchored in a relatively straight and uniform channel about 15 m deep over 4 days of intense observations from 19 to 24 may 1994 the progression from neap tides to spring tides caused a severe reduction in stratification from an initial top to bottom salinity difference of 18 to 3 6 while low gradient richardson numbers ri occurred only in the weakly stratified bottom layer on flood during neap tides low ri spanned the water column on the second half of ebb during spring tides turbulent dissipation rates varied roughly inversely to ri during neap conditions turbulent mixing was intense only in the bottom layer on flood where eddy diffusivities k reached more than 10 2 m 2 s 1 while mixing was weak in the centre of the halocline near 5 m during spring tides strong mixing occurred throughout the water column during the latter part of ebb with typical k 5 10 3 m 2 s 1 and typical vertical salt flux of 0 0015 kg m 2 s 1 if it existed in isolation this flux would eliminate the haline stratification on a time scale of 4 h ebb time bands of low ri and intense mixing showed increasing phase lag with increasing distance from the bottom the spring tide ebb time internal turbulent stress was relatively uniform with depth with a typical magnitude of 0 4 pa 4 dyne cm 2 turbulent overturning thorpe scales ranged from below 1 cm in the neap tide halocline to a few metres in the weakly stratified bottom layer on flood the strong spring tide ebb time mixing had thorpe scales of typically 0 5 m throughout the water column ozmidov scales tracked thorpe scales except in the weakly stratified layer above the bottom

modulation of the semidiurnal cycle of turbulent dissipation by wind driven upwelling in a coastal embayment with two 25 hour series of turbulent microstructure and currents observations carried out in august 2013 during spring chaos 1 and neap tides chaos 2 we investigated the semidiurnal cycle of turbulent dissipation in an embayment affected by coastal upwelling ria de vigo nw iberia at the time of sampling the bay hosted a net wind driven bi directional positive exchange flow and thermal stratification turbulent kinetic energy tke dissipation e at the interface between upwelled and surface waters was enhanced by two orders of magnitude during the ebbs 10 6 w kg 1 with respect to the floods 10 8 w kg 1 this pattern was caused by the constructive interference of the shear associated with the upwelling and tidal currents the vertical structure of the tidal currents was consistent with a deformation of tidal ellipses by stratification which was tightly coupled to the intensity of upwelling this two pronged interaction resulted in a modulation of the semidiurnal cycle of turbulent dissipation by coastal upwelling thus as a result of the upwelling relaxation conditions experienced during chaos 1 depth integrated interior tke dissipation rates were higher by a factor of 2 compared to chaos 2 by using a simple model we determined that observed variations in turbulent mixing had a limited influence on the tidal variations of stratification which were dominated by straining and advection the mixing mechanism described here is potentially relevant for the ecology of upwelling bays as it can stimulate the transport of nutrients from deep upwelled waters to the sun lit surface layers where primary production takes place

intratidal variations in stratification and mixing in the hudson estuary this study represents a detailed examination of intratidal variations in boundary layer structure in a relatively narrow straight section of the hudson river estuary during high river discharge the presence of active mixing is inferred from estimates of the richardson number the maximum height above bed to which mixing occurs is comparable during flood and ebb however the regions of mixing differ in density structure during the flood active mixing is confined to a well mixed near bed layer whereas during the ebb active mixing occurs throughout regions of significant stratification tidal straining maintains stratification during the ebb while it promotes the growth of a uniform bottom mixed layer during the flood in general maximum and minimum near bed stratification occur during late ebb and flood respectively reflecting the dominant role played by tidal straining in determining intratidal variation vertical stretching was also found to be an important term in the stratification balance during the ebb

the vertical structure of turbulent dissipation in shelf seas abstract the free fall fly profiler has been used to determine the variation in energy dissipation in the water column over a tidal cycle at mixed and stratified sites in the irish sea it was found that exhibits a strong m4 variation with a pronounced phase lag that increases with height above the bed in mixed conditions this m4 signal which extends throughout the water column is reasonably well reproduced by turbulent closure models of the vertical exchange in the summer stratified situation the m4 signal in is confined to about 40 m above the seabed with phase delays of more than 4 h relative to the seabed the lowest levels of dissipation 10 5 w m 3 measured in the pycnocline are significantly above the system noise level and much higher than predicted by a model using the mellor yamada level 2 closure scheme my2 0 however when allowance is made for the diffusion of tke the model my2 2 simulates the depth time distribution of dissipation in the stratified case satisfactorily if

a dynamic equation for the potential energy anomaly for analysing mixing and stratification in estuaries and coastal seas abstract in this paper a time dependent dynamic equation for the potential energy anomaly is rigorously derived from dynamic equations for potential temperature and salinity the continuity equation and the equation of state for sea water the terms locally changing are a the advection b the depth mean straining c the non mean straining d the vertical advection e the vertical mixing f surface and bottom density fluxes g inner sources of density e g due to absorption of solar radiation and the non linearity of the equation of state and h horizontal divergence of horizontal turbulent density fluxes in order to derive the equation in concise form a vertical velocity linearly varying with depth with respect to depth proportional vertical coordinates had to be defined the evaluation of the terms in the equation is then carried out for a one dimensional tidal straining study and a two dimensional estuarine circulation study comparisons to empirical estimates for these terms are made for the one dimensional study it is concluded that the equation provides a general reference for empirical bulk parameterisations of stratification and mixing processes in estuaries and coastal seas and that it is a tool for complete analysis of the relevant terms from numerical models

the role of advection in determining the temperature structure of the irish sea abstract the temperature structure of the irish sea is investigated using a 3 yr simulation with a high resolution 1 8 km three dimensional baroclinic model the proudman oceanographic laboratory coastal ocean modelling system and ctd and advanced very high resolution radiometer observations this paper focuses on the extent to which horizontal advection determines the temperature structure it is found that it has a significant effect on the depth mean temperatures throughout the region and on the vertical profiles in seasonally stratified areas such as the celtic sea and western irish sea there is depth mean advective heating during the summer in these stratified regions whereas in well mixed regions advection tends to reduce the amplitude of the seasonal cycle through an analysis of the terms in the temperature equation the warming of the cool pool waters of the western irish sea can be attributed to the advection of partially well mixed waters into the stratified region from the north th

the effect of periodic stratification on floc size distribution and its tidal and vertical variability geum estuary south korea abstract to understand the spatiotemporal variability in the floc size distribution the factors controlling floc size and the timing of flocculation with sediment flux in the periodically stratified geum estuary korea simultaneous measurements of current salinity suspended sediment concentration ssc and floc size distributions were analyzed and floc size was compared with stratification turbulence ssc and sediment flux salinity measurements indicated that the water column was stratified during ebb and well mixed during flood consistent with the process of strain induced periodic stratification coincident with periodic stratification was an ebb flood asymmetry in floc size where coarser multimodal and variable distributions occurred during the stratified ebb whereas finer unimodal and relatively constant distributions occurred during the well mixed flood vertically flocs coarsened downward during the stratified ebb and were vertically homogenous during the well mixed flood the ebb flood asymmetry in floc size was due to the ebb flood asymmetry in stratification and turbulence furthermore while floc size decreased with turbulence the ssc increased with turbulence as a consequence large flocs occurred with relatively small ebb directed fluxes whereas small flocs occurred with relatively large flood directed fluxes overall these observations indicate that the variability of the floc size distribution in periodically stratified estuaries is different than in well mixed estuaries and that ebb directed fluxes may be more flocculated than flood directed fluxes in these systems

comparison of fluid geochemistry and microbiology of multiple organic rich reservoirs in the illinois basin usa evidence for controls on methanogenesis and microbial transport abstract microbial methane in sedimentary basins comprises approximately 20 of global natural gas resources yet little is known about the environmental requirements and metabolic rates of these subsurface microbial communities the illinois basin located in the midcontinent of the united states is an ideal location to investigate hydrogeochemical factors controlling methanogenesis as microbial methane accumulations occur 1 in three organic rich reservoirs of different geologic ages and organic matter types upper devonian new albany shale up to 900 m depth pennsylvanian coals up to 600 m depth and quaternary glacial sediments shallow aquifers 2 across steep salinity gradients and 3 with variable concentrations of so 4 2 for all three organic rich reservoirs aqueous geochemical conditions are favorable for microbial methanogenesis with near neutral ph so 4 2 concentrations 80 c in nutrient poor environments necessitating the re introduction of microbes into the subsurface via groundwater transport

evidence of tidal straining in well mixed channel flow from micro structure observations 1 this study presents for the first time micro structure observations in tidally energetic weakly stratified regimes obtained in the wadden sea a tidal shallow coastal area in the south eastern north sea characterised by barrier islands separated by tidal gulleys the tidal currents are typically overlaid by a weak horizontal density gradient due to freshwater run off from land the observations in an energetic tidal channel clearly show the expected effects of tidal straining destratification during flood and increased stratification during ebb microstructure observations are consistent with the tidal straining dynamics during the flood relatively high values of viscous dissipation are observed whilst during ebb the values are substantially smaller it is also shown that the tidal cycle of stratification and destratification depends on the position in the tidal channel in parts of the channel increased stratification occurs during full flood a phenomenon which can only be explained by advection of stratified water masses formed outside the tidal channel the observations presented here show the general significance of the tidal straining process for tidally energetic weakly stratified regimes

field observations of hypersaline runoff through a shallow estuary abstract this study investigates a rare situation at the mond river estuary in the persian gulf in which the classical estuarine density gradient coincides with hypersaline runoff entering from saline soils upstream of the estuary after severe precipitation this builds a unique estuarine setting where two salt water masses one originating from the coastal ocean and the other being discharged from upstream confine a range of almost freshwater in the middle of estuary this freshwater lens estuary fle situation includes two saltwater sources with opposing senses of estuarine circulation therefore the tidal damping by the strong river flood can occur especially during neap tide when high unsteadiness number 0 04 signified ebb oriented condition which was induced by straining residual lateral circulation near the fle mouth transition from well mixed to weak strain induced periodic stratification regimes indicated the importance of the spring neap tidal variations close to the mouth a 13 66 day periodic tidal asymmetry from the triad k1 o1 m2 ebb dominance during spring tide and flood dominance in neap tide was overcome by higher harmonics

measurements of the turbulent energy dissipation rate e and an evaluation of the dispersion process of the changjiang diluted water in the east china sea 1 the large amount of fresh water discharged from changjiang spreads over the east china sea continental shelf as low salinity water particularly in the summer season to understand the primary production in the east china sea it is necessary to clarify the dispersion process of the changjiang diluted water cdw since the high production area appears to cover the cdw in this study the dispersion process of the cdw was evaluated through the estimation of the turbulent energy dissipation rate e in the cdw and calculation of the divergence in the cdw using satellite tracked drifters a large value of e was observed just above the bottom and in the surface layer whose magnitude was sometimes greater than 10 7 w kg 1 on the other hand a small value of e less than 10 8 w kg 1 was found just beneath the cdw the small value of e leads to a small value of the vertical eddy diffusivity of less than 10 5 m 2 s 1 the vertical advection of the bottom water is likely to be a major process for increasing the salinity since the horizontal diffusion would not play a significant role if the usual magnitude of the horizontal diffusivity is assumed calculation of divergence in the surface layer using the drifters data also suggests that upward velocity of o 10 5 m s 1 over the cdw area is expected the vertical transport from the bottom layer could increase the salinity of the cdw and also carry the nutrients into the euphotic zone resulting in biological production

combined effects of wind tide and horizontal density gradients on stratification in estuaries and coastal seas abstract the aim of this one dimensional water column study is to combine modified versions of three characteristic parameters for periodic tidal flow under the influence of a longitudinal buoyancy gradient the horizontal richardson number the inverse strouhal number and the inverse ekman number into a parameter space study including constant wind forcing from various directions it is shown how the underlying dynamical equations can be cast into nondimensional form depending mainly on these three nondimensional parameters plus the relative wind speed and the wind direction idealized model simulations are carried out for the whole realistic range of horizontal richardson and inverse strouhal numbers for various latitudes showing the amplitude of the tidally induced stratification for a wide range of scenarios it is found that classical threshold values for the horizontal richardson number indicating the switch from periodic stratification to permanent stratification are valid only for special cas

boundary mixing in lakes 1 modeling the effect of shear induced convection 1 recent studies in lakes have shown evidence for a strong influence of shear induced stratification on mixing in turbulent bottom boundary layers bbls on sloping topography these observations suggest that the periodic near bottom shear resulting from internal wave motions may lead to alternating periods of gravitationally stable and unstable stratification in the bbl with relevant implications for turbulence and mixing in the entire basin the impact of these processes for basin scale mixing is investigated here in a three dimensional processes oriented modeling study using a well investigated system lake alpnach switzerland as an example consistent with available observations our results indicate that the bbl becomes gravitationally unstable in areas with upslope flow covering a substantial fraction of the total bottom area of the lake while near bottom convection associated with the unstable stratification in these areas results in strong turbulence its contribution to net mixing is negligible since the bbl is already well mixed conversely in areas with downslope flow the near bottom shear generates stable stratification leading to a suppression of turbulence but also to larger mixing rates due to an enhanced mixing efficiency overall bbl mixing is found to dominate basin scale mixing these mechanisms are likely to be important for a large class of stratified natural waters in which boundary layer mixing is energized by periodic internal waves or basin scale motions

effects of tidal straining on the semidiurnal cycle of dissipation in the rhine region of freshwater influence comparison of model and measurements 1 the rhine region of freshwater influence in the north sea is a region of the coastal ocean that exhibits strong cross shore density gradients interacting with the progressive kelvin tidal wave which generates mixing when mixing is low enough to allow stratification to develop the stratification modifies the tidal currents and generates a strong tidal shear that will strain the density gradients generating a semidiurnal cycle of stratification observations of turbulent kinetic energy dissipation during calm conditions show that the bottom half of the water column has a predominantly m 4 cycle higher values of occurring during flood flow are associated with the residual flow in that direction following the strong peak in dissipation near the bed enhanced dissipation is apparent extending up the water column in the upper part of the water column shows a predominantly m 2 variation with the maximum occurring at a time when vertical stability is negligible and the effect of tidal straining is to create instability in the water column as the cross shore shear forces higher salinity water over fresher water the inference is that potential energy released by straining is responsible for convective motions and hence the enhanced turbulent activity observed the cycle of stratification and dissipation has been simulated using a one dimensional mellor yamada levels 2 and 2 2 turbulence closure model the model correctly predicts the amplitude and phase of the stratification while the model is in qualitative agreement in the description of the dissipation and its variability the absolute values of appear to be too high

morphology of crab predation scars on recent and fossil turritellid gastropods abstract when nonlethal attempted predation breaks the aperture of a gastropod shell the break is preserved as a scar which is frequently visible in the fossil record such scars are very frequently observed on fossil and living turritellidae a family of high spired marine gastropods but little is known about which predators make such scars or how they do so if the form of these scars on fossil shells could be better interpreted biologically a large data set of predation scars might become available for analysis we experimented with live turritellids turritella banksi and four species of crabs from the family xanthidae panopeus sp eurypanopeus planus leptodius taboganus and xanthodius sternberghii in panama in order to investigate factors contributing to the breakage morphology resulting from crab predation on turritellid shells qualitative examination of scar morphology resulting from attacks by different crab species shows that particular crab species can cause distinctively shaped scars although some shapes of scars can be created by more than one crab species multivariate analysis of these scars reveals that scar morphologies arising from different crab species fall on overlapping continua in morphospace incorporating the shapes of fossil scars into these analyses reveals that fossil scars are similar to many of those created in the aquaria and that scar shape can be accurately predicted by predator species in particular scars caused by panopeus can be very similar to some fossil scars although the particular crab species used in the experiments probably do not prey on turritellids in the wild the data on causes of break scar morphology and crab turritellid predation behavior allow information of predation stored in the scars on fossil turritellids to be used to explore the history of predation on this important group of gastropods

recent developments in the study of ocean turbulence abstract this review describes the changes in the understanding of turbulence and mixing in the upper layers of the ocean the abyssal ocean and in continental shelf seas that have come in the past 25 years largely through advances in instruments and methodology

saline intrusion in partially mixed estuaries abstract restricting interest to partially mixed estuaries earlier studies of tidally averaged linearised theories relating to the vertical structure of salinity and velocities accompanying saline intrusion are extended to take account of tidal straining and associated convective overturning the applicability of these theories is evaluated by reference to a single point numerical model in which the time varying cycle of depth averaged tidal current amplitude u and a temporally and vertically constant saline gradient sx are specified this model highlights the importance of convective overturning in counteracting unstable density structures introduced by tidal straining by omitting overturning in the model results agree closely with linearised theoretical derivations however incorporating overturning substantially increases tidally averaged surface to bed differences for both residual currents u and salinity s the vertical structure of tidal currents is a maximum and hence the effect of tidal straining in shallow macro tidal estuaries the propagation of tidal elevations and currents remains insensitive to saline intrusion in partially mixed estuaries the applicability of the model was evaluated by simulation of recent measurements by rippeth et al j phys oceanogr 31 2001 2458 to explore the generality of estuarine responses the model was run for a wide range of values of saline intrusion lengths l and water depths d additional sensitivity analyses were made for changes in u and bed stress coefficient k response frameworks are shown for u s potential energy anomaly work done by bed friction and internal shear rates and efficiency of saline mixing and ratios of relative mixing by diffusion to overturning by equating the rate of mixing associated with vertical diffusion with river flow q an expression for saline intrusion length l d 2 k u u o uo river flow velocity was derived this formulation agrees with an earlier derivation based on flume tests and showed reasonable agreement with observed values in six estuaries eight cases however in funnel shaped estuaries axial migration of the intrusion introduces major variations in d u and uo thereby complicating the application of the above expression for l moreover the time lag involved in the adjustment of l to changes in u and q may explain much of the complexity encountered in the observations

semidiurnal switching of stratification in the region of freshwater influence of the rhine observations in the rhine region of freshwater influence rofi system in the north sea show evidence of large semidiurnal oscillations in stability superimposed on a mean stratification occurring throughout the stratified region at times of reduced mixing the amplitude of this semidiurnal variation is of the same order as the mean stability and frequently results in conditions being mixed or nearly mixed once per tide it is deduced that this semidiurnal variation results primarily from cross shore tidal straining which interacts with the density gradient to induce stratification this conceptual picture of the contributing processes has been tested in a one dimensional point model forced by the observed slopes and the local density gradients the model exhibits the same qualitative behavior as the observations produces oscillations in stratification of the amplitude observed and confirms the critical role of cross shore tidal straining the large cross shore shear nder stratified conditions is identified with the changes in ellipse configuration which are observed between mixed and stratified conditions the occurrence of semidiurnal variations in stability in the rhine rofi is thus inferred to be a consequence of the development of mean stability whenever the horizontal density gradients relax in conditions of low stirring

boils and turbulence in a weakly stratified shallow tidal sea abstract measurements of turbulence are made in a weakly but variably stratified region of tidal straining in the eastern irish sea using turbulence sensors profiling vertically through the water column on the fast light yo yo fly profiler and horizontally on the autonomous underwater vehicle auv autosub the tidal currents exceed 1 m s 1 at the location of the measurements in water of a depth of about 43 5 m and result in turbulence extending from the seabed to the surface with a cycle period that is half that of the tides as previously observed the time of onset of enhanced turbulence that is measured by the sensors on fly and autosub as the speed of tidal currents increases are in good agreement as are their mean levels boils on the sea surface are identified using the autonomously recording inverted echo sounder version 2 aries ii a two beam upward pointing side scan sonar mounted on a rig resting on the seabed the boils have mean horizontal dimensions of about 25 m they are continually

gas potential of new albany shale devonian mississippian in the illinois basin a study to update and evaluate publicly available data relating to present and potential gas production from new albany shale in the illinois basin was conducted cooperatively by the indiana illinois and kentucky geological surveys illinois basin consortium and was partially funded by the gas research institute deliverables included a plate of stratigraphic cross sections and six basin wide maps at a scale of 1 1 000 000 the new albany shale is an organic rich brownish black shale present throughout the illinois basin gas potential of the new albany shale may be great because it contains an estimated 86 tcf of natural gas and has produced modest volumes since 1858 from more than 60 fields mostly in the southeastern part of the basin reservoir beds include organic rich shales of the grassy creek shale clegg creek and blocher shale members limited geologic and carbon isotope data indicate that the gas is indigenous and thermogenic t sub max data suggest that the gas generation begins at r sub o values of 0 53 and may begin at r sub 0 values as low as 0 41 in some beds new albany shale reservoirs contain both free gas in open pore space and gas adsorbed on clay and kerogen surfaces natural fracturing ismore essential for effective reservoir permeability fractures are most common near structures such as faults flexures and buried carbonate banks based on limited data fractures and joints have preferred orientations of 45 225 degrees and 135 315 degrees commercial production requires well stimulation to connect the well bore with the natural fracture system and to prop open pressure sensitive near borehole fractures current stimulations employ hydraulic fracture treatments using nitrogen and foam with sand as a propping agent less

flow and turbulence structure in a hypertidal estuary with the world s biggest tidal bore turbulent and flow structure associated with breaking tidal bores are deliberately investigated on the basis of field measurements high resolution velocity and hydrographic data are collected in the middle qiantang estuary by a vertical array of acoustic doppler velocimeters and optical backscatter sensors collaborated with a bottom mounted acoustic doppler current profiler besides obvious variations in diurnal and spring neap tidal cycles the estuarine dynamics is featured by extreme asymmetry in flood and ebb tides the flood tide is abnormally accelerated to generate tidal bores at the first 10 min or more with breaking or undular configurations at the front the occurrence of peak flow velocity turbulent kinetic energy tke and tke dissipation rate e is definitely associated with breaking bores with their values several times to 2 orders of magnitude larger than the corresponding secondary peak values during the maximum ebb flows flow and turbulence dynamics are significantly affected by the tidal bore froude number a sandwich e structure is clear exhibited with the maximum value at the surface secondary maximum near the bed and the minimum at the intermediate dual tke sources are indicated by an approximate local balance between shear production and dissipation near the bottom and a top down tke dissipation using the modified froude scaling in the vertical water column the highly elevated dissipation by breaking bores is comparable to that by intense breaking waves in the surf zone and the former potentially penetrates the entire water column to produce extreme sediment resuspension events in combination with intense bottom shear stress

quantifying the contributions of tidal straining and gravitational circulation to residual circulation in periodically stratified tidal estuaries abstract this numerical modeling study quantifies for the first time the contribution of various processes to estuarine circulation in periodically stratified tidal flow under the impact of a constant horizontal buoyancy gradient the one dimensional water column equations with periodic forcing are first cast into nondimensional form resulting in a multidimensional parameter space spanned by the modified inverse strouhal number and the modified horizontal richardson number as well as relative wind speed and wind direction and the residual runoff the along tide momentum equation is then solved for the tidal mean velocity profile in such a way that it is equated to the sum of the contributions of tidal straining resulting from the temporal correlation between eddy viscosity and vertical shear gravitational circulation resulting from the depth varying forcing by a constant horizontal buoyancy gradient wind straining and depth mean residual flow resulting from net freshwater runoff this definitio

semi diurnal tidal ellipse variability in a region of freshwater influence 1 new insight into a key process instrumental in the onset of stratification in tidally driven systems is presented for the first time using long term field observations from the liverpool bay region of freshwater influence rofi tidal ellipses respond to changes in stratification becoming more circular with greater vertical structure and reveal variability in response to tidal straining an inflection point that occurs before the maximum vertical density difference at the time of maximum positive bed surface cross shore shear is identified as the crucial point where the water column abruptly decouples the surface and bed layers then rotate in clockwise and anti clockwise directions respectively the maximum vertical stratification is observed when the maximum negative bed surface along shore shear is reached decoupling to a two layer system is therefore not the smooth progression predicted using turbulence closure models but rather a very sudden switch highlighting the importance of the role of reynolds stresses

asymmetric tidal straining across an inlet lateral inversion and variability over a tidal cycle abstract tidal straining is a phenomenon of temporal variations in stratification and mixing resulting from the interaction of a longitudinal salinity gradient with the vertical shear of the horizontal tidal velocity as a result the theory predicts stronger and weaker stratification during ebb low tide and flood high tide respectively in contrast to this well known temporal asymmetry in this study we document in situ measurements demonstrating a lateral asymmetry and lateral inversion of tidal straining at barataria pass a narrow 600 m wide tidal inlet of barataria bay in southeastern louisiana during flood the eastern side of the channel had strong stratification of 4 psu salinity change over a 1 5 m thin layer while the western side had a 2 psu change over a 12 m water column this strong lateral difference decreased as flood continued until near the end of the flood when it reached vertically well mixed condition across the channel during ebb it was just the opposite such that the western side became stratified while the eastern end was well mixed this resulted to a small correlation coefficient of 0 05 for stratification between the west and east sides although the central channel and east side have a high correlation coefficient of 0 88 the tidally averaged salinity was higher on the western end than the eastern end except in a narrow boundary layer close to the eastern shore this is an apparent contradiction to what the coriolis effect would produce in classical estuarine dynamics our hypothesis for the observed difference arises from the influence of the river water coming out of the mississippi river through the southwest pass of the birdfoot delta this water mass may have played a role in the observed complicated lateral inversion of the tidal straining this study underlines the complexity of estuarine dynamics proximal to large deltaic systems and we anticipate that these results will underscore the need for a modeling study to further investigate this dynamic process

interactions between organic acids anions in formation waters and reservoir mineral phases abstract formation waters from the norwegian continental shelf have been analysed for short chain organic acid anions acetic acid is the major component and the observed levels are in the range 0 3 20 mm in uncontaminated formation water samples studies of individual reservoirs and formations over time and area show a quite uniform distribution of the acids within each unit for large volumes of fluid the organic acid anion levels are generally low in reservoirs containing biodegraded oil statistical analysis has been used to find correlations between the organic and inorganic ion compositions and the reservoir conditions and shows a dominating negative correlation between the organic acid anion levels and the levels of inorganic ions indicating that the main effect of the organic ions is to increase the stability of mineral phases laboratory experiments have also been performed and show similar effects for sandstone samples under oxidizing conditions the buffering capacity of the organic acid species is also important as it prevents the development of extreme ph conditions caused by pyrite oxidation in carbonate systems few systematic effects were observed the results both for the natural systems and the laboratory experiments show that the presence of organic acids in the formation waters does not contribute to the development of secondary porosity in the reservoir rock the observed relationships rather indicate decreasing concentration levels of inorganic ions in the formation waters with increasing levels of organic anions and thus an increase in mineral stability

the cycle of turbulent dissipation in the presence of tidal straining in regions of large horizontal density gradient tidal straining acts to produce a periodic component of stratification that interacts with turbulent mixing to control water column structure and flow a 25 h series of measurements of the rate of dissipation of turbulent kinetic energy e in the liverpool bay region of freshwater influence rofi have revealed the form of this interaction and indicate substantial differences from regions where horizontal gradients are weak in the rofi system there is a pronounced difference between flood and ebb regimes during the ebb the water column stratifies and strong dissipation is confined to the lower half of the water column by contrast during the flood stratification is eroded with complete vertical mixing occurring at high water and high values of dissipation 3 mw m23 extending throughout the water column the cycle of dissipation is therefore predominantly semidiurnal in the upper layers whereas near the bottom boundary the principal variation is at the m4 frequency as observed in regions of horizontal uniformity toward the end of the flood phase of the cycle tidal straining produces instabilities in the water column that release additional energy for convective mixing confirmation of increased vertical motions throughout the water column during the late flood and at high water is provided by measurements of vertical velocity and the error velocity from a bottom mounted acoustic doppler current profiler

observations and modelling of periodic stratification in the upper york river estuary virginia abstract two current meter moorings were deployed simultaneously for 61 days during the winter of 1989 90 in the upper york river estuary virginia at a separation along the estuary axis of one tidal excursion both moorings were equipped with four current meters measuring conductivity temperature and current velocity and a surface conductivity temperature recorder giving a detailed picture of the vertical structure of the water column the time series of the vertical density structure measured by both moorings show closely similar periods of mixed and stratified conditions along the estuary comparison with the tidal currents over the same period demonstrates lower stratification or complete mixing to be associated with the strong spring tidal currents while significant stability develops during weaker currents such a spring neap signal is caused by the modulation of tidal mixing energy which is in competition with the stratifying estuarine circulation superimposed on this tidal signal are more irregular remixing events associated with peaks in the surface wind stress a numerical model used successfully in earlier work in a shelf sea region is applied to the upper york river regime the model employs a mellor yamada level 2 turbulence closure scheme to relate the efficiency of vertical turbulent mixing to the local water column stability running the model with a depth dependent horizontal density gradient results in no significant stratification predicted for the entire deployment by incorporating depth variability of the horizontal density gradient estimated from the observations from the two moorings the model reproduces the qualitative features of the observed evolution of stratification

the importance of tidal and lateral asymmetries in stratification to residual circulation in partially mixed estuaries measurements collected in the york river estuary virginia demonstrate the important impact that tidal and lateral asymmetries in turbulent mixing have on the tidally averaged residual circulation a reduction in turbulent mixing during the ebb phase of the tide caused by tidal straining of the axial density gradient results in increased vertical velocity shear throughout the water column during the ebb tide in the absence of significant lateral differences in turbulent mixing the enhanced ebb directed transport caused by tidal straining is balanced by a reduction in the net seaward directed barotropic pressure gradient resulting in laterally uniform two layer residual flow however the channel shoal morphology of many drowned river valley estuaries often leads to lateral gradients in turbulent mixing tidal straining may then lead to tidal asymmetries in turbulent mixing near the deeper channel while the neighboring shoals remain relatively well mixed as a result the largest lateral asymmetries in turbulent mixing occur at the end of the ebb tide when the channel is significantly more stratified than the shoals the reduced friction at the end of ebb delays the onset of the flood tide increasing the duration of ebb in the channel conversely over the shoal regions where stratification is more inhibited by tidal mixing there is greater friction and the transition from ebb to flood occurs more rapidly the resulting residual circulation is seaward over the channel and landward over the shoal the shoal channel segregation of this barotropically induced estuarine residual flow is opposite to that typically associated with baroclinic estuarine circulation over channel shoal bathymetry

a new model linking atmospheric methane sources to pleistocene glaciation via methanogenesis in sedimentary basins methane ch 4 is an important greenhouse gas and amplifier of climate change however the causes of atmospheric ch 4 variations over glacial interglacial cycles remain unresolved we propose that microbial methanogenesis along the shallow margins of sedimentary basins provides a source of atmospheric ch 4 temporally connected with both advance and retreat of continental ice sheets extensive biodegradation of hydrocarbons in the antrim shale formation michigan united states is associated with an active subsurface consortium of fermentative and methanogenic microorganisms this activity was initially stimulated when saline formation waters were diluted by meltwater derived from overriding pleistocene ice sheets during glaciation ch 4 produced by this community accumulated in the shale at a rate of 1 tg ch 4 per 1000 yr as a result of ice coverage and increased hydrostatic pressure we estimate that at present the antrim shale contains only 12 25 of the cumulative mass of ch 4 generated in the shale over the pleistocene indicating that ch 4 that had accumulated during glaciation was subsequently released following ice sheet retreat while release from the antrim shale represents only a small part of the global ch 4 budget when extended to other glaciated sedimentary basins subsurface methanogenesis may generate a substantial previously unrecognized source of atmospheric ch 4 during deglaciation

circular plumes in lake pontchartrain estuary under wind straining circular shaped density plumes of low turbidity low fecal indicator escherichia coli and enterococci concentrations and high salinity have been observed near the industrial canal in lake pontchartrain north of the city of new orleans a conceptual model in polar coordinates and a numerical model are developed together with data analysis to illustrate the dense plume it is demonstrated that the northward expansion of the plume occurs under northerly winds the northward expansion of the plume occurs under northerly winds that drive downwind flow at the surface and upwind radial flow at the bottom northerly wind induced straining similar to tidal straining promotes vertical stratification as a result the water becomes stratified near a thin bottom layer 1 m within which density currents are facilitated the stability of the stratified plume suppresses wind induced turbulent mixing inside the plume the bottom water outside of the plume is more effectively stirred by the wind the result being that the suspended sediment concentration outside of the plume area is much higher than inside this contrast in mixing makes the plume visible from the surface by satellites even though the stratification is at the bottom laterally wind stress produces a torque vorticity in areas of non uniform depth such that upwind flow is developed in deep water and downwind flow in shallow water the continuity requirement produces an upwind flow along the axis of the industrial canal ic the upwind flow is balanced by the downwind flow over the shallower peripheral areas along the coast

very near bottom tidal straining in a sea strait 1 large vertical temperature differences up to 3 c m 1 are observed very near 1 m s 1 the upper sensor at 0 42 m above the bottom mab follows typical temperature variations higher up in the water column being 90 out of phase with the main along channel current in contrast the lower sensor at 0 08 mab is approximately 30 50 more in phase with the free stream current the data confirm the mechanism of tidal straining but much closer to the bottom than previously observed and in a 3 d version with cross flow influences in this mechanism turbulent shear flow and horizontal density gradients create stable stratification during ebb and unstable conditions during flood here stratification provides a slippery boundary flow condition and periodic support of short scale internal waves with periods down to 50 s

biogeochemistry of the forest city basin coalbed methane play abstract hydrogeochemical and microbial analyses of co produced formation waters and gas in the forest city basin were coupled to determine the origin of methane in shallow coal seams and make comparisons to commercial shale and coal gas plays in the adjacent illinois michigan and cherokee basins forest city basin coals contain dilute meteoric waters cl

turbulence dynamics at the shoal channel interface in a partially stratified estuary abstractturbulence observations at the shoal channel interface in south san francisco bay are described and analyzed in this work profiles of turbulent kinetic energy tke shear production and dissipation rate are estimated from adcp beam velocities over a nine day period during winter 2009 when the estuary was partially stratified the tke shear production profiles are consistent with a tidally driven bottom boundary layer except late in the ebb when shear production intensifies and the region of maximum production is located in the upper half of the water column a simple scaling for the depth averaged tke shear production based on bed friction performs well except during this late ebb period when it fails to predict the peak in shear production which suggests that a mechanism other than bottom drag drives turbulence dynamics during this period the profiles of tke dissipation rates also display a late ebb peak a lateral circulation develops at the shoal channel interface during this late ebb perio

creation and tidal advection of a cold salinity front in storfjorden 2 supercooling induced by turbulent mixing of cold water 1 measurements near the edge of fast ice in freemansundet svalbard reveal mixing processes associated with tidal advection of a sharp front in salinity including possible supercooling induced by double diffusion in a fully turbulent water column the front translated back and forth with the semidiurnal tide between an area of mobile drifting ice in storfjorden proper and the narrow sound covered by fast ice water on each side of the front was near its salinity determined freezing temperature instruments deployed about 400 m into the sound from the fast ice edge measured current temperature conductivity and turbulence quantities through several tidal cycles turbulence data illustrate that as the steep horizontal salinity density gradient advected past the measurement site vertical shear near the fast ice base induced marked flood ebb asymmetry in turbulent mixing as fresher water entered the sound on the flood phase inward transport of denser water near the upper boundary was retarded leading to statically unstable conditions and enhanced turbulence the opposite occurred during ebb tide as denser water underran lighter transient episodes of supercooling accompanied frontal passage on both flood and ebb phases the most likely explanation for a zone of supercooled water within the strongly mixed frontal region is that during mixing of fresher slightly warmer but still at freezing water from outside with saltier colder water in the sound the former constituent lost heat faster than gaining salt this interpretation differing turbulent diffusivities for heat and salt challenges strict application of reynolds analogy for highly turbulent shear flow

ftir absorption indices for thermal maturity in comparison with vitrinite reflectance r0 in type ii kerogens from devonian black shales abstract ftir absorbance signals in kerogens and macerals were evaluated as indices for thermal maturity two sets of naturally matured type ii kerogens from the new albany shale illinois basin and the exshaw formation western canada sedimentary basin and kerogens from hydrous pyrolysis artificial maturation of the new albany shale were characterized by ftir good correlation was observed between the aromatic aliphatic absorption ratio and vitrinite reflectance r 0 ftir parameters are especially valuable for determining the degree of maturity of marine source rocks lacking vitrinite with increasing maturity ftir spectra express four trends i an increase in the absorption of aromatic bands ii a decrease in the absorption of aliphatic bands iii a loss of oxygenated groups carbonyl and carboxyl and iv an initial decrease in the ch 2 ch 3 ratio that is not apparent at higher maturity in naturally matured samples but is observed throughout increasing r 0 in artificially matured samples the difference in the ch 2 ch 3 ratio in samples from natural and artificial maturation at higher maturity indicates that short term artificial maturation at high temperatures is not fully equivalent to slow geologic maturation at lower temperatures with increasing r 0 the carboxyl carbonyl aromatic carbon ratio generally decreases except that kerogens from the exshaw formation and from hydrous pyrolysis experiments express an intermittent slight increase at medium maturity ftir derived aromaticities correlate well with r 0 although some uncertainty is due to the dependence of ftir parameters on the maceral composition of kerogen whereas r 0 is solely dependent on vitrinite

characterization of the origin of coalbed gases in southeastern illinois basin by compound specific carbon and hydrogen stable isotope ratios abstract coalbed gases and waters from exploratory and production gas wells in the southeastern illinois basin were sampled to assess geochemically the origin of coalbed gases with an emphasis on the springfield and seelyville coal members that are commercially targeted for coalbed methane production in indiana on line analyses of hydrocarbon gases methane to butanes c 1 c 2 c 3 n c 4 i c 4 and co 2 yielded gas concentrations plus d and 13 c values the low thermal maturity of indiana coals with vitrinite reflectance r 0 0 6 is in agreement with an overwhelmingly biogenic isotopic signature of coalbed gases containing 96 methane generated via bacterial co 2 reduction in contrast thermogenic gas was generated in the stratigraphically equivalent coal beds in western kentucky s rough creek graben zone where higher maturities of up to r 0 0 8 were reached owing to tectonic and hydrothermal activity no secondary biogenic methane was observed in more mature western kentucky coal beds where greater burial depth limits the recharge of meteoric water biogenic and thermogenic coalbed gases represent two end members that are compositionally and isotopically distinct microbial biodegradation of thermogenic c 2 hydrocarbon gases in indiana coal beds preferentially targets c 3 and introduces isotope fractionation whereby remaining c 3 is enriched in deuterium and 13 c

stratification in the presence of an axial convergent front causes and implications abstract flood tide salinity stratification in the presence of an axial convergent front is revealed with new data the data were collected during sampling campaigns in the conwy estuary north wales the location of a previous study for axial convergent fronts the stratification with a maximum observed vertical density difference of 5 kg m 3 begins as soon as the saline intrusion arrives and lasts throughout the flood tide the flood stratification is shown to be caused by a tidal straining type process the along channel shear is modified by cross channel currents developed during the formation of the convergent front such that surface currents are smaller than those at mid depth the modified shear interacts with the horizontal density gradient to form the stratification on the flood tide implications for the turbulence and sediment transport regime in the estuary are discussed

tidal variations of flow convergence shear and stratification at the rio de la plata estuary turbidity front 1 intratidal variability of density and velocity fields is investigated at the turbidity front of the rio de la plata estuary south america current velocity and temperature salinity profiles collected in august 1999 along a repeated transect crossing the front are analyzed horizontal and vertical gradients stability of the front convergence zones and transverse flow associated to the frontal boundary are described strong horizontal convergence of the across front velocity and build up of along front velocity shear were observed at the front in the proximity of the front enhanced transverse or along front flow created jet like structures at the surface and near the bottom flowing in opposite directions these structures persisted throughout the tidal cycle and were advected upstream downstream by the flood ebb current through a distance of 10 km during peak flood the upper layer flow reversed from its predominant downstream direction and upstreamflow occupied the entire water column outside the peak flood two layer estuarine circulation dominated changes in density field were observed in response to tidal straining tidal advection and wind induced mixing but stratification remained throughout the tidal cycle this work demonstrates the large spatial variability of the velocity field at the turbidity front it provides evidence of enhanced transverse circulation along the frontal boundary and reveals the importance of advective and frictional intratidal processes in the dynamics of the central part of the estuary

noble gas tracing of groundwater coalbed methane interaction in the san juan basin usa the san juan basin natural gas field located in northwestern new mexico and southwestern colorado in the usa is a case type coalbed methane system groundwater is thought to play a key role in both biogenic methane generation and the co2 sequestration potential of coalbed systems we show here how noble gases can be used to construct a physical model that describes the interaction between the groundwater system and the produced gas we collected 28 gas samples from producing wells in the artesian overpressured high production region of the basin together with 8 gas samples from the underpressured low production zone as a control stable isotope and major species determination clearly characterize the gas in the high production region as dominantly biogenic in origin and the underpressured low producing region as having a significant admix of thermogenic coal gas 3he 4he ratios increase from 0 0836ra at the basin margin to 0 318ra towards the center indicating a clear but small mantle he signature in all gases coherent fractionation of water derived 20ne 36ar and crustal 4he 40ar are explained by a simple rayleigh fractionation model of open system groundwater degassing low 20ne concentrations compared to the model predicted values are accounted for by dilution of the groundwater associated gas by desorbed coalbed methane this rayleigh fractionation and dilution model together with the gas production history allows us to quantify the amount of water involved in gas production at each well the quantified water volumes in both underpressured and overpressured zones range from 1 7 103 m3 to 4 2 105 m3 with no clear distinction between over and underpressured production zones these results conclusively show that the volume of groundwater seen by coal does not play a role in determining the volume of methane produced by secondary biodegradation of these coalbeds there is no requirement of continuous groundwater flow for renewing the microbes or nutrient components we furthermore observe strong mass related isotopic fractionation of 20ne 22ne and 38ar 36ar isotopic ratios this can be explained by a noble gas concentration gradient in the groundwater during gas production which causes diffusive partial re equilibration of the noble gas isotopes it is important for the study of other systems in which extensive groundwater degassing may have occurred to recognize that severe isotopic fractionation of air derived noble gases can occur when such concentration gradients are established during gas production excess air derived xe and kr in our samples are shown to be related to the diluting coalbed methane and can only be accounted for if xe and kr are preferentially and volumetrically trapped within the coal matrix and released during biodegradation to form ch4

impact of nonlinear waves on the dissipation of internal tidal energy at a shelf break the vertical and temporal structure of the dissipation of turbulent kinetic energy within the internal tide at a location 5 km shoreward of the shelf break on the malin shelf has been determined using a combination of the free falling light yo yo profiler and acoustic doppler current profilers two distinct internal wave regimes were encountered period i in which large amplitude high frequency nonlinear internal waves niws occurred around neap tides and period ii in which the internal wave spectral continuum was not dominated by any particular frequency band around spring tides empirical orthogonal function analysis shows that for the low frequency waves 76 of the variance was described by mode 1 rising to 95 for the high frequency waves during period i the dissipation and vertical mixing were characterized by the niws and 70 of the dissipation occurred in the bottom boundary layer during period ii the depth integrated dissipation was more evenly distributed throughout the tidal cycle whereas vertical mixing was greatly enhanced during a single hour long episode of elevated thermocline dissipation coincident with weakened stratification during both periods i and ii 30 of the total measured dissipation occurred within the thermocline when averaged over 12 4 hours the remainder occurred within the bottom boundary layer bbl tidal average values for depth integrated dissipation and vertical eddy diffusivity for period i ii were 1 1 10 2 w m 2 4 0 10 2 w m 2 and 5 cm2 s 1 12 cm2 s 1 respectively decay rates and internal damping are discussed and vertical heat fluxes are estimated observed dissipation rates are compared with a simple model for bbl dissipation

constraining the timing of microbial methane generation in an organic rich shale using noble gases illinois basin usa abstract at least 20 of the world s natural gas originates from methanogens subsisting on organic rich coals and shales however in situ microbial methane production rates are unknown methanogens in the upper devonian new albany shale in the illinois basin extract hydrogen from low salinity formation water to form economic quantities of natural gas because of this association constraining the source and timing of groundwater recharge will enable estimation of minimum in situ metabolic rates thirty four formation water and gas samples were analyzed for stable isotopes oxygen and hydrogen chloride tritium 14 c and noble gases chloride and 18 o spatial patterns reveal a plume of water with low salinity 0 7 to 2154 mm and 18 o values 0 14 to 7 25 penetrating 1 km depth into evapo concentrated brines parallel to terminal moraines of the laurentide ice sheet suggesting glacial mediated recharge however isotopic mixing trends indicate that the recharge endmember 7 18 o is higher than the assumed bulk ice sheet value 18 o and similar to modern local precipitation 7 5 to 4 5 18 o continental paleoprecipitation records reveal that throughout the pleistocene 18 o of precipitation in the region ranged from 10 to 5 suggesting that the dilute groundwater was primarily sourced from paleoprecipitation with minor contributions from glacial meltwater for the majority of samples the atmosphere derived 4 he contribution is negligible and the 4 he is dominated by a crustal radiogenic source with near complete transfer of dissolved noble gases to the gas phase in addition mantle derived helium is negligible for all samples 18 o data however all groundwater in the study area was influenced by pleistocene recharge estimated in situ microbial methane production rates range from 10 to 1000 tcf ma 10 4 to 10 6 times slower than average laboratory rates from coals findings from this study have implications for targeting undeveloped microbial gas accumulations improving natural gas reservoir estimates the potential of in situ methanogen stimulation and understanding biologic cycling of carbon in subsurface reservoirs

observations of tidal straining within two different ocean environments in the east china sea stratification and near bottom turbulence tidal straining describes the straining effect induced by the vertical shear of oscillatory tidal currents that act on horizontal density gradients it tends to create tidal periodic stratification and modulate the turbulence in the bottom boundary layer bbl here we present observations of current hydrology and turbulence obtained at two mooring stations that are characterized by two typical hydrological environments in the east china sea ecs one is located adjacent to the changjiang river s mouth and the other is located over a sloping shelf which is far from the freshwater sources tidal straining induces a semidiurnal switching between stable and unstable stratification at both stations near bottom high frequency velocity measurements further reveal that the dissipation rate of turbulent kinetic energy tke is highly elevated during periods when unstable stratification occurs a comparison between the tke dissipation rate e and the shear production p further reveals that the near bottom mixing is locally shear induced most of the time except during the unstable stratification period within this period the magnitude of dissipation exceeds the expected value based on the law of the wall by an order of magnitude the buoyancy flux that calculated by the balance method is too small to compensate for the existing discrepancy between the dissipation and shear production another plausible candidate is the advection of tke which may play an important role in the tke budget during the unstable stratification period

a numerical study of sea breeze driven ocean poincare wave propagation and mixing near the critical latitude abstract near the vicinity of 30 latitude the coincidence of the period of sea breeze and the inertial period of the ocean leads to a maximum near inertial ocean response to sea breeze this produces a propagating inertial internal poincare wave response that transfers energy laterally away from the coast and provides significant vertical mixing in this paper the latitudinal dependence of this wave propagation and its associated vertical mixing are investigated primarily using a nonlinear numerical ocean model three dimensional idealized simulations show that the coastal oceanic response to sea breeze is trapped poleward of 30 latitude however it can propagate offshore as poincare waves equatorward of 30 latitude near 30 latitude the maximum oceanic response to sea breeze moves offshore slowly because of the near zero group speed of poincare waves at this latitude the lateral energy flux convergence plus the energy input from the wind is maximum near the critical latitude leading to increa

near bed shear stress turbulence production and dissipation in a shallow and narrow tidal channel near bed highly resolved velocity profiles were measured in the lower 0 03 m of the water column using acoustic doppler profiling velocimeters in narrow tidal channels in a salt marsh the bed shear stress was estimated from the velocity profiles using three methods the log law reynolds stress and shear stress derived from the turbulent kinetic energy tke bed shear stresses were largest during ebbing tide while near bed velocities were larger during flooding tide the reynolds stress and tke method gave similar results while the log law method resulted in smaller bed shear stress values during ebbing tide shear stresses and turbulent kinetic energy followed a similar trend with the largest peaks during ebbing tide the maximum turbulent kinetic energy was on the order of 1 10 2 m2 s2 the fluid shear stress during flooding tide was approximately 30 of the fluid shear stress during ebbing tide the maximum tke derived shear stress was 0 7 n m2 and 2 7 n m2 during flooding and ebbing tide respectively and occurred around 0 02 m above the bed turbulence dissipation was estimated using the frequency spectrum and structure function methods turbulence dissipation estimates from both methods were maximum near the bed 0 01 m both the structure function and the frequency spectrum methods resulted in maximum dissipation estimates on the order of 4 10 3 m2 s3 turbulence production exceeded turbulence dissipation at every phase of the tide suggesting that advection and vertical diffusion are not negligible however turbulence production and dissipation were within a factor of 2 for 77 of the estimates the turbulence production and dissipation decreased quickly away from the bed suggesting that measurements higher in the water column cannot be translated directly to turbulence production and dissipation estimates near the bed copyright 2015 john wiley sons ltd

cleats and their relation to geologic lineaments and coalbed methane potential in pennsylvanian coals in indiana abstract cleats and fractures in pennsylvanian coals in southwestern indiana were described statistically analyzed and subsequently interpreted in terms of their origin relation to geologic lineaments and significance for coal permeability and coalbed gas generation and storage these cleats can be interpreted as the result of superimposed endogenic and exogenic processes endogenic processes are associated with coalification i e matrix dehydration and shrinkage while exogenic processes are mainly associated with larger scale phenomena such as tectonic stress at least two distinct generations of cleats were identified on the basis of field reconnaissance and microscopic study a first generation of cleats that developed early on during coalification and a second generation that cuts through the previous one at an angle that mimics the orientation of the present day stress field the observed parallelism between early formed cleats and mapped lineaments suggests a well established tectonic control during early cleat formation authigenic minerals filling early cleats represent the vestiges of once open hydrologic regimes the second generation of cleats is characterized by less prominent features i e smaller apertures with a much less pronounced occurrence of authigenic mineralization our findings suggest a multistage development of cleats that resulted from tectonic stress regimes that changed orientation during coalification and basin evolution the coals studied are characterized by a macrocleat distribution similar to that of well developed coalbed methane basins e g black warrior basin alabama scatter plots and regression analyses of meso and microcleats reveal a power law distribution between spacing and cleat aperture the same distribution was observed for fractures at microscopic scale our observations suggest that microcleats enhance permeability by providing additional paths for migration of gas out of the coal matrix in addition to providing access for methanogenic bacteria the abundance distribution and orientation of cleats control coal fabric and are crucial features in all stages of coalbed gas operations i e exploration and production understanding coal fabric is important for coal gas exploration as it may be related to groundwater migration and the occurrence of methanogenic bacteria prerequisite to biogenic gas accumulations likewise the distribution of cleats in coal also determines pathways for migration and accumulation of thermogenic gas generated during coalification

observations of turbulence in a partially stratified estuary the authors present a field study of estuarine turbulence in which profiles of reynolds stresses were directly measured using an adcp throughout a 25 h tidal day the dataset that is discussed quantifies turbulent mixing for a water column in northern san francisco bay that experiences a sequence of states that includes a weak ebb and flood that are stratified followed by a strong and eventually unstratified ebb and flood these measurements show that energetic turbulence is confined to a bottom mixed layer by the overlying stratification examination of individual reynolds stress profiles along with profiles of richardson number and turbulent froude number shows that the water column can be divided into regions based on the relative importance of buoyancy effects using the measured turbulence production rate p the dissipation rate e is estimated the observed turbulence had values of e nn 2 20 all of the time and e nn 2 200 most of the time suggesting that the observed motions were buoyancy affected turbulence rather than internal waves however at times turbulent froude numbers in much of the upper water column were less than one indicating important stratification effects taken as a whole the data show that stratification affects the turbulent velocity variance q2 most severely that is observed reductions in u9w9 are largely associated with small values of q2 rather than with a dramatic reduction in the efficiency with which turbulent motions produce momentum fluxes finally the dataset is compared to predictions made using the popular mellor yamada level 2 5 closure these comparisons show that the model tends to underestimate the turbulent kinetic energy in regions of strong stratification where the turbulence is strongly inhomogeneous and to overestimate the turbulent kinetic energy in weakly stratified regions the length scale does not appear to compensate for these errors and as a result similar errors are seen in the eddy viscosity predictions it is hypothesized that the underestimation of q2 is due to an inaccurate parameterization of turbulence self transport from the near bed region to the overlying stratification

variability of geochemical properties in a microbially dominated coalbed gas system from the eastern margin of the illinois basin usa this study outlines gas characteristics along the southeastern margins of the illinois basin and evaluates regional versus local gas variations in seelyville and springfield coal beds our findings suggest that high permeability and shallow 100 250 m depths of these indiana coals allowed inoculation with methanogenic microbial consortia thus leading to widespread microbial methane generation along the eastern marginal part of the illinois basin low maturity coals in the illinois basin with a vitrinite reflectance ro 0 6 contain significant amounts of coal gas 3 m 3

microstructure observations of turbulent mixing in a partially mixed estuary part i dissipation rate variations of turbulent mixing in the water column and in the benthic boundary layer were observed with a microstructure profiler in the hudson river estuary during two cruises in summer and fall of 1995 variability patterns of stratification shear richardson number ri and turbulent dissipation rates e were similar to those of earlier observations with strong turbulence in the weakly stratified bottom layer weak turbulence in the halocline during neap tides and low ri and high e spanning the water column during spring ebbs depthintegrated turbulent dissipation rates re dz approximately equaled the work done by the tidal pressure gradient force adjusted for the change in tidal kinetic energy alternatively a suitable scaling for re dz is also provided by the product of bottom shear stress and depth average velocity t by a relationship that fails at slack tide however at heights above bottom of z 0 3 m and again excepting slack tides observed e were highly correlated with a law of the wall dissipation rate eb 5 kz ratios e eb were close to 1 here the friction 3 u

evidence of high vertical wave number behavior in a continuously stratified reservoir boadella spain high vertical wave number modes clearly dominate the internal wave field during the stratification period in boadella reservoir in northeast spain in this period the extraction of hypolimnetic water due to summer irrigation brought the surface level down by 6 m in one month and the epilimnetic water progressively occupied the whole water column the temperature profile with the exception of a few meters at the surface layer presented an almost constant temperature gradient of about 0 7 c m the period of the main vertical mode is 24 h with an amplitude of around 1 m thermistor chain records and meteorological data allow us to deduce that this mode is at least a third vertical mode forced by the wind which normally has a typical periodicity of 24 h however when the wind changes direction from south to north the circulation cells developed due to this forced nonstationary oscillation are destroyed when this occurs the bulk richardson number is ri db 1 similar vertical structures as a response to wind forcing should be expected in similar systems although this has not been reported in the literature

stratification and turbulence in subtidal channels through intertidal mudflats 1 field observations in san francisco bay of subtidal channels that drain through intertidal mudflats indicate that substantial periodic stratification develops in very shallow flows with distinct asymmetries between flood and ebb conditions the greatest variability in salinity and stratification occurs during the wet winter months very strong longitudinal salinity gradients develop across the intertidal zone between salty subtidal water and fresher water draining into the marsh upstream tidal straining of the longitudinal gradient creates the periodic stratification stratifying through ebbs and destratifying through floods the stratification can be very strong in very shallow flows with n2 0 1 s 2 in about 0 5 m water depth analysis of the data with dimensionless numbers shows that at times the stratification is strong enough to suppress turbulence and mixing the asymmetry in stratification between ebbs and floods results in asymmetries in eddy viscosity and eddy diffusivity with lower values during the stratified ebbs because of the intertidal elevation of the system the strong longitudinal salinity gradient is regenerated each time the mudflats and marsh drain the system approaches a tidally periodic steady state where the flow dynamics are dominated by the sharp front of salty water that advects into the intertidal zone on the rising tide and exits the system by each lower low water the field data are supported by a numerical model with simple mudflat channel bathymetry both the field and numerical work indicate that the shallow low energy flows in mudflat channels transition between strongly stratified suppressed turbulence and relatively unstratified active turbulence each tidal cycle

enhanced turbulence due to the superposition of internal gravity waves and a coastal upwelling jet to vertical aspect ratios of 10 2 to 10 3 median value 300 these patches are clearly defined by regions of low richardson number and occur where and when the linear superposition of the three dominant shear constituents near inertial m2 and thermal wind interferes constructively this is most pronounced at the base of the coastal jet where the thermal wind shear is largest while the effect of the turbulence stress divergence on the jet is small compared to geostrophy 1 it is significant in the second order force balance governing secondary circulation the timescale associated with the decay of the thermal wind shear via turbulence stress is o 10 days we confirm that the vertical salt flux due to mixing is comparable to the net ekman transport of salt onto the shelf within the bottom boundary layer because numerical models of coastal circulation lack turbulence in midwater column any vertical transport of scalars including salt and heat must be achieved inshore of the 40 m isobath this is inconsistent with the observations presented in this study in which significant vertical turbulent salt transport is found to exist across the entire shelf

cretaceous marine nutrients greenhouse carbonates and the abundance of turritelline gastropods abstract modern marine carbonate sediments accumulate where carbonate producing organisms are abundant and siliciclastic input is low such accumulations occur today in two main environments and may be characterized as warm low nutrient wln also known as tropical or chlorozoan carbonates or cool high nutrient chn also known as cool water temperate nontropical foramol or heterozoan carbonates few carbonates form today in warm high nutrient whn conditions because such environments are very rare modern wln environments occur mainly in low latitudes where nutrients from upwelling or terrestrial runoff are low modern chn environments occur mainly in middle to high latitudes where nutrients mostly from upwelling are abundant however whn depositional environments may have been widespread during the cretaceous and paleogene during these times upwelled nutrient rich waters would in general have been warmer than they are today nutrients in these waters supported diverse carbonate produc

microbial generation of economic accumulations of methane within a shallow organic rich shale although methane of bacterial origin is ubiquitous in marine and freshwater sediments economic accumulations of bacterial gases occur mainly at depths of several kilometres in tertiary basins that had high sedimentation rates1 2 here we present an integration of geochemical and isotopic data from gas and water extracted from the upper devonian antrim shale along the northern margin of the michigan basin which demonstrates that significant volumes of bacterial gas have been generated in organic rich shales at depths of less than 600 metres the antrim shale is mainly a self sourced reservoir in contrast to conventional gas deposits that have migrated from a source to a reservoir and has become one of the most actively exploited gas reservoirs3 in the united states the gas forming processes operating at shallow depths in the antrim shale are not unique4 and an understanding of these processes should lead to the identification and development of other economic non conventional gas deposits around the world

extent and limits of biodegradation by in situ methanogenic consortia in shale and formation fluids abstract consortia of microbes degrade recalcitrant organic matter in deep subsurface reservoirs such as shales and coals under anaerobic conditions into simple c molecules such as co 2 and acetate these substrates are subsequently metabolized by methanogens into economic quantities of natural gas in sedimentary basins world wide this study explores organic matter in the devonian new albany shale illinois basin usa and associated fluids to investigate the extent of organic matter biodegradation and evaluate the potential for stimulating in situ gas production identification of labile compound classes such as n alkanes fatty acids and phenols in produced waters of the new albany shale and low biodegradation indices in the shale core samples indicate limited biodegradation together with detectable acetate concentrations up to 225 1 m these observations suggest that both the supporting microbial consortia and methanogens are limited in extent and activity by comparison the new albany shale is much less biodegraded than the microbial ch 4 producing michigan basin antrim shale powder river basin coals or san juan basin coals in the new albany shale the extent of biodegradation generally becomes more varied with higher salinities suggesting diverse microbial adaptations to degrade om at high salinities enhancement of in situ ch 4 production may be most effective if targeted at stimulating production of the supporting microbial consortia as well as methanogens

friction dominated exchange in a florida estuary abstract the typically observed gravitational circulation in estuaries with lateral variations in bathymetry consists of a combined distribution of vertically and horizontally sheared flows the distribution features inflow at depth and outflow at the surface and along the sides however theoretical results of density driven exchange flows dominated by frictional effects display a laterally sheared distribution with inflow occupying the deepest portion of the cross section and outflow over the shoals the main purpose of this investigation was to obtain observational evidence in support of theoretical results a tidal cycle survey was conducted on february 24 2009 to collect current velocity and hydrographic profile data along a cross estuary transect observations from hillsborough bay were compared qualitatively to numerical model and analytical solution results the observed residual exchange flow pattern compared favorably with the results from a numerical model and an analytical solution that used a condition controlled by friction the relative importance of friction was explored at tidal and subtidal timescales intratidally frictional influences were observed in the spatial distribution of tidal current amplitude and phase as well as in potential energy anomaly stratification variations subtidally frictional influences were observed through the spatial distributions of tidally averaged stratification turbulent kinetic energy dissipation and eddy viscosity values the main finding of this study was that relatively weak tidal currents

summertime tidal flushing of barataria bay transports of water and suspended sediments the edges of the inlet the net flux of total suspended sediment out of the bay was determined to be 8800 t of which 20 was organic matter demonstrating a significant source of organic matter to the base of the coastal ocean detrital food chain the time evolution and net fluxes of water and suspended sediments showed that the net flow resembles conventional estuarine circulation patterns with net outward flow on the surface and shallow ends of the inlet and with net inward flow in the center and at the bottom of the center of the inlet the west side has a much larger outward flow than the east side while the east side is fresher these differences suggest that the louisiana coastal current from around the bird s foot delta derived from the mixing of shelf water with the mississippi river freshwater may have entered the bay this must have been mostly from the east side during the survey which resulted in a smaller outward flow on the eastern side a numerical experiment further confirmed this assumption and the model was verified by field observations on 5 may 2010

numerical analysis of stratification and destratification processes in a tidally energetic inlet with an ebb tidal delta stratification and destratification processes in a tidally energetic weakly stratified inlet in the wadden sea south eastern north sea are investigated in this modeling study observations of current velocity and vertical density structure show strain induced periodic stratification for the southern shoal of the tidal channel in contrast to this in the nearby central region of the channel increased stratification is already observed directly after full flood to investigate the processes leading to this different behavior a nested model system using getm is set up and successfully validated against field data the simulated density development along a cross section that includes both stations shows that cross channel stratification is strongly increasing during flood such that available potential energy is released in the deeper part of the channel during flood an analysis of the potential energy anomaly budget confirms that the early onset of vertical stratification during flood at the deeper station is mainly controlled by the stratifying cross channel straining of the density field in contrast to this in the shallow part of the channel the relatively weak cross channel straining is balanced by along channel straining and vertical mixing an idealized analytical model confirms the following hypothesis the laterally convergent flood current advecting laterally stratified water masses from the shallow and wide ebb tidal delta to the deep and narrow tidal channel has the tendency to substantially increase cross channel density gradients in the tidal channel this process leads to stratification during flood

diagnostic criteria for reconstruction of tidal continental shelf regimes changing the paradigm abstract a biogeochemical flux paradigm is presented which provides a conceptual and numerical framework for reconstructing the dynamical and biogeochemical regimes of ancient tide driven continental shelves the paradigm links turbulence primary production and suspended particulate matter flux in the water column to microbiological and isotopic proxies in the sediment record and identifies the diagnostic signatures of sediments deposited in stratified frontal and mixed dynamic regimes of tidal shelves the critical governing processes in the water column and at the sediment water interface are temperature particulate organic carbon supply and benthic oxygen consumption which exhibit strong gradients across tidal mixing fronts the diagnostic proxies in the sediment record are microplanktonic e g dinoflagellate cysts and microbenthic e g foraminifera and their associated stable isotopic properties advanced numerical models are available which incorporate biophysical coupling in the water column and benthic boundary layer and which simulate biogeochemical and ecological processes and organic fluxes to the seabed these models offer potential advances for interpreting microbiological and isotopic proxies of biogeochemical regime preserved in the fine sediment record quaternary shelf deposits provide the best potential validation of the biogeochemical flux paradigm since most quaternary species are living today but the paradigm is applicable to ancient tidal shelf deposits the paradigm is particularly applicable to the fine sediment record which potentially preserves the most complete history of shelf evolution

biogenic methane generation in the degradation of eastern australian permian coals abstract hydrocarbons extracted from one sydney basin coal have a distribution of aromatic and aliphatic hydrocarbons consistent with a high degree of biodegradation as previously demonstrated for bowen basin coals on the same basis hydrocarbons from two other coals from the sydney basin are less biodegraded and display an additional minor series of alkylaromatic degradation products all these degradation reactions generate co2 which on biogenic reduction to ch4 is reservoired in association with the permian coal seams of the sydney and bowen basins australia variations in access to oxygenated waters nutrients and micro organisms are held to be largely responsible for the extent of biodegradation of coal hydrocarbons and for the reduction of co2