organic carbon and sedimentation rate further evidence for anoxic deep water conditions in the cenomanian turonian atlantic ocean abstract the relationship between organic carbon content and sedimentation rate based on data derived from recent to miocene sediments deposited under oxic and anoxic sea water conditions is used to characterize the different depositional environments these data are compared with data of cenomanian turonian dsdp sediments from the atlantic ocean the results may support deposition in anoxic environments due to extremely reduced deep water circulation as the main cause for the formation of cenomanian turonian black shales in the north atlantic rather than increased supply of organic matter due to higher oceanic productivity

a review of prokaryotic populations and processes in sub seafloor sediments including biosphere geosphere interactions article i nfo sub seafloor biosphere marine microbiology prokaryotic diversity dark energy biosphere geosphere interactions a general review of the sub seafloor biosphere is presented this includes an update and assessment of prokary otic cell distributions within marine sediments current deepest 1922 m and the impact of this on global sub seafloor biomass estimates these global estimates appear relatively robust to different calculation approaches and our updated estimate is 5 39 10 29 cells taking into consideration new data from very low organic matter south pacific gyre sediments this is higher than other recent estimates which is justified as several sediments such as gas hydrate deposits and oil reservoirs can have elevated cell concentrations the proposed relationship between elevated cell concentrations and milankovitch cycles in sequential diatom rich layers at some sites demonstratesnotonlyadynamicdeepbiosphere butalsothatthedeepbiosphereisanintegralpartofearthsys tem processes over geological time scales cell depth distributions vary in different oceanographic provinces and this is also reflected in contrasting biodiversity despite this there are some clear common sub seafloor prokary otes for bacteria these are the phyla chloroflexi gammaproteobacteria planctomycetes and the candidate phylum js1 and for archaea uncultivated lineages within the phylum crenarchaeota miscellaneous crenarchaeotal group and marine benthic group b euryarchaeota sagmeg marine benthic group d thermoplasmatales associated groups and thaumarchaeota marine group i in addition spores viruses and fungi have been detected but their importance is not yet clear consistent with the direct demonstration of active prokaryotic cells prokaryotes have been enriched and isolated from deep sediments and these reflect a subset of the total diversity including spore formers that are rarely detected in dna analyses activities are generally low in deep marine sediments 10 000 times lower than in near surface sediments however depth integrated activity calculations demonstrate that sub surface sediments can be responsible for the majority of sediment activity up to 90 and hence are biogeochemically important unlike near surface sediments competitive metabolisms can occur together and metabolism per cell can be 1000 times lower than

anaerobic oxidation of methane and sulfate reduction along the chilean continental margin anaerobic oxidation of methane aom and sulfate reduction sr were investigated in sediments of the chilean upwelling region at three stations between 800 and 3000 m water depth major goals of this study were to quantify and evaluate rates of aom and sr in a coastal marine upwelling system with high organic input to analyze the impact of aom on the methane budget and to determine the contribution of aom to sr within the sulfate methane transition zone smt furthermore we investigated the formation of authigenic carbonates correlated with aom we determined the vertical distribution of aom and sr activity methane sulfate sulfide ph total chlorins and a variety of other geochemical parameters depth integrated rates of aom within the smt were between 7 and 1124 mmol m 2 a 1 effectively removing methane below the sediment water interface single measurements revealed aom peaks of 2 to 51 nmol cm 3 d 1 with highest rates at the shallowest station 800 m the methane turnover was higher than in other diffusive systems of similar ocean depth this higher turnover was most likely due to elevated organic matter input in this upwelling region offering significant amounts of substrates for methanogenesis sr within the smt was mostly fuelled by methane aom led to the formation of isotopically light dic 13c 24 6 vpdb and of distinct layers of authigenic carbonates 13c 14 6 vpdb

carbonate apron models alternatives to the submarine fan model for paleoenvironmental analysis and hydrocarbon exploration sediment gravity flow deposition along the deep water flanks of carbonate platforms typically does not produce submarine fans rather wedge shaped carbonate aprons develop parallel to the adjacent shelf slope break the major difference between submarine fans and carbonate aprons is a point source with channelized sedimentation on fans versus a line source with sheet flow sedimentation on aprons two types of carbonate aprons may develop along relatively gentle 4 platform margin slopes aprons form immediately adjacent to the shallow water platform and are referred to as carbonate slope aprons along relatively steep 4 15 platform margin slopes redeposited limestones accumulate in a base of slope setting by passing an upper slope via a multitude of small submarine canyons and are referred to as carbonate base of slope aprons both apron types are further subdivided into inner and outer facies belts inner apron sediments consist of thick mud supported conglomerates and megabreccias facies f as well as thick coarse grained turbidites facies a interbedded with subordinate amounts of fine grained peri platform ooze facies g outer apron sediments consist of thinner grain supported conglomerates and turbidites facies a as well as classical turbidites facies c with recognizable bouma divisions interbedded with approximately equal proportions of peri platform ooze facies g seaward aprons grade laterally into basinal facies of thin base cut out carbonate turbidites facies d that are subordinate to peri platform oozes facies g carbonate base of slope aprons grade shelfward into an upper slope facies of fine grained peri platform ooze facies g cut by numerous small canyons that are filled with coarse debris as well as intraformational truncation surfaces which result from submarine sliding in contrast slope aprons grade shelfward immediately into shoal water platform margin facies without an intervening by pass slope the two carbonate apron models presented here offer alternatives to the submarine fan model for paleoenvironmental analysis and hydrocarbon exploration for mass transported carbonate facies

quaternary evolution of the ria de vigo spain abstract the seismic analysis of the sedimentary infill of the ria de vigo reveals that the infill comprises of a fifth order sequence 18 ky bounded by a major discontinuity surface within the holocene sequence several seismic units have been identified the oldest one originated after the wurm glaciation and the origin of its top discontinuity was eroded during the regression sea level at 120 m of this last glacial episode isotopic stages 2 to 4 18 ky b p in some places below this ria infill another sequence can be identified this older sequence is associated with the pre existing faults and troughs in the deepest parts of the ria and it was deposited during the wurm glaciation miocene pleistocene eroded sediments were moved from the rias area to deeper zones where they accumulated in the lowstand system tract the next sea level rise occured in two steps producing two transgressive units separated by an hiatus surface this cycle was interrupted by the next sea level fall to 55 m marked on the seismic records by another recognizable boundary the next sea level rise took place in several steps back stepping with some stillstands generating another transgressive system tract in the ria from that time on the relative sea level continued rising depositing the most recent unit which represents the highstand system tract separating the highstand system tract and the transgresive unit below there is a consensed section the analysis of 29 cores provides groundtruthing for our hypothesis the identified main lithologic units show a transgressive sequence a granitic rock base an overlying second unit locally wedge shaped and most recently a marine muddy unit covering the present ria seafloor

stratigraphy and palaeogeography of the godavari supergroup in the south central pranhita godavari valley south india abstract the proterozoic succession of the pranhita godavari valley pg valley defined as the godavari supergroup occurs in two nw se trending belts flanking the margins of the valley the supergroup comprises several unconformity bound sequences of group status and exhibits strong regional variations in the distribution of different groups deposited in widely variable conditions ranging from fan alluvial setting to deep water slope base of slope and basinal environments the lithologic assemblages indicate that the basin experienced multiple rifting and tectonic environment varied from an unstable rift setting to a stable shelf regime the godavari supergroup with a complex history of stratigraphic evolution is still plagued by multiple problems of classification and correlation that have long been the major impediments to basin analysis in the present paper the attributes of the genetically linked stratal packages the unconformities separating them and their correlation in the south central part of the valley have been critically evaluated to address the problems of regional stratigraphy the godavari supergroup is bounded by two inter regional unconformities separating it from the underlying crystalline cratonic basement and from the overlying gondwana sedimentary rocks of late paleozoic mesozoic age the supergroup in turn is subdivided into four major sequences by three regional unconformities in the south central part of the valley comparison of stratigraphic profiles at different points indicates that the thickness of the unconformity bound sequences or of individual formations and also the number of formations increases from the central part of the valley towards the southeast the stratigraphic relationships collectively suggest that the basin deepened and opened in a southeasterly direction the basin in its southeastern part appears to have been much wider than the present day outcrop width and to have been continuous with other adjoining proterozoic basins the central part of the basin was a relatively stable and positive area compared to the southern part where the basin floor repeatedly moved across the base level to the order of several hundred meters the amplitude of movements indicates tectonically controlled subsidence and uplift

sedimentation during carbonate ramp to slope evolution in a tectonically active area bowland basin dinantian northern england the bowland basin northern england contains a series of carbonates and terrigenous mudstones deposited during the ivorian to early brigantian two regional depositional environments are indicated by facies and facies associations wackestone packstone and calcarenite facies indicate deposition in a carbonate ramp environment while lime mudstone wackestone calcarenite and limestone breccia conglomerate facies often extensively slumped represent a carbonate slope environment stratigraphic relations suggest that the depositional environment evolved from a ramp into a slope through the dinantian two main sediment sources are indicated by the sequence an extra basinal terrigenous mud source and a supply of carbonate from the margins of the basin deposition from suspension and from sediment gravity flows in situ production and remobilization of sediment during sedimentary sliding were important processes operating within the basin periods of enhanced tectonic activity in the late chadian to early arundian and late asbian to early brigantian are indicated by basin wide horizons of sedimentary slide and mass flow deposits both intervals were marked by a decline in carbonate production resulting from inundation and uplift emergence the first of these intervals separates deposition on a seafloor with gentle topography carbonate ramp from a situation where major lateral thickness and facies variations were present and deposition took place in a carbonate slope environment the second interval marks the end of major carbonate deposition within the bowland basin and the onset of regional terrigenous sedimentation

characterisation and preliminary quantification of the methane reservoir in a coastal sedimentary source san simon bay ria de vigo nw spain abstract ria de vigo is a river valley flooded by the sea with a bay san simon bay at its innermost part the accumulation of holocene sediment in san simon bay has been studied by the integration of 1 large scale high resolution seismic data and 2 detailed geochemical analysis of a gravity core in san simon bay the majority of the seismic records are obscured by acoustic turbidity which represents gassy sediments but on records from rande strait it is possible to distinguish two quaternary seismic sequences an upper pleistocene sequence sq1 and a holocene sequence sq2 only sq2 is recognized in san simon bay where it is comprised of two seismic units the upper unit represents the hst sediment i e the period of highest sea level a gravity core taken within the gassy zone at 10 m water depth provided 3 55 m of fine grained sediments muds from the youngest seismic unit 4 m thick geochemical analysis show high values 4 to 10 of toc sediment and porewater analyses indicate a distinct sulphate methane transition zone smtz between 60 and 80 cm where sulphate is depleted to 0 4 mm the top of the acoustic turbidity the gas front at 80 cm corresponds to the lower limit of the smtz the methane cannot have been derived from the underlying metamorphic and granitic rocks but was probably derived by microbial degradation of the organic matter in the holocene sediments we estimate that the sediments of the bay contain approximately 1 8 10 6 m 3 of organic carbon and 275 ton of methane

observations from a yakutat eddy in the northern gulf of alaska 1 the impingement of deep basin eddies on the shelf in the gulf of alaska has been implicated as an important mechanism for cross shelf exchange the influence of eddies on biological processes has been confirmed with data from the sea viewing wide field of view sensor showing elevated chlorophyll associated with eddies altimetry data suggest that an eddy formed in the winter of 2003 near yakutat alaska and propagated along the shelf break reaching the head of the gulf by spring this eddy was sampled during may and september of 2003 the eddy core water was warm salty and high in nitrate relative to basin water of the same density but was similar to historical water properties from the shelf near yakutat this suggests a shelf origin for the eddy core water

factors determining the spatial distribution of gas charged sediments in the continental shelf off southeastern korea abstract a high resolution seismic survey piston coring and water column measurements were conducted on the continental shelf off southeastern korea to examine the gas charged sediments and gas seepage in the water column we examined the factors controlling the spatial distribution of the acoustic blanking zone in recent korea strait shelf mud kssm deposits along the southeastern shelf the molecular composition methane hydrogen and carbon dioxide isotope values of headspace gases and the geochemical composition of core sediments were measured in 11 piston cores dissolved methane concentrations in the water column were observed at 6 of the 11 coring sites acoustic blanking of the recent kssm deposits was limited to areas with water depths between 30 and 105 m along the continental shelf except line l 13 133 m depth the flanks of the acoustic blanking zone can be classified as distinct disturbed and step types based on their configurations the highest methane concentration in the headspace gases was about 2 7 2 4 m below the seafloor which matched the top of the acoustic blanking zone no obvious methane was found in piston cores obtained from the gas free zone the 13 c ch4 ranged from 56 5 to 106 3 relative to vienna pee dee belemnite v pdb and the d ch4 ranged from 194 to 207 relative to vienna standard mean ocean water v smow these isotope values show that methane is produced by the microbial reduction of co 2 ch 4 oxidation results in a significantly heavier carbon isotope fraction from 91 2 to 56 5 in the 2 m below the seafloor with constant 13 c co2 isotope fractionation no obvious differences were seen in the total carbon tc 2 438 3 378 total nitrogen tn 0 159 0 548 total organic carbon toc 1 139 2 078 or caco 3 2 825 15 258 in the continental shelf whereas the toc was high and the caco 3 low distant from the nakdong river distinct dissolved methane anomalies 21 87 ppm were seen in the water column in the acoustic blanking and gas seepage zones this level was about 15 times higher than the dissolved methane level 1 5 ppm in the water column in the gas free zone these results indicate that the acoustic blanking zone accumulates biogenic methane methane seepage was observed in the water column throughout the acoustic blanking zone of the continental shelf off southeastern korea

ramgundam sandstone a middle proterozoic shoal bar sequence abstract much of the abundant precambrian sedimentary record of india appears to have been deposited in shallow coastal environments one example of this the ramgundam sandstone is a moderately thick 120 m unit composed of lens shaped bodies of arkosic sandstone and interbedded sandstone and shale outcrops are mostly limited to small quarry pits but sufficient exposures are present to show major bedding characteristics principal components are wave built bars and interbar fillings the bars 1 to 5 m thick and tens of meters wide contain laterally accreting sets of trough cross bedded sandstone interbar deposits are composed of horizontal to subhorizontal beds of ripple laminated sandstones and interlaminated shales siltstone and sandstone containing wavy lenticular and flaser laminae shallow water subaerial exposure and probable tidal fluctuations are recorded on the bar surfaces by flat topped ripples and interference ripples water recession marks and desiccation cracks

fe illites in a proterozoic deep marine slope deposit in the penganga group of the pranhita godavari valley their origin and environmental significance ferric illite fe illite developed extensively in a siliciclastic submarine fan sequence within a lime mud succession in the proterozoic penganga group at adilabad south india the minerals formed as de novo by preferentially replacing high k minerals like alkali feldspar and mica it is compositionally intermediate between glauconite and illite in a continuously varying chemical spectrum the mineral exhibits an r1 or r3 type stacking sequence with low 10 interlayering of illite and smectite i s all the analyzed grains are characterized by high potash content and it has been inferred that the maturity is dictated by high ak in the microenvironment this occurrence strongly calls into question the environmental significance that has so far been attributed to glauconitic minerals

carbonate ramp depositional systems abstract the classification tectonic settings stratigraphy and early diagenesis of carbonate ramp systems are reviewed carbonate ramps are common in all geological periods but were dominant at times when reef constructing organisms were absent or inhibited ramps can be subdivided into inner mid and outer ramp environments the mid ramp zone extends from fair weather wave base to normal storm wave base although the water depths which these boundaries represent vary an additional outer slope environment occurs on distally steepened ramps as with siliciclastic shelves a range of wave storm and tide dominated ramps can be recognised and this forms the most convenient basis for ramp classification the carbonate productivity profile of ramps differs from that of rimmed shelves with the inner ramp showing lower production rates than comparable shallow water facies on rimmed shelves the zone of greatest organic carbonate sediment production appears to have shifted from the mid ramp to the inner ramp since the late jurassic carbonate ramps occur in most types of sedimentary basin but are best developed where subsidence is flexural and gradients are slight over large areas as in foreland and cratonic interior basins and along passive margins the featureless depositional profiles of many ramps means that sequence geometries are best observed on regional seismic lines some show low angle sigmoidal or shingled clinoforms suggesting that ramps may seldom be homoclinal but possess subtle slope geometries which reflect depositional environments because of their low angle slopes ramps respond differently to rimmed shelves during relative sea level changes although results seem to be strongly dependent on the rate of relative sea level change during a minor fall shallow ramp facies belts will simply shift basinwards in a forced regression in contrast the whole surface of a steep sloped flat topped rimmed shelf may be exposed so that sediment production ceases or is drastically reduced during a major fall shallow ramp bounded basins may empty completely conversely ramps also flood gradually whereas rimmed shelves do so more rapidly homoclinal ramps develop no resedimented lowstand deposits rimmed shelves and distally steepened ramps in contrast may develop lowstand talus or turbiditic wedges distally steepened ramps may behave more like homoclinal ramps during minor base level falls and like rimmed shelves during major base level falls many ramps consist of layered successions of several ramp sequences stacked one upon the other ramp stacks of this sort may show gross vertical accretion but individual ramp sequences seldom appear to develop in a keep up style apart from minor organic buildups as with many rimmed shelves steepening of the outer ramp margin due to tectonism slope inheritance or differential sedimentation may promote the development of a distally steepened ramp or rimmed shelf a wide variety of organisms have constructed buildups in mid and outer ramp environments isolated buildups may seed early in ramp development accrete to wave base or sea level and continue growth by stacking through successive ramp sequences so that depositional and diagenetic features within them are in concert with those of the shallow ramp the location of isolated buildups on ramps is governed by tectonism halokinesis antecedent topography or by the subtle slope geometry of the previous ramp sequence diagenesis on ramps shows some major variations compared with diagenesis on steep sloped flat topped carbonate platforms ramp bounded basins may form prolific petroleum sourcing and reservoiring systems and offer a range of subtle stratigraphic play types and lateral facies variations which determine reservoir quality and distribution isolated buildups in the mid and outer ramp environments represent one of the commonest petroleum reservoirs in ramp systems and tend to have their foundations in transgressive systems tracts grainstone and packstone reservoirs are widespread and range from shoreline carbonate sandbodies to major detached shoal complexes or shoals over offshore highs grainstone sandbodies occur in both highstand systems tracts and in prograding lowstand wedges

geochemical characteristics of chilean upwelling sediments at 36 s abstract we examined the cycling of c s fe and the trace elements te ag cd mo mn re and se in chilean upwelling sediments from water depths of 127 2744 m at 36 s the sediments are clay rich with moderate total organic carbon toc and low biogenic caco3 contents for the shallow site within oxygen minimum zone omz which shows some thioploca biomass highest observed toc contents correspond to highest productivity induced toc flux and or enhanced preservation under o2 deficient conditions pyrite formation may be restricted by the concurrent presence of elemental s in thioploca and or the availability of reactive iron in the other sediments 370 m water depth pyrite formation is strongly restricted due to the low availability of metabolizable toc for sulfate reduction calculations of the degree of bacterially mediated remineralization suggest that more nitrogen than toc was remineralized by sulfate reduction than expected from redfield c n relationship manganese is significantly depleted due to reductive dissolution of mn hydr oxides within all sediments mo is only significantly enriched vs the lithogenic background in the shallow core implying anoxic conditions contents of re and mo as well as re mo ratios indicate suboxic conditions for the other cores cd and re contents decrease with increasing water depth reflecting decreasing reducing conditions in contrast authigenic ag contents increase with water depth suggesting enrichment controlled by seawater availability and or opaline input and not redox conditions elevated se contents in all cores may be either due to bio detrital input or diffusion from seawater accumulation rates of toc ts and te off chile are in the same order as those from the gulf of california and lower than those off peru distinct seasonality in upwelling and dilution of chilean sediments with terrigenous material lead to rather low metabolizable toc contents and lower te trapping capacity

the influence of coastal origin eddies on oceanic plankton distributions in the eastern gulf of alaska abstract mesoscale anticyclonic eddies that form along the eastern continental margin of the gulf of alaska in winter transport large quantities of coastal water to the open ocean the continuous plankton recorder cpr sampled repeated transects that passed close to or through these eddies in 2000 and 2001 shelf species of diatoms and calanoid copepods were recorded in cpr samples within or in close proximity to eddies and they persisted through the sampling period several oceanic taxa also showed higher concentrations within the eddies significantly reduced occurrences of shelf taxa in oceanic samples were seen in 2001 coinciding with a weaker eddy than observed in 2000 images of ocean temperature and chlorophyll a prepared from satellite observations reveal changes in offshore transport mechanisms by eddies between winter and spring and between southern and northern eddies in this region these effects may also explain some of the seasonal and geographical variability in the cpr samples between april and june of 2000 the haida eddy straddled the low chlorophyll a waters of the gulf and the high chlorophyll a waters of the continental margin and entrained each water type into the neighbouring region the resulting swirling patterns were clearly observed in seawifs imagery sitka eddies failed to penetrate into low chlorophyll a waters during these months and were less effective in stirring high and low chlorophyll a concentrations surface currents in surrounding waters simulated by the ocean surface current simulations oscurs model were used to provide some insights into the likely fate of planktonic organisms once they leave the eddies we show that these eddies may play a significant role in distributing plankton species

formation of the haida 1998 oceanic eddy 1 two large mesoscale anticyclonic eddies formed along the west coast of the queen charlotte islands of western canada in early 1998 altimetry measurements from topex poseidon and ers 2 satellites suggest that these eddies first appeared near cape st james at the southern tip of the islands the eddies merged in june to form haida 1998 the highest eddy ever observed in the region currents near cape st james in winter follow complicated patterns attributed to tidal rectification and pressure driven outflow from hecate strait the adjustment of these flows to the bathymetric features likely contributes to the formation of haida eddies eddies that first appear farther north along the west coast of the queen charlotte islands are set up by other processes such as baroclinic instability

a synoptic survey of young mesoscale eddies in the eastern gulf of alaska eddies in the gulf of alaska are important sources of coastal water and associated nutrients iron and biota to the high nutrient low chlorophyll central gulf of alaska three primary eddy formation regions along the eastern boundary of the gulf have been identified from south to north haida sitka and yakutat in the spring of 2005 three eddies one of each type were sampled soon after their formation the subsurface eddy core water in all three eddies was defined by high iron concentrations and low dissolved oxygen compared with surrounding basin water the sitka and yakutat core waters also exhibited a subsurface temperature maximum mesothermal water coincident in depth with the iron maximum suggesting that eddies may play a role in the formation of temperature inversions observed throughout the gulf of alaska the data suggest different formation regions with the yakutat eddy forming in shallow shelf water with riverine input while the sitka and haida eddies appear to form in deeper water

preservation of elemental and isotopic source identification of sedimentary organic matter the amount and type of organic matter in the sediments of lakes and oceans contribute to their paleoenvironmental and paleoclimatological records only a small fraction of the initial aquatic organic matter survives destruction and alteration during sinking and sedimentation selective degradation modifies the character of the surviving small fraction of organic matter which becomes incorporated in bottom sediments organic matter alterations can continue to sub bottom depths of hundreds of meters corresponding to millions of years source and paleoenvironmental information nonetheless remains preserved in the molecular elemental and isotopic compositions of organic matter c n and 13c values of total organic matter in particular appear to retain paleoenvironmental information for multi myr time periods

late diagenetic changes in manganese ores of the upper proterozoic penganga group india sedimentary manganese oxide ores occur interstratified with chert and minor calcareous shale and enclosed in limestone in the transgressive upper proterozoic marine sequence of the penganga group andhra pradesh india this sequence shows no evidence of volcanism or hydrothermal activity and is not metamorphosed the manganeseoxide ores show unusual coexistence of the primary sedimentary minerals todorokite and birnessite with their diagenetic conversion products manganite braunite and bixbyite the mineralogical characters and chemical compositions of the important phases as well as their texture are discussed braunite occurs in two generations one earlier or contemporaneous with bixbyite and the other later than bixbyite the different stages of conversion of todorokite and birnessite to braunite and todorokite to manganite are preserved the general observations suggest that bixbyite was also produced by conversion of todorokite birnessite pyrolusite and cryptomelane conventionally considered as diagenetic products of mn oxide hydroxide precursors deposited as primary sediments are here rarely present and then only as supergene derivatives this documentation of diagenetic transformation of primary todorokite and birnessite directly to braunite and bixbyite should be considered while extrapolating the progenitor mineralogy of the metamorphosed manganese oxide ores that abound in the precambrian

fabric development in proterozoic bedded chert penganga group adilabad india sedimentologic implications laterally persistent horizons of interstratified chert limestone manganese ores within a siliceous micritic limestone are interpreted to have been deposited on a distally steepened deep carbonate ramp in the proterozoic penganga group of the pranhita godavari valley south india the association of intraformational limeclast conglomerate turbidites and pelagic micritic limestone indicates deposition of the chert bearing horizons at the toe of a distally steepened ramp slope environment the chert is characterized by a wide variety of fabrics including cryptocrystalline and microcrystalline quartz equant megaquartz and chalcedony cryptocrystalline and microcrystalline quartz are most common they occur mainly as lenses of mosaic quartz and define laminae megaquartz in contrast occurs as irregular patches as fenestroids within cryptocrystalline or microcrystalline fabrics or in complex aggregates and disrupted mosaics the origin of the cryptocrystalline and microcrystalline quartz by maturation parallels the morphological evolution of many deep sea cherts that form by maturation of biogenic opal a to quartz chert through an opal ct stage within the quartz stage the microcrystalline fabric formed by pervasive space centered grain growth of cryptocrystalline crystals and porphyroid growth resulted in porphyrotopic fabric the megaquartz formed directly from pore water in continuity with maturation of opal ct the disrupted and curdled mosaics attest to high fluid activity within semilithified sediments the variation in fabric types at the microscale suggests fabric evolution at low temperature the presence of chert clasts in debris flow conglomerates and development of a plethora of water escape structures suggest maturation of silica at shallow burial depth maturation at shallow burial was favored by very low detrital content of these cherts and a mg rich pelagic carbonate depositional geochemical milieu estimated sedimentation rates from 2 to 10 mm ky super 1 closely match the average sedimentation rate from modern pelagic siliceous deposits

bacterial populations and processes in sediments containing gas hydrates odp leg 146 cascadia margin abstract bacterial populations and activity were quantified at three sites in the cascadia margin accretionary wedge off the west canadian american coast odp leg 146 at two sites sediments contained gas hydrates site 889 890 had a discrete zone of hydrate approximately 10 m above a bottom simulating reflector bsr at 225 m below sea floor mbsf and site 892 had disseminated hydrate in the upper 20 mbsf and a bsr at 74 mbsf site 888 was a control site without gas hydrates the control site 888 and top approximately 90 mbsf of site 889 890 had bacterial distributions similar to previous pacific ocean sites in the upper approximately 30 m of site 892 however bacterial populations were much lower suggesting inhibition by the high concentrations of h 2 s within the hydrate zone below this depth bacterial populations rose to concentrations consistent with other sites the control site was dominated by so 4 reduction and rates of ch 4 oxidation in the top 90 m were low 0 002 0 033 nmol cm 3 d 1 at site 889 890 bacterial populations and activity were stimulated in the discrete hydrate zone ch 4 oxidation rates increased in the middle of this zone to 134 5 nmol cm 3 d 1 ca 9 times the average rate at other depths resulting in a significant 10 increase in the total bacterial population the anaerobic process es responsible for this oxidation remain unclear despite so 4 reducing bacteria previously associated with ch 4 oxidation also being stimulated in this zone fluid flux into accretionary wedge sediments may be an important process in providing electron acceptors to maintain these relatively high rates of ch 4 oxidation this first microbiological study of gas hydrates indicates that bacterial processes are influenced by gas and fluid venting and they play a major role in geochemical changes within these deep 200 mbsf sediments

facies associations and depositional environment of the proterozoic carbonate hosted microbanded manganese oxide ore deposit penganga group godavari rift basin india the late proterozoic penganga group pranhita godavari valley south india is a sequence of siliciclastic carbonate sediments that was deposited in an intracratonic rift basin the nonvolcanogenic sedimentary succession containing listric basin marginal faults shows abrupt facies changes from craton to trough with rich resources of stratabound microbanded manganese oxide ore hosted within the trough facies the cratonic sediments of the basin are mature siliciclastics with clear imprints of tide dominated shallow subtidal deposition the siliciclastics show rapid lateral transition into a thick sequence of nonstromatolitic and micritic ribbon limestone containing sediment gravity flow deposits in the basal part and a persistent horizon of bedded manganese ore deposit in the upper part the limestone is overlain by a thick sequence of basinal shale facies through an interbedded sequence of limestone and shale uniformly thick laterally persistent parallel sided and thin bedded limestone shale rhythmites and parted limestone rhythmites characterize the ribbon limestones except for widespread thin parallel lamination structures formed by waves tidal currents and storm induced currents are totally absent the limestone is brown and argillaceous in the basal part and changes upwards to siliceous and pyritiferous light gray to dark gray varieties and finally to carbonaceous black limestone the physical attributes of the limestone suggest complete drowning of the carbonate interval in response to higher sea level caused by tectonic subsidence and a deep water slope and basinal environment of carbonate deposition under continued transgression the bedded manganese oxide ore interstratified with chert simulating iron formation typically shows micron scale to millimeter scale thin parallel laminae of almost uniform thickness texture and composition the manganese ore and chert laminae chiefly have a cryptocrystalline to microcrystalline fabric consisting of grains of clay size to fine silt size todorokite and birnessite are the most abundant sedimentary manganese oxides they were locally transformed to braunite and bixbyite during diagenesis the centimeter thick laminated chert beds are analogous to microbanded chert mesobands of the iron formations and are composed of microcrystalline quartz the sedimentary attributes of the ore chert sequence suggest deposition from suspension fallout in a low energy environment penecontemporaneous deformation of the ore deposit indicates slow downslope movement of the semiconsolidated sediments over the basin floor the sedimentology and stratigraphy of the penganga group suggest a sediment gravity flow influenced deep water base of slope basinal environment of manganese accumulation during the thermal subsidence stage of basin development within an extensional rift setting on the basis of sedimentological evidence and parallel studies done on iron formations a possible hydrothermal source of manganese is also considered this represents a significant departure from the prevalent views of high energy shallow water shelf environments of manganese deposition and terrigenous sources of metal in cratonic basins hosting several important manganese deposits and thus it has important implications for genesis of manganese ores

carbonate platform facies models various types of carbonate platforms are characterized by distinctive profiles facies and evolutionary sequences ramps may be homoclinal or distally steepened and may have fringing or barrier shoal water complexes of ooid pellet sands or skeletal banks homoclinal ramps pass seaward into deeper water without major break in slope and lack deep water breccias distally steepened ramps may be low energy and characterized by widespread shallow subwave base mud blankets or high energy with coastal beach dune complexes and widespread skeletal sand blankets slope facies may contain abundant breccias of slope derived clasts rimmed shelves have relatively flat tops and marked break in slopes at the high energy shallow shelf edge where they pass into deep water such shelves may be aggraded with peritidal facies extending over much of the shelf or incipiently drowned depending on magnitude of sea level fluctuations they may be accretionary or bypass types that include gullied slope escarpment and high relief erosional forms intrashelf basins occur on some shelves controlling distribution of reservoir and source beds isolated platforms are surrounded by deeper water and may be located on rifted continental margins or on submarine volcanoes most have high relief rimmed margins platforms that have been subjected to rapid sea level rise may be incipiently drowned and characterized by raised rims elevated patch or pinnacle reefs and widespread subwave base carbonate or fine clastic blankets completely drowned shelves develop where the shelf is submerged to subphotic depths terminating shallow water deposition and commonly resulting in blanketing of the shelf by deeper water facies some margins show extensive down to basin faulting that is contemporaneous with carbonate deposition or associated with thick prograding clastic sequences the various types of platforms change in response to variations in sedimentation subsidence or sea level rise and may form distinctive evolutionary sequences the relatively few models presented appear to accommodate most geological examples some of which contain major reservoir facies

deep water dolomites from the proterozoic penganga group in the pranhita godavari valley andhra pradesh india abstract the middle to upper proterozoic penganga group of the pranhita godavari valley south india contains a thick slope to basin limestone shale succession the limestone the chanda limestone is well bedded and micritic with several interbeds of slope related autoclastic debris flow lime clast conglomerates in its lower part the limestone near its base encloses a 30 m thick interval of rhythmically alternating centimeter to decimeter thick beds of limestone and dolomitic limestone the autoclasts of debris flow conglomerates within the dolomite bearing sequence shows similar rhythmic repetition of dolomite and limestone dolomite crystals range in size from micrite to 225 m and have planar s to idiotopic p fabrics a dolomitic bed contains 40 45 dolomite the dolomite content ommonly decreases from the base to the top of a bed with decline in frequency and size of coarse dolomite rhombs the dolomite is nonstoichiometric ca0 54 0 56 mg0 46 0 44co3 and has cloudy core with inclusions of micritic calcite and dolomite the d13cpdb values of limestone and dolomite vary between 2 0 to 3 4 and 3 8 to 4 3 respectively the d18opdb values of limestone and dolomite range from 6 0 to 7 6 and 0 4 to 8 7 respectively stratigraphic petrographic and geochemical studies suggest dolomitization in normal marine pore water during shallow burial diagenesis rhythmicity is attributed to recurrent episodes of dolomitization with mg2 derived mainly from dissolution of precursor high magnesium calcite a mass balance calculation suggests that 9 mole mgco3 in the precursor would provide sufficient mg2 normal grading of dolomite rhombs suggests that upward movement of mg enriched pore water dolomitized a thin interval of lime tone wherever the mg content reached the threshold for dolomitization

modeling the residual circulation of a coastal embayment affected by wind driven upwelling circulation of the ria de vigo nw spain 1 the barotropic and baroclinic nontidal circulations of the ria de vigo nw spain are studied using the three dimensional finite difference model hamsom hamburg shelf ocean model the external forcing acting on the system includes winds freshwater inflow in the inner part of the ria and all over its surface and heat exchange with the atmosphere modeled velocity is compared with data from an aanderaa dcm12 acoustic doppler current meter at one site of the ria and predicted salinity and temperature fields contrasted with seabird conductivity temperature depth data along the main channel of the ria all predicted current salinity and temperature fields are in good agreement with experimental data a new residual circulation pattern of the ria de vigo and its dependence on coastal winds is described according to this pattern the typical estuarine two layer circulation overlays in the outer part of the ria with a lateral circulation resulting in a three dimensional dynamics the lateral circulation is induced by interaction between the southward northward alongshore coastal jet associated with upwelling downwelling winds and the topographic configuration of the ria

reinterpretation of the quaternary sedimentary infill of the ria de vigo nw iberian peninsula as a compound incised valley abstract seismic data have been used to investigate the stratigraphy of the galician rias for more than two decades here we present a new interpretation of the sedimentary infill of an incised valley ria de vigo nw iberian peninsula based on high resolution seismic profiles core sediment analysis and radiocarbon 14 c data the new data indicate that the stratigraphic architecture of the galician rias result from multiple incision infill phases and therefore they are reclassified as compound rather than simple incised valleys seven seismic units were identified one of tertiary age u 1 four of pleistocene age u 2 u 5 which are interpreted as 4 th order sequences deposited between mis 11 and mis 2 and late pleistocene u 6 and holocene u 7 units corresponding with post glacial sedimentation the sedimentary infill overlies a highly faulted irregular granitic and metamorphic basement the inherited morphology is shown to be important for controlling the pathway and evolution of the fluvial network as well as preservation of the sedimentary deposits during several glacial interglacial cycles the presence of a rocky barrier at the mouth of the ria is a distinctive feature that conditions sedimentation and exchange of sediment between the ria and the adjacent shelf for the first time faults and tilted blocks affecting late pleistocene mis 3 deposits have been identified the new data presented here provide the opportunity to reconstruct the evolution of the sedimentary infill of a ria with especially high resolution during the last post glacial transgression

the holocene gas system of the ria de vigo nw spain factors controlling the location of gas accumulations seeps and pockmarks abstract high resolution seismic profiles cored sediment gravity cores and vibro cores and radiocarbon data from suitable shell material were used to establish the stratigraphic context of gassy sediments in the ria de vigo mapping of seeps and pockmarks were compared with the stratigraphy of gassy sediment to ascertain the relationships in the ria de vigo three holocene seismic units were identified the first two uh 1 and uh 2 correspond to the transgressive systems tract tst and the third uh 3 corresponds to the highstand systems tract hst the presence of acoustic turbidity within the three seismic units indicates that generation of gas took place during the relative sea level rise that occurred after the younger dryas event i e during the tst and mainly during the hst the relative sea level variations are shown to be a major factor controlling the type fine vs coarse grained and distribution of facies lateral and vertical that generate and or allow gas accumulation or escape acoustic turbidity due to presence of free gas was recognized in the three holocene units with a total of 12 acoustic turbidity fields mapped in the ria de vigo no significant variations in the spatial distribution or extent of the gas fields or in their vertical location below the present sea bed have been observed during the seismic surveys carried out in the ria during the last decade furthermore gas escape from the sediment and active pockmark has been observed therefore we conclude that gas escape seeps pockmark generation and maintenance and methane formation are in equilibrium the combined analysis of high resolution seismic profiles sediment temperature pore water salinity and hydrostatic pressure revealed 1 a direct relationship between depth of free gas and hydrostatic pressure and 2 a direct link between sediment temperature and number of acoustic plumes from seabed to water column more acoustic plumes were detected in summer when the temperature of the sediment was higher than in winter indicating a seasonal variation of seeps within the ria salinity showed no significant differences between sea bottom water and sediment pore water and was therefore not a major factor controlling gas front variations

methanogenesis and sulfate reduction in marine sediments a new model a number of studies have shown that methanogens are active in the presence of sulfate under some conditions this phenomenon is especially exemplified in carbonate sediments of the southern australian continental margin three sites cored during ocean drilling program odp leg 182 in the great australian bight have high concentrations of microbially generated methane and hydrogen sulfide throughout almost 500 m of sediments in these cores the sulfate reducing and methanogenic zones overlap completely that is the usual sulfate methane transition zone is absent amino acid racemization data show that the gassy sediments consist of younger carbonates than the low gas sites high concentrations of the reduced gases also occur in two odp sites on the margin of the bahamas platform both of which have similar sedimentary conditions to those of the high gas sites of leg 182 co generation of these reduced gases results from an unusual combination of conditions including 1 a thick quaternary sequence of iron poor carbonate sediments 2 a sub seafloor brine and 3 moderate amounts of organic carbon the probable explanation for the co generation of hydrogen sulfide and methane in all these sites as well as in other reported environments is that methanogens are utilizing non competitive substrates to produce methane within the sulfate reducing zone taken together these results form the basis of a new model for sulfate reduction and methanogenesis in marine sediments the biogeochemical end members of the model are 1 minimal sulfate reduction 2 complete sulfate reduction followed by methanogenesis and 3 overlapping sulfate reduction and methanogenesis with no transition zone

geochemical and microbial context of the gassy sediments in the ria de vigo nw of spain abstract multidisciplinary data coming from high resolution seismic profiles gravity core analysis sedimentology and geochemistry radiocarbon dating and metagenomics has been used to establish the stratigraphic framework as well as the geochemical and microbial context of the gassy sediments in the ria de vigo gas appears within the muddy sediments that compound the youngest holocene seismic unit of the infilling of the ria and is mainly methane generated by microbial decomposition in a rich organic matter depositional environment within the ria the biogeochemical zonation is affected by the quality quantity and age of the buried organic matter the 16s ribosomal rna gene amplicon clone libraries sequencing suggests that prokaryotic composition may be linked to the sediment type uncultured archaea common to the marine sub sea floor of the miscellaneous crenarchaeotal group mcg and marine benthic group d mbgd are dominant in these organic rich estuarine and shallow marine sediments the presence of methanomicrobia and anme in the sulphate methane transition zone smtz from the estuarine gassy area indicates anaerobic oxidation of methane aom in these sediments desulfobacteraceae are also homogeneously distributed along the sediment columns even in the methanogenic zone suggesting the existence of a cryptic sulphur cycle driven by iron under the smtz the older sediments retrieved from the non gassy outer area of the ria contain less organic matter and appear to represent mainly a sulphate reducing environment where smtz is not yet reached at the bottom of the sediment core methanogens of various affiliations and traces of methane are detected in this sulphate reducing environment with an unusual increase in concentration towards the sediment surface microbial methanogenesis in the sulphate reducing zone of surface sediments a process recently described in organic rich sediments able to supply methanogenic substrates could reconcile these observations

basinal autoclastic mass flow regime in the precambrian chanda limestone formation adilabad india abstract the precambrian chanda limestone formation in adilabad district andhra pradesh india transcribes carbonate mass flow deposition in an extensional basin within the godavari rift valley system in the lower part of the formation conglomerate beds which commonly interrupt the sequence of thin bedded lime mud turbidites register pulsatory reinvigoration of mass flow processes these gravelly mass flow products are significant because they indicate i deposition far below the wave base ii intrabasinal origin iii disassociation with any depositional slope like reef margins iv occurrence in discrete vertical packages v clear signals for sediment gravity flows and vi invariable facies successions the conglomerate bodies are extensive parallel to the depositional strike but restricted across the latter the main mode of accretion is dominantly vertical aggradation only the muddy turbidites spreading far and wide into the basin the course of temporal evolution of the mass flows are also interpreted from vertical facies transitions although every flow eventually gave rise to turbidity currents their initial states differred in three distinct ways which are correlated with gradient of the translational slope gradual decrease in declivity of the depositional substrate through time is interpreted

proterozoic deep marine volcanic eruptive product obsidian glass in chanda limestone penganga group india its implications for basin evolution and metallogenesis abstract subaqueous effusive to low explosive eruptive product composed of blocky and fluidal fragments of glass and sparsely crystalline glass of obsidian lava are present in fine grained volcaniclastic debris flow conglomerate and turbidite occurring in a deep marine sequence of thin and even bedded grey siliceous micritic limestone of the chanda limestone the siliceous limestone conformably encloses stratiform manganese deposit obsidian glass fragments consist of coherent and amorphous glassy groundmass containing patches of devitrified fibrous glass oriented microlites and microphenocrysts of quartz and k feldspars euhedral crystals resorbed rounded and embayed crystals spherical vesicles microlites and flow texture are distinctive volcanic features observed in these fragments chemically similar to obsidian lava the blocky fragments represent the products of hydroclastic fragmentation whereas the fludial one may indicate formation under steam explosion gravitational instability of the resulting slurry of glass fragments and the ambient sea water sediments at the site of eruption causes gravity flows that carries the fragments along with epiclasts away from vent to a distal site for final deposition this study records maiden evidence of volcanism in the chanda limestone of the neoproterozoic penganga group and infers rifting and partial melting of the sialic crust and silicic lava flow during deposition of limestone unusually rich in silica the inferred tectono sedimentary setting facies association and mode of occurrence of the manganese deposit suggest a hydrothermal source for the metal

a seeping sea floor in a ria environment ria de vigo nw spain the occurrence of gas accumulations in the ria de vigo nw spain have been characterized by the authors in previous research pockmarks frequently appear on seismic and sonar records covering ca 45 of the sea floor of the study area which indicates that gas expulsion is not an uncommon phenomena in the coastal ria environment here we report the occurrence of gas seepage for the first time along the coast of nw spain side scan sonar echo sounder and high resolution seismic techniques were used for mapping gas expulsion features some expulsion pockmarks sit over elongated features that represent bottom marks created by anthropogenic activity thus these anthropogenic sites may act as preferential venting zones for gas as well as being potential hazards on a muddy sea bed such as that of the ria where gas accumulates just below the surface of the sea floor

shallow to deep water deposition in a cratonic basin an example from the proterozoic penganga group pranhita godavari valley india abstract the unmetamorphosed proterozoic succession dominated by deep water lithographic limestone and shale in the western flank of the pranhita godavari valley is designated as the penganga group the succession in different parts of the valley includes the pranhita sandstone 25 400 m the chanda limestone 300 m and the sat nala shale 2000 m in ascending order the pranhita sandstone and the chanda limestone reveal considerable variations in the character of the stratal packages and depositional settings from mancherial in the south to adilabad in the north the sat nala shale in both southern and northern outcrop belts is completely devoid of sand it is brown to purple in colour and resembles present day deep water mud deposits in the mancherial area the pranhita sandstone consists of 25 400 m thick conglomerate pebbly red arkose and quartzose sandstone succession of coastal alluvial fan to shallow shelf origin the chanda limestone is micritic and locally includes interbedded lenses of cross stratified quartzose sandstone in the lower part the depositional milieu varies from shallow shelf to below wave base outer ramp carbonate platform around adilabad the pranhita sandstone 25 m lacks the conglomerate pebbly arkose association at the base and comprises only quartzose sandstone and shale of shoreface to muddy shelf settings the chanda limestone is essentially micritic but in contrast to the mancherial area includes several interbedded intervals of slope related autoclastic debris flow limestone conglomerates and calciturbidites and represents deep outer ramp to slope and basinal settings a predominantly deep water micritic limestone and deep water shale succession suggests that the penganga basin evolved to a vast deep epicratonic sea connected to an open ocean the absence of a coastal alluvial fan association at the lower part of the pranhita sandstone and presence of a slope to basinal association in the chanda limestone in the northern outcrop belts around adilabad indicates a northward transition from coastal to deep shelf slope and basinal conditions and a northerly paleoslope of the basin

platform to basin facies evolution the carbonates of late proterozoic vendian gourma west africa abstract during the vendian uppermost proterozoic the gourma basin was an east west intracratonic feature that opened eastward into an ocean the southern edge of the gourma basin preserves facies belts representing land to deep basin environments shoreward facies consist of tidal belts of microbially laminated dolomites or porostromata dolomites interstratified with clastic carbonates of local origin landward supratidal facies consist of finely laminated sedimentary rocks with sizeable desiccation features but no evaporites the upper part of this unit is affected by a periodic emersion resulting in a complex succession of paleosol profiles offshore of the tidal facies are lagoon facies rocks that were protected from the open sea by a carbonate buildup composed of a framework of stromat lite bioherms associated with detrital dolomites indicating a high energy environment slope facies consist of a thick succession of ribbon limestones with some graded beds synsedimentary slumping and faulting of the ribbon limestones indicate a significant depositional slope allochthonous deposits are sporadically interstratified with the ribbon limestones and consist of muds with small grains derived from the platform deposits polygenic breccias conglomerates composed only of ribbon limestone intraclasts and oolitic calcarenites shelf facies rocks are pervasively dolomitized but original textures are well preserved including fine grained structures the ribbon limestones on the other hand are not dolomitized except for dolomitic layers caused by pressure solution the distribution of facies in gourma basin carbonates corresponds well with ideal models of wilson 1975 comparisons with present day facies distributions are less satisfactory especially considering the dominance of cyanobacteria over all environments for which there is no modern day counterpart comparisons with the modern environment however suggest a subtropical semi arid climate during deposition of the gourma strata

significance of styles of epicontinental shale sedimentation in the belt basin mid proterozoic of montana u s a abstract within the strongly shale dominated sediment fill of the mid proterozoic belt basin a large variety of shale types can be distinguished sedimentological investigations of several formations have yielded valuable data about lateral associations of shale types as well as about the principal mechanisms of shale deposition depositional environments of shales in the belt basin range from red bed mudstones of ancient flood plains to deep water mudstones in a turbidite setting graded silt mud couplets are ubiquitous in most shales of the belt basin however they differ in detail and can be related to a variety of depositional processes such as low density turbidity currents sheet floods storms and wave reworking in several formations there is evidence that microbial mats colonized the sediment surface and probably protected the sediment surface from erosion shales from comparable phanerozoic settings generally lack the silt mud couplets that are so commonly observed in shales from the belt basin and other proterozoic epicontinental basins this difference probably reflects the proliferation of bioturbating organisms in the phanerozoic the absence of benthic microbial mats in phanerozoic shales is probably due to the evolution of metazoan grazers towards the end of the proterozoic

persistently declining oxygen levels in the interior waters of the eastern subarctic pacific abstract fifty years of measurements at ocean station papa osp 50 n 145 w show trends in the interior waters of the subarctic pacific that are both impacted by short term few years to bi decadal atmospheric or ocean circulation oscillations and by persistent climate trends between 1956 and 2006 waters below the ocean mixed layer to a depth of at least 1000 m have been warming and losing oxygen on density surfaces found in the depth range 100 400 m 26 3 27 0 the ocean is warming at 0 005 0 012 c y 1 whereas oxygen is declining at 0 39 0 70 mol kg 1 y 1 or at an integrated rate of 123 mmol m 2 y 1 decrease of 22 over 50 years during this time the hypoxic boundary defined as 60 mol o 2 kg 1 has shoaled from 400 to 300 m in the alaska gyre the 26 2 isopycnal occasionally ventilates whereas at osp 26 0 has not been seen at the ocean surface since 1971 as the upper ocean continues to stratify to interpret the 50 year record at osp the isopycnal transport of oxygenated waters within the interior of the subarctic pacific is assessed by using a slightly modified no parameter broecker w 1974 no a conservative water mass tracer earth and planetary science letters 23 100 107 the highest nitrate oxygen signature in interior waters of the north pacific is found in the bering sea gyre western subarctic gyre and east kamchatka current region as a consequence of winter mixing to the 26 6 isopycnal by mixing with low no waters found in the subtropics and okhotsk sea this signature is diluted as waters flow eastward across the pacific evidence of low no waters flowing north from california is seen along the coasts of british columbia and se alaska oxygen in the subsurface waters of the alaskan gyre was supplied 60 by subarctic and 40 by subtropical waters during woce surveys whereas such estimates are shown to periodically vary by 20 at osp other features discernable in the osp data include periods of increased ventilation of deeper isopycnals on an 18 year cycle and strong short term few month variability caused by passing mesoscale eddies the potential impacts of declining oxygen on coastal ecosystems are discussed

a note on cross shelf exchange in the northern gulf of alaska the continental shelf of the gulf of alaska goa is a complex system characterized by large freshwater runoff and strong winds the goa supports one of the world s richest ecosystems including numerous species of fishes marine mammals and sea birds the mechanisms that provide nutrients to support this ecosystem are not well understood the rivers and streams that provide freshwater to the shelf are low in nitrate and the regional winds favor downwelling high concentrations of nitrate are available in the deep basin of the goa but these must be introduced to the shelf in order to support the high productivity we present evidence for cross shelf exchange due to three different mechanisms episodes of downwelling relaxation result in a flux of saline nutrient rich water onto the shelf at depth eddies formed in the northeastern goa propagate along the shelf break influencing cross shelf exchange by carrying shelf origin water from the formation region into the basin and by interacting with the shelf break circulation bathymetric steering in the many canyons that incise the goa shelf results in flow into the canyons where strong tidal mixing results in cross isobath movement of water properties

topographically induced baroclinic eddies near a coastline with application to he northeast pacific abstract a mathematical model is developed to describe the interaction between variable bottom topography and a steady horizontally sheared baroclinic coastal current the topography modeled in this study consists of an offshore seamount and a seaward protrusion of the continental slope the fluid motions are assumed to conserve potential vorticity on the f plane and expressions for the pressure density velocity and mass transport fields are obtained using a normal mode analysis and the appropriate green s function for the horizontal problem the theory is applied to the northeast pacific ocean in an attempt to model the anticyclonic eddy which has been observed by tabata west of sitka alaska the numerical calculations of the model and the observed location dimensions velocities and transports of the sitka eddy are in good agreement the axial velocities and dimensions of the calculated eddy field are largest for upstream surface and bottom currents of approximately 5 7 and 1 2 cm s 1 respectivel