seismic and geologic characteristics of drowning events on carbonate platforms carbonate platform drowning events are recorded in upper jurassic lower cretaceous carbonates of the baltimore canyon area offshore united states east coast and lower miocene carbonates of the pearl river mouth basin offshore people s republic of china cursory examination of seismic data from both areas would indicate that the platforms fit the classic definition of a drowning unconformity however detailed lithologic and paleontologic data indicate that the depositional erosional hiatuses vary widely in each area from 0 25 m y in the baltimore canyon area to a condensed section with no hiatus in the pearl river mouth basin the sedimentary sequence produced during drowning the drowning sequence in both areas is gradational with underlying shallow platform carbo ates and in some places overlying deeper marine limestones and shales the generalized facies models proposed for drowning events suggest that they may appear as instantaneous or gradual changes in geologic and seismic data however high resolution seismic data across the baltimore canyon and pearl river mouth basins show that some drowning sequences may be masked on conventionally processed seismic sections because of this resolution problem a distinction should be made between seismically and geologically defined unconformity surfaces the geologic characteristics of drowning events on carbonate platforms include a gradational lower and sometimes upper contact chemical sedimentation glauconite phosphate open marine shelf sediments and a variable hiatus at the upper boundary late growth shelf margin reefs also can be a diagnostic feature of drowned platforms seismic characteristics include horizontal to sub horizontal basinal marine onlap basin parallel reflector continuity above the carbonate sequence boundary and downlap of later highstand units evidence from platforms adjacent to continental margins baltimore canyon indicates that drowning occurs in a step wise fashion advancing more or less symmetrically across the platform from shelf to nearshore environments data from isolated open ocean atolls pearl river mouth basin indicate that they respond differently to drowning showing an asymmetric decrease in shallow platform environments as drowning occurs proper recognition by a geoscientist of the seismic and geologic characteristics of drowning events can lead to better reservoir and seal estimates and the correct reconstruction of the depositional tectonic and eustatic histories of an area

seismic models of a prograding carbonate platform vercors south east france abstract the southern vercors area south east france displays superb exposures of a lower cretaceous prograding carbonate platform these exposures allow a geometrical sequence analysis to be combined with detailed outcrop observations two dimensional seismic modelling techniques were used to determine the seismic response of three of these exposures the montagnette the archiane valley and the rocher de combau the outcrops display prograding and retreating series of clinoforms interfingering with fine grained basinal sediments photographs from different angles of view combined with detailed field observations were used to construct lithological profiles with a resolution of f few metres p wave velocities and bulk densities were measured from cored hand specimens and selected values were assigned to lithostratigraphic units in each profile the vertical incidence modelling technique was then used to compute migrated time sections of reflectivity these were convolved with source wavelets of different frequencies to produce the final synthetic seismic profiles none of the modelled outcrop geometries is correctly portrayed seismically at 25 50 hz frequencies not only do the synthetic seismic profiles show less detail they also show misleading geometries in most seismic models for instance pseudo unconformities are present that correspond to rapid changes in dip and in facies in outcrop furthermore the seismic tool is not able to resolve between stratal patterns punctuated by unconformities and those with gradual shifts in facies belts accompanied by complex interfingering relationships this implies that an unconformity recognized on a real seismic line should not automatically be assumed to represent a true stratal unconformity large outcrops are often used to show the validity of sequence stratigraphy a concept mainly based on the analysis of seismic lines this study however shows that there is no one to one relationship between geometries observed in outcrop and in seismic lines seismic modelling of outcrops therefore provides a means for evaluating outcrop geometries in terms of seismic stratigraphy

seismic stratigraphy of devonian reef complexes northern canning basin western australia seismic stratigraphic modeling of the devonian reef complexes of the northern canning basin suggests that the model of reef development proposed by playford can be applied judiciously to the subsurface reefs playford proposed based on outcrop geology that reef development occurred on the edge of upright backstepping platforms during the frasnian and on the edge of advancing platforms during the famennian the modeling shows four diagnostic characteristics 1 tuned trough on peak seismic wavelet character of the advanced famennian reefs 2 distinct trough wavelet character of backstepped frasnian platform underlying the advancing reefs 3 local dips on migrated seismic data that can be correlated with marginal slope facies and 4 pull up on seismic sections beneath the carbonate buildups the most useful of these characteristics is the trough on peak wavelet pattern defining the front of the famennian reefs as the famennian back reef facies is a known hydrocarbon reservoir the trough on peak pattern can provide valuable clues to the famennian reef geometry

palaeoceanography of lower cretaceous alpine platform carbonates shallow water carbonate platforms evolve in an interplay of local regional and global dynamics thereby generating a detailed record of environmental conditions platform sediments constitute important archives of past environmental change and as such they are increasingly analysed for the information they contain on wider scale patterns the integration of this information with palaeoceanographic data obtained from deeper water sediments provides a powerful tool to develop a more diverse and complete image of past environmental change in this contribution the remains of the northern tethyan carbonate platform of early cretaceous age cropping out in the western swiss jura and the helvetic alps are used to illustrate the advantage of a palaeoceanographic approach in their interpretation the tools applied are biostratigraphy and chemostratigraphy carbon and strontium isotopes in order to obtain a precise age model for correlation with hemi pelagic sediments in adjacent basins sequence stratigraphy as a means to reconstruct the impact of sea level change phosphorus stratigraphy as a proxy for trophic levels facies and microfacies analyses as a gateway to gain better insight into the carbonate factory and the detailed analysis of platform drowning unconformities as an indicator of wider scale palaeoenvironmental change it is shown that changes from periods of predominantly photozoan to predominantly heterozoan carbonate production near the berriasian valanginian boundary and middle early aptian coincided with substantial increases in phosphorus burial rates and major carbon isotope excursions furthermore the majority of the platform drowning episodes occurred just prior to and during episodes of major palaeoenvironmental change and last but not least the platform itself may have influenced the chemistry of adjacent basins and the carbon isotope composition of associated carbonates by the export of periplatform ooze

influence of antarctic ice sheet lowering on the southern hemisphere climate modeling experiments mimicking the mid miocene a coupled global atmosphere ocean model is used to study the influence of the antarctica ice sheet in a configuration that mimics that of the early miocene on the atmospheric and oceanic circulations based on different climate simulations of the present day ctr and conducted with distinct antarctic ice sheet topography ais exp it is found that the reduction of the antarctic ice sheet topography ais induces warming of the southern hemisphere and reduces the meridional thermal gradient consequently the atmospheric transient low level eddy heat flux overline v prime t prime and the eddy momentum flux overline u prime v prime are reduced causing the reduced transport of heat from the mid latitudes to the pole the stationary flow and transient wave anomalies generate changes in the ssts which modify the rate of deep water formation strengthening the formation of the antarctic bottom water substantial changes are predicted to occur in the atmospheric and oceanic heat transport and a comparison between the total heat transport of the atmosphere ocean system as simulated by the ais exp and the ctr runs shows that the reduction of the ais height leads to reduced southern hemisphere poleward and increased equatorward heat transport these results are in agreement with reduced storm track activities and baroclinicity

the influence of changing orbital parameters and surface boundary conditions on climate simulations for the past 18 000 years abstract general circulation model experiments at 3000 year intervals for the past 18 000 years were made to estimate the magnitude timing and pattern of the climatic response to prescribed changes of orbital parameters date of perihelion axial tilt eccentricity and glacial age lower boundary conditions ice sheets land albedo sea ice and sea surface temperature the experiments used the community climate model ccm of the national center for atmospheric research ncar the response of monsoon circulations and tropical precipitation to the orbitally produced solar radiation changes was much larger than the response to changes of glacial age boundary conditions the continental interior of eurasia was 2 4 k warmer in summer and summer monsoon precipitation of north africa south asia was increased by 10 20 between 12 000 and 6000 yr bp before present when perihelion occurred during northern summer rather than in winter as now and the earth s axial tilt was larger than now southern hemisphe

influence of glacial ice sheets on the atlantic meridional overturning circulation through surface wind change coupled modeling studies have recently shown that the existence of the glacial ice sheets intensifies the atlantic meridional overturning circulation amoc however most models show a strong amoc in their simulations of the last glacial maximum lgm which is biased compared to reconstructions that indicate both a weaker and stronger amoc during the lgm therefore a detailed investigation of the mechanism behind this intensification of the amoc is important for a better understanding of the glacial climate and the lgm amoc here various numerical simulations are conducted to focus on the effect of wind changes due to glacial ice sheets on the amoc and the crucial region where the wind modifies the amoc first from atmospheric general circulation model experiments the effect of glacial ice sheets on the surface wind is evaluated second from ocean general circulation model experiments the influence of the wind stress change on the amoc is evaluated by applying wind stress anomalies regionally or at different magnitudes as a boundary condition these experiments demonstrate that glacial ice sheets intensify the amoc through an increase in the wind stress at the north atlantic mid latitudes which is induced by the north american ice sheet this intensification of the amoc is caused by the increased oceanic horizontal and vertical transport of salt while the change in sea ice transport has an opposite though minor effect experiments further show that the eurasian ice sheet intensifies the amoc by directly affecting the deep water formation in the norwegian sea

controls and predictability of carbonate facies architecture in a lower jurassic three dimensional barrier shoal complex djebel bou dahar high atlas morocco spatial information on lithofacies from outcrops is paramount for understanding the internal dynamics external controls and degree of predictability of the facies architecture of shallow water carbonate platform tops to quantify the spatial distribution and vertical stacking of lithofacies within an outer platform shoal barrier complex integrated facies analysis and digital field technologies have been applied to a high relief carbonate platform exposed in the djebel bou dahar lower jurassic high atlas morocco the outer platform is characterized by subtidal cross bedded coarse grainstone to rudstone grading into supratidal pisoidal packstone rudstone with tepees that together formed a 350 to 420 m wide shoal barrier belt parallel to the margin this belt acted as a topographic high separating a restricted lagoon from the subtidal open marine region low energy tidal flats developed on the protected flank of the barrier facing the lagoon lithofacies patterns were captured quantitatively from outcrop and integrated into a digital outcrop model the outcrop model enabled rapid visualization of field data and efficient extraction of quantitative data such as widths of facies belts in addition the spatial heterogeneity was captured in multiple time slices i e during different phases of cyclic base level fluctuations in general the lateral continuity of lithofacies is highest when relative water depth increased during flooding of the platform top establishing low energy subtidal conditions across the whole platform and when the accommodation space was filled with tidal flat facies heterogeneity increased during deposition of the relief building bar facies that promoted spatial diversification of depositional environments during the initial phases of accommodation space creation cycles commonly are composed of a thin transgressive tidal flat unit followed by coated grain rudstone bar facies lateral to the bar facies pisoidal grainstone beach deposits accumulated these bar and beach deposits were overlain by subtidal lagoonal facies or would grow through the maximum flooding and highstand there the bars either graded into supratidal pisoidal facies with tepees when accommodation space was filled or were capped by subaerial exposure due to a sea level fall modified embedded markov analysis was used to test the presence of common ordering in vertical lithofacies stacking in a stationary interval constant depositional mode analysis of individual sections did not reveal any ordering which may be related to the limited thickness of these sections composite sections however rejected the null hypothesis of randomness the addition of stratigraphically significant information to the markov analysis such as exposure surfaces and lateral dimensions of facies bodies strengthens the verdict of unambiguous preferential ordering through careful quantitative reconstruction of stratal geometry and facies relationships in fully integrated digital outcrop models accurate depositional models could be established that enhanced the predictability of carbonate sediment accumulation 2009 the authors journal compilation 2009 international association of sedimentologists

the global climate response to lowering surface orography of antarctica and the importance of atmosphere ocean coupling abstracta global climate model is used to study the effect of flattening the orography of the antarctic ice sheet on climate a general result is that the antarctic continent and the atmosphere aloft warm while there is modest cooling globally the large local warming over antarctica leads to increased outgoing longwave radiation which drives anomalous southward energy transport toward the continent and cooling elsewhere atmosphere and ocean both anomalously transport energy southward in the southern hemisphere near antarctica poleward energy and momentum transport by baroclinic eddies strengthens anomalous southward cross equatorial energy transport is associated with a northward shift in the intertropical convergence zone in the ocean anomalous southward energy transport arises from a slowdown of the upper cell of the oceanic meridional overturning circulation and a weakening of the horizontal ocean gyres causing sea ice in the northern hemisphere to expand and the arctic to cool comparison wi

seismic modeling of an early jurassic drowned carbonate platform djebel bou dahar high atlas morocco 1 along the southern margin of the high atlas of morocco early jurassic carbonate platforms developed in a transtensional rifted basin one of these platforms was the djebel bou dahar platform which grew to a vertical relief of 500 m drowned and was onlapped by deep marine sediments exceptional outcrops allowed detailed lithologic models of the platform to be made with a resolution of a few meters we constructed a geometric model from building blocks comprising particular rock types we estimated p wave velocities and bulk densities of these lithologies from hand specimens and we assumed these properties to be homogeneous and invariant throughout the building blocks we constructed time sections of reflectivity from vertical incidence and image ray migration techniques these time sections were convolved with 25 to 200 hz ricker wavelets to produce synthetic seismic sections large scale features such as the platform and the drowning unconformity were resolved regardless of the frequencies and modeling techniques used broad low frequency wavelets resulted in profiles with apparent truncation of bedding and basinward shifts in the point of onlap smaller scale features such as reef knolls and sand bodies at the platform margin were poorly resolved at frequencies up to 50 hz however we inferred the existence and location of these smaller features from the disturbance and distortion of other reflections this distortion is most obvious in image ray migration modeling because of the refraction of rays through the small scale features geometric effects caused lateral amplitude variations due to wavelet interference along the unconformity reflection where no petrophysical changes were modeled the seismic models of the djebel bou dahar in its present stage may be good analogs for seismic lines of deeply buried carbonate platforms to make a comparison with real seismic lines of less deeply buried platforms possible we reconstructed both the geometry and petrophysical properties and the subsequent seismic modeling of the djebel bou dahar carbonate platform as it was prior to the onset of deep burial at this stage impedance contrasts were at their maximum this study clearly demonstrates the significance of modeling the seismic response of a detailed outcrop based stratigraphic model and shows that seismic modeling of specific sequence stratigraphy can reveal what features are likely to be resolved under ideal conditions and what pitfalls may await the interpreter

controls on sonic velocity in carbonates compressional and shear wave velocities v p and v s of 210 minicores of carbonates from different areas and ages were measured under variable confining and pore fluid pressures the lithologies of the samples range from unconsolidated carbonate mud to completely lithified limestones the velocity measurements enable us to relate velocity variations in carbonates to factors such as mineralogy porosity pore types and density and to quantify the velocity effects of compaction and other diagenetic alterations

hysteresis of glaciations in the permo carboniferous the hysteresis of glaciations in the permo carboniferous is simulated in an iterative energy balance model with 84 scenarios of varied orbital elements co2 levels and glacial modes gondwana attains its glacial maximum when co2 is roughly the same or slightly higher than the preindustrial level once glaciated it maintains a large ice covered area under higher co2 levels up to 4 to 6 times of the preindustrial value followed by an abrupt disappearance of ice beyond this threshold the solutions arrived at by iteration indicate the existence of parameter thresholds of glaciations and deglaciations which lead to the alternating extremes of glacial coverage dominated by the large seasonal cycle over land the concentrated geography of the permo carboniferous is more sensitive to the co2 forced climate than sparsely distributed land our conceptual modeling agrees well with the recent geological evidence of intercalations of three glaciations and two lacunae of deglaciations in this geologic time

seismic modelling of outcrop carbonate systems an application to the cretaceous platform to basin system of the maiella mountain central apennines italy

tertiary oxygen isotope synthesis sea level history and continental margin erosion tertiary benthic and planktonic foraminiferal oxygen isotope records are correlated to a standard geomagnetic polarity time scale making use of improved chronostratigraphic control and additional oligocene isotope data synchronous changes in both benthic and planktonic 18o values which occurred in the oligocene to miocene 36 5 2 ma are interpreted in part to represent ice growth and decay the inferred ice growth events correlate with erosion on passive continental margins as interpreted from seismic and chronostratigraphic records this association is consistent with a link between oligocene to miocene erosional events and rapid 15 m m y glacioeustatic lowerings of about 50 m high benthic foraminiferal 18o values suggest the presence of continental ice sheets during much of the oligocene to recent 36 0 ma substantially ice free conditions probably existed throughout the paleocene and eocene 66 36 ma the mechanisms and rates of sea level change apparently were different between the early and late tertiary with glacioeustatic changes restricted to the past 36 m y pre oligocene erosion on passive continental margins was caused by eustatic lowerings resulting from global spreading rate changes we apply a model which suggests that large areas of the continental shelves were subaerially exposed during such tectonoeustatic lowstands stimulating slope failure and submarine erosion the different mechanisms and rates of eustatic change may have caused contrasting erosional patterns between the early and late tertiary on passive continental margins this speculation needs to be confirmed by examination of data from several passive margins

influence of the antarctic ice sheet on southern high latitude climate during the cenozoic albedo vs topography effect sensitivity of high latitude temperature distribution to the variation of the antarctic ice sheet is investigated using an atmospheric general circulation model coupled with a mixed layer ocean results suggest that cooling effect of the ice sheet is not negligible compared to the effect of atmospheric co2 decrease such as during the cenozoic cooling effect of high surface albedo of the ice sheet is enhanced over the ice sheet and slightly suppressed over the circumpolar ocean by the high topography of the ice sheet high surface albedo and high topography of the ice sheet each causes increase and decrease of southward atmospheric heat transport respectively whose contribution to maintaining the heat balance anomaly of atmosphere ocean system is comparable to that of local adjustment of radiation balance

a 170 year sr ca and ba ca coral record from the western pacific warm pool 1 what can we learn from an unusual coral record 1 a long lived porites coral from new ireland 2 5 s 150 5 e papua new guinea has been analyzed for sr ca and ba ca by laser ablation inductively coupled plasma mass spectrometry la icpms the top 50 years of the core are examined here and compared with instrumental sea surface temperatures ssts for the small range of sst in the warm pool sr ca shows an unusually large response the best estimate for the slope of a temperature calibration is 0 23 0 03 mmol mol per c this strong slope is partly a result of the high resolution sampling but is also due to additional biological mediation of the sr ca ratios which appears to be associated with seasonal changes of calcification rate and limits the prediction skills of this proxy thermometer high density bands synchronous with high sr ca ratios suggest that calcification may be enhanced by beneficial small amounts of nutrients available during el nino transient ba enrichment synchronous with colder than average sst is also observed during el nino and attributed to the shoaling of the new ireland coastal undercurrent the high level of enrichment in particular during upwelling triggered by westerly wind bursts suggests an origin from both dissolved seawater and marine biological barium

sensitivity of a general circulation model to changes in northern hemisphere ice sheets sensitivity experiments with a general circulation model demonstrate the role of ice sheet size on the local regional and global climate model experiments isolate the effects of albedo height and area of the ice sheets and show how the national center for atmospheric research community climate model 1 responds to changes in the size of northern hemisphere ice sheets a flat ice sheet with full glacial areal extent but no elevation is used to study albedo effects a full ice sheet with full glacial areal extent and elevation is used to represent height effects an ice sheet with half the glacial area of the others but the full glacial elevation is used to represent area effects all of the sensitivity experiments have 1 interactive sea surface temperatures calculated by a slab ocean and 2 modern boundary conditions except for the ice sheets the experiments show that both the full and flat ice sheets lower the global mean surface temperatures gmt by 2 5 c and that the gmt is dependent upon the area rather than the height of the ice sheets high ice sheets maintain colder temperatures than lower ice sheets over the ice sheets themselves but compensating warmer temperatures occur downstream from the high ice sheets the downstream warmer temperatures are the result of 1 glacial anticyclones that cause subsidence and reduced cloud cover during summer as well as reduced soil moisture and 2 increased southwesterly flow across the atlantic ocean that results in increased southerly advection of warm air during winter a dynamical effect of the high ice sheets during summer is to change the wave number of the planetary waves in the midlatitudes whereas a thermodynamic effect of the flat ice sheets during summer is to lower the geopotential heights throughout the northern hemisphere in general northern hemisphere ice sheets induce both a local response over the ice sheets and a regional response downstream from the ice sheets but have little impact on the southern hemisphere except where sea ice expands

role of large particles in the transport of elements and organic compounds through the oceanic water column abstract during the past decade data from a variety of sources have been obtained which show conclusively that the relatively rare large particles sinking through the water column are responsible for the majority of the downward vertical mass flux in the sea this finding has important implications for understanding the transfer distribution and fate of elements and organic compounds in marine waters the large 100 m detrital particles responsible for vertical flux are primarily biogenic and range in size and composition from small discrete fecal pellets and plankton hard parts to large amorphous aggregates or snow which contain both organic and inorganic constituents depending on size shape and density these particles sink at rates ranging from 1000 m day 1 several methods have been developed for sampling these particles of which in situ sediment trapping has probably furnished the most comprehensive qualitative and quantitative information on the role large particles play in material transport flux studies have highlighted the importance of marine heterotrophs in packaging fine suspended particulate matter into large rapidly sinking particles which accelerate the movement of incorporated materials to depth large particle production via biological packaging is not restricted to the euphotic zone but can occur at all depths and information is now accruing on rates of production of large particles in the water column chemical analyses of sedimenting particles collected in sediment traps and those sampled by other means have allowed quantifying vertical fluxes and residence times of elements radionuclides and organic compounds natural and anthropogenic in various oceanic regimes pertinent studies dealing with the above aspects are reviewed and several areas for future research are suggested

evolution of a high relief carbonate platform slope using 3d digital outcrop models lower jurassic djebel bou dahar high atlas morocco abstract seismic scale continuous exposures of an early jurassic carbonate platform located in the southern high atlas of morocco provide detailed quantitative information about the lithofacies and stratal geometries of an aggrading retrograding and prograding steeply inclined slope system that evolved in an active intracratonic rift basin aggradational backstepping and progradational outer platform to slope transects are each characterized by distinct lithological features and stratal patterns a digital outcrop model was constructed using real time kinematic global positioning system and lidar storing information on recorded stratal surfaces including lithofacies information from these data 3d models of the slope system could be built in the study area four stages of slope development are recognized stage 1 corresponds to a low relief carbonate platform characterized by widespread sub wave base depositional conditions stage 2 developed as an aggrading to retrograding platform during which the platform built considerable relief in addition a massive organically bound upper slope fringe formed dominated by coral sponge microbial boundstone the in situ boundstone consisted of irregular wedges that accreted on the upper slope down to 140 m below the platform break after a period of sediment starvation stage 3 involved a major backstepping of sediment deposition seventy meter high clinoforms prograded from the center of the platform across the platform top until reaching the relict platform break of stage 2 subsequently progradation occurred basinward as indicated by the spilling of lobes down the existing slope deposits of stage 2 limited progradation along with the reestablishment of the in situ coral sponge microbial boundstone on the upper slope led to the progressive steepening of the slope profile to dips of 23 clinoforms are planar in the upper part of the slope and concave upwards in the lower part with a total relief of over 460 m stage 4 is observed only in the lower slope and adjacent basin coarse detrital carbonate deposits form lens shaped aprons that grade basinward into kilometer scale lobes that alternate with thick wedges of basinal fallout sediments syndepositional extensional faults operating in the subsurface affected sedimentation along the lower slope profile throughout the four stages and are expressed in the stratal patterns through growth strata in monoclinal folds three dimensional digital outcrop models reconstructing the architecture of the slope system allowed quantification of slope geometries and stratal relationships and spatial distribution of sedimentary bodies the digital model enhanced the understanding of the depositional changes during slope evolution and also led to observations on for example angular relationships that would have been difficult to detect without the digital approach the evolution from aggrading to retrograding prograding and retrograding was strongly influenced by synsedimentary tectonics within an intracratonic rift basin the tectonics generated irregular distribution of accommodation space variably interfering with eustatic sea level movement this in combination with the character of lithofacies types and style of sedimentary processes along the slope such as in situ boundstone growth and gravity driven resedimentation processes directly controlled carbonate slope architecture and stratigraphy of the djebel bou dahar and influenced stratal anatomy and lithofacies distribution

three dimensional geological and synthetic seismic model of early permian redeposited basinal carbonate deposits victorio canyon west texas the lower permian outcrops of victorio canyon in the sierra diablo mountains in west texas show undisturbed stratigraphy of carbonate toe of slope and basinal deposits these rocks consist of a vertical stack of carbonate debris flow deposits and hyperconcentrated density flow deposits thick skeletal ooid grainstone deposited as basin floor fans various reworked muddy carbonate deposits and fine siliciclastic siltstones this succession was deposited within five sequences that include the upper hueco formation late wolfcampian through the bone spring formation middle leonardian using a ground based light detecting and ranging generated high resolution digital outcrop model dom aged as a template we mapped and digitized the stratigraphy of the toe of slope and basinal deposits on the 5 cm 2 in precision dom on the basis of the digitized stratigraphic contacts several regional surfaces were constructed and a 3 d geocellular model was built facies information within this model is extrapolated from measured section data using both a kriging algorithm and stochastic simulations using impedance values extracted from a subsurface analog a 3 d impedance model was created for both the kriged and the stochastic models both models incorporate fine scale stratigraphic architecture in addition the stochastic impedance model incorporates spatially correlated noise resulting in more realistic synthetic seismograms three dimensional synthetic seismograms were calculated at 20 40 and 80 hz the reservoir prone facies is skeletal ooid grainstone deposited as a 1 5 km 750 m 0 93 mi 2460 ft basin floor fan up to 15 m 49 ft thick this basin floor fan is subtly imaged in vertical seismic section at a frequency below 80 hz it is however better recognized on time slices with peak frequency as low as 20 hz and even better delineated on horizon slices that parallel the stratigraphy

an early middle eocene antarctic summer monsoon evidence of fossil climates abstract a warmer and mostly ice free south polar region prevailed during the early middle eocene indicative of a low latitudinal temperature gradient climatic models mostly fail to reconstruct such a low gradient demonstrating our poor understanding of the mechanisms involved in heat transfer here we describe a new phenomenon that shaped the southern high latitude climate during the early middle eocene the antarctic summer monsoon our palaeoclimatic reconstruction is based on 25 morphotypes of fossil dicotyledonous leaves from the early middle eocene fossil leaf assemblage of fossil hill from king george island the antarctic peninsula we use a novel clamp climate leaf analysis multivariate program calibration which includes new climatic parameters that allow us to characterise better the seasonality in precipitation our reconstruction indicates a warm humid temperate climate with strong seasonality in temperature and precipitation seasonality in precipitation indicates a rainfall rate of 6 4 1 30 mm day during summer summer daily rate of precipitation sdr and a summer precipitation representing more than 60 3 8 28 of annual rainfall ratio of summer precipitation rsp which fulfils the definition of a summer monsoon in the modern world this implies a seasonal alternation of high and low pressure systems over antarctica during the early middle eocene such a climate regime would have impacted upon global atmospheric circulation and heat transfer this climatic regime presents a challenge for climatic models and their ability to reconstruct accurately palaeoclimates at high southern latitudes and thereby understand latitudinal heat transfer in a greenhouse earth regime

accommodation and supply a dual control on stratigraphic sequences abstract it is widely accepted that both eustatic and tectonically controlled regional changes of sea level have contributed to the record of stratigraphic sequences i suggest that environmental change be added as a third autonomous control sedimentologic principles clearly indicate that sequences and their systems tracts are controlled by the interplay of two rates the rate of change in accommodation space available for sedimentation and the rate of sediment supply sea level has direct control on accommodation but its influence on sediment supply is remote and easily overshadowed by environmental factors for instance the record of the most recent sea level rise is a transgressive systems tract where supply is low it is a prograding highstand systems tract in deltas where the supply is high examples of sequence boundaries generated by changes in sediment supply include tectonically driven shifts in sediment input into basins changes in ocean currents pulsating supply from failure of submarine slopes and drowning of carbonate platforms by environmental stress furthermore the stratigraphic sequences in fluviatile continental basins are physically removed from sea level induced changes in accommodation and must have formed by changes in the rate and pattern of supply subaerial exposure of marine sediments at the sequence boundary is a most important criterion for recognizing sea level cycles as opposed to supply cycles other criteria include downstepping of shelf breaks and characteristic patterns in the spacing of time lines within sequences some third order cycles ca 0 5 3 ma duration meet these criteria others do not cycle stacking patterns and the shifting facies belts on cratons indicate that many second and third order cycles lack pronounced exposure unconformities and represent gradual changes superimposed on more rapid shorter oscillations seismic data yield poor images of these gradational changes because they lack resolution seismic models of outcrops show that near the limits of resolution the seismic tool tends to portray rapid changes in facies and bed thickness i e lateral variations in supply as unconformities seismic unconformities do not necessarily correspond to unconformities in outcrop the succession of stratigraphic sequences is best considered a composite record of sea level and environmental change separating the various controls requires carefully planned experiments work on the oxygen isotope curve another proxy for sea level has set an example in this respect

facies architecture of an early jurassic carbonate platform slope jbel bou dahar high atlas morocco the plateau of jbel bou dahar high atlas morocco represents an outstanding example of a well preserved early mesozoic carbonate platform field observations of the large scale sedimentary structures and the arrangement of lithofacies associations within the mid and lower slope were combined with detailed microfacies analyses and gamma ray measurements the investigations were carried out in order to 1 resolve the sequence stratigraphic architecture of the slope and 2 reconstruct the sediment export patterns and depositional processes related to high frequency sea level variations the slope strata are arranged into two different types of lithofacies associations that are bounded by characteristic discontinuities and show a specific occurrence along the investigated slope section the steep 8 18 mid slope is dominated by thick to medium bedded limestone successions that were able to maintain a high angle of repose due to their detrital composition the sediment packages are bounded by basal low angle unconformities and wedge out within the lower slope or thin into the adjacent basin as single carbonate layers the sediments are characterized by coarse grained and poorly sorted floatstones to rudstones which show low gamma ray values and contain components derived mainly from the platform edge the facies associations were interpreted as debris flow deposits that were exported during sea level lowstands when the flattened platform top and extended parts of the margin were exposed and sediment production was restricted mainly to the outer margin and upper slope section in contrast the gently dipping lower slope 4 18 is characterized by rhythmic limestone marl alternations that show increasing thicknesses with increasing distance from the platform from the lower part of the mid slope down the successions onlap the debris flow deposits forming a characteristic interfingering pattern of different lithofacies associations the medium to thin bedded carbonates of the limestone marl alternations comprise two different microfacies types 1 mudstones to wackestones with high gamma ray values and a mixed shallow marine to deep marine microfacies overlain by 2 well sorted arenitic packstones to grainstones with low gamma ray values and a shallow marine component composition the latter points to the platform interior and margin as primary source areas the shallow marine material was exported during sea level highstands when the carbonate factory included the entire platform margin and the flooded lagoons of the platform top during these periods the exported fine grained sediments could maintain only low angles of repose thus they bypassed the steep upper and mid slope sections developed into turbidites during downslope transport and were finally redeposited on the gently dipping lower slope and in the adjacent basin the architecture of the investigated slope area shows a characteristic interfingering of different facies associations that can be interpreted as genetic highstand and lowstand stratigraphic sequences the arrangement of these sequences along the slope as well as their composition were controlled by high frequency sea level fluctuations affecting the primary sediment production source area export patterns and sedimentary processes resulting in a shift of the depositional centers during different sea level stands

egyptian tethyan margin in the mesozoic evolution of a mixed carbonate siliciclastic shelf edge from western desert to sinai abstract during the mesozoic the egyptian margin was an oblique margin with nw se oriented transform fault system in the eastern mediterranean basin mesozoic margins were characterized by mixed carbonate siliciclastics platforms where subsidence and eustasy were the main parameters controlling the facies distribution and geometries of the platform to basin transition geometries and facies on the platform slope basin profile are today well constrained on the levant side but are still poorly known on the egyptian side geometries and stratigraphic architecture of the egyptian margin have been mapped thanks to a regional seismic and well data base provided by a joint industrial academic group upmc total this study brings new understandings on the seismo stratigraphic architecture of the platform slope basin system in a key area from western desert to nile delta and the levant margin mapping of the top jurassic and top cretaceous horizons shows the seismic geomorphology of the margin and the hinge line from western desert to sinai during the jurassic carbonate platforms were prograding with a distal thickening of the external platform with gentle slope profiles paleo valleys and embayments since the cretaceous the margin shows an aggrading and retrograding mixed carbonate siliciclastic platform succession with abrupt nw se trending paleo shelf edges affected by large gravity slide scars and distally steepened segments this structure of the platform edges is strongly controlled by the inherited tethyan transform fault directions along the hinge line embayments are interpreted as megaslides the basin filling is characterized by an alternation of chaotic seismic facies and high amplitude reflectors onlapping the paleoslopes mass transport complex mtc deposits are characterized by the mobilization of thick sedimentary sections up to 3500 m in thickness as a mixed amalgamation of debris flows internally preserved blocks and or compressively deformed distal allochthonous masses transported materials have been sourced from the dismantling of the mesozoic mixed carbonate siliciclastic platform and they can spread down slope over areas as large as 70 000 km 2 based on stratigraphic correlations with global sea level changes platform instability could have been triggered by the gravitational collapse of the carbonate siliciclastic platform under its own weight after successive subaerial exposures periods which were able to enable karstification processes

sequence development in an isolated carbonate platform lower jurassic djebel bou dahar high atlas morocco influence of tectonics eustacy and carbonate production six sequences each with a distinct shape stratal anatomy bounding surfaces lithofacies composition and stacking patterns were identified in the djebel bou dahar isolated carbonate platform which developed in a half graben in the high atlas marine rift basin during the early jurassic sequences are believed to have a complex multi process origin as the result of tectonic processes regional subsidence and fault block rotation overprinting the eustatic signal in addition changing carbonate producers with shifting centres of deposition and relative accumulation potential influenced the filling and anatomy of the accommodation space that had been created by the hybrid tectonic and eustatic processes sequences i and ii are tabular in shape and interpreted to primarily reflect the combined processes of uniform tectonic subsidence and eustacy which resulted in a ramp to shelf carbonate system with lagoon and tidal flat lithofacies onset of a rifting episode around the base of the lotharingian and fault block rotation were responsible for the development of the angular unconformity with the underlying sequence ii and the progressive development of a shallow water isolated platform and the wedge shaped depositional sequences iii to vi during the lotharingian and pliensbachian concurrent variations in palaeoceanographic conditions and in the carbonate factory caused a shift towards a flat topped platform geometry with high energy coated grains and sub photic sponge microbial deposition around the margin the aggradational to retrogradational character of sequences iii iv and vi is inferred to be a result of the combined effect of in situ carbonate production coupled with high rates of creation of accommodation space in the hanging wall environmental effects on carbonate production were the most probable cause for drowning of the djebel bou dahar platform during the toarcian but also influenced the filling and geometry of sequences iv to vi this study suggests that the commonly accepted model of pulse driven sequence development in half grabens may not be fully applicable for the djebel bou dahar where a more continuous rotation at sequence scale associated with subsidence pulses of the djebel bou dahar block seems to have interacted with eustacy and carbonate production to generate sequences continuous rotation is suggested by the presence of regular patterns of diverging strata and numerous minor exposure surfaces in lagoonal deposits of sequences v and vi in addition this study demonstrates that eustacy and variations in the carbonate factory might be as significant as tectonics in sequence development and platform drowning in carbonate platforms within rift basins this observation could imply that the effects of eustacy and the carbonate factory might not have been correctly recognized in other similar carbonate platforms due to a tectonic biased interpretation

vertical resolution of thick beds thin beds and thin bed cyclothems resolution has two common definitions the first of these relates to determination of the position of a reflector in time or space resolution by this definition is inversely related to pulse width or sharpness of the wavelet and potentially directly related to bandwidth of the frequency spectrum the second definition of resolution relates to determination of the spacing of close features such as thin beds this type of resolution is determined by the rayleigh criterion which inversely relates resolution to wavelength resolution of thin beds improves with shorter wavelength or higher frequency in comparison to the rayleigh criterion the ricker and widess criteria improve the resolution potential of thin beds but they rely on models of the reflectors and require that the beds be isolated it is a long established and widely accepted axiom that the best wavelets contain bandwidths of at least a couple of octaves this results in wavelets with reduced side lobes and ringiness a more easily interpret

climate sensitivity to continental ice sheet size and configuration abstract a version of the ncar community climate model ccm has been used to carry out a study of climate sensitivity to the size and distribution of continental ice sheets by comparing two perpetual season fixed sea surface temperature sst winter simulations of the climate at a glacial maximum the two simulations differ only in the size and configuration of the continental ice sheet boundary conditions these conditions represent minimum and maximum ice sheet extent for a glacial maximum as presently constrained by the geologic evidence all other boundary conditions are as specified by climap 1981 for winter surface conditions during the last glacial maximum lgm the ice sheets specified by climap 1981 used by many previous researchers to simulate ice age climate conditions are intermediate in size between the minimum and maximum ice sheets used for the simulations described here the maximum case simulation resulted in unexpected positive temperature anomalies over the northern hemisphere

climate and ice sheet mass balance at the last glacial maximum from the genesis version 2 global climate model abstract at the last glacial maximum lgm about 21 000 calendar years ago 21 ka bp the overall mass balance of the laurentide and eurasian ice sheets should have been close to zero since their rate of change of total ice volume was approximately zero at that time the surface mass balance should have been zero or positive to balance any iceberg iceshelf discharge and basal melting but could not have been strongly negative in principle this can be tested by global climate model cgm simulations with prescribed ice sheet extents and topography we describe results from a suite of 21 ka bp simulations using a new global climate model genesis version 2 0 a with ssts prescribed from climap 1981 and predicted by a mixed layer ocean model and with ice sheets prescribed from clih the ice 4g peltier 1994 and climap 1981 reconstructions in common with previous gcm simulations using mixed layer oceans substantial cooling over and around antarctica occurs due to normal gcm dynamics and polar sea ice feedback without recourse to changes in thermohaline circulation we find slightly enhanced cooling and cloudiness over low latitude land masses compared to over ocean especially over the andes which decreases the disagreement with land based data and suggests that tropical climap ssts may only be 1 2 c too warm the gcm used here is well suited for ice sheet mass balance studies because i the surface can be represented at a finer resolution than the atmospheric gcm ii an elevation correction accounts for spectral distortions of the atmospheric gcm topography iii a simple post processing correction for refreezing of meltwater is applied and iv the model s precipitation and mass balances for present day greenland and antarctica are realistic however for all reasonable combinations of ssts and ice sheet configurations the predicted annual surface mass balances of the lgm laurentide and eurasian ice sheets are implausibly negative possible reasons for this discrepancy are discussed including increased ice age aerosols higher climap like ice sheet profiles in the few thousand years preceding lgm and a surge of the southern laurentide just before lgm to fleetingly produce the ice 4g profile at 21 ka bp

marine snow latitudinal distribution in the equatorial pacific along 180 1 marine snow ms distribution from the surface to 1000 m depth was determined in the equatorial pacific using the underwater video profiler during the etude du broutage en zone equatoriale cruise in fall 1996 the latitudinal transect was carried out at 17 stations along the 180 meridian from 8 s to 8 n during a cold phase of el nino southern oscillation higher ms concentrations were found below the equatorial zone than poleward at the equator the estimated integrated ms carbon m 2 in the upper kilometer was 5 7 g m 2 while both southward and northward between 1 and 8 the mean integrated ms carbon was about 2 7 g m 2 in the upper 50 m the ms carbon was twofold lower than the combined carbon of autotrophic and heterotrophic protists and four times lower than the mesozooplankton carbon biomass both measured concurrently during the cruise different water bodies had different ms content the highest concentrations were found in the south equatorial current the south equatorial counter current and the north equatorial countercurrent tropical waters at the south in the south subsurface countercurrents and the warm northern superficial waters had the lowest ms biomass mechanistically a latitudinal conveyor belt a poleward divergence of upwelled waters that return to the equator after being downwelled at north and south convergent zones may partially explain the vertical distribution of particulate matter observed during the studied period

pore networks in holocene carbonate sediments abstract permeabilities of holocene carbonate sediments from florida and the bahamas determined for 74 samples by water flow rates in a variable head permeameter range from 0 6 to 57 000 millidarcies porosities range from 40 to 78 percent these parameters are related to depositional texture as follows table the muddiest sediments i e the finest grained have the highest porosities but lowest permeabilities this negative correlation between porosity and permeability is the reverse of the situation found in carbonate rocks even as young as pleistocene high porosity and low permeability in sediments show a strong correlation with percentage of fines 62 m from capillary pressure curves it is inferred that many of the pore throats of muddy carbonate sediments are less than 1 m in diameter at least after drying measured permeabilities are used to calculate cementation rates for simple models of upper phreatic zone cementation the calculated rates would require excessive time to produce the degree of cementation seen in late pleistocene rocks of florida and the bahamas the amounts of rainfall and evaporation are the most important factors in the degree of cementation

impact of the north american ice sheet orography on the last glacial maximum eddies and snowfall the present work evaluates the influence of the north american ice sheet orography on the last glacial maximum lgm 21 000 years ago atmospheric circulation and snowfall in the northern mid latitudes focusing on the north atlantic sector three atmospheric general circulation model experiments are analysed a control and an lgm climate simulation and an lgm run with flat ice sheets over northern north america this ice sheet orography affects lee cyclogenesis over north america and forces differences in stationary waves and therefore in the baroclinicity of the mean flow as a result the atlantic storm track is reinforced in the flat ice sheet experiment compared to the lgm one this in turn has a profound impact on snowfall over northern europe implying a coupling between the two ice sheets

response of thermohaline circulation and thermal structure to removal of ice sheets and high atmospheric co2 concentration 1 the thermohaline circulation thc response to ice sheet removal and quadrupling of atmospheric co2 in a coupled model and the equilibrium thermal structure are examined after thc weakening diffusion of heat and salt to the northern north atlantic at deep layers increases the temperature and salinity there in response to co2 quadrupling resulting convective instability induces the exchange of warmer saltier water in deep layers and cooler fresher water near the surface this contributes to a gradual increase in the thc intensity culminating in its complete and rapid recovery due to positive haline feedback overcoming negative thermal feedback on the thc removal of ice sheets prolongs the overall recovery and strengthens the final thc due to precipitation changes over the northern north atlantic and labrador sea bottom water and high latitude sea surface temperatures are higher without ice sheets leading to a smaller meridional temperature gradient as indicated by cenozoic reconstructions

recent advances in observing the physical oceanography of the western mediterranean sea the mediterranean sea has been investigated intensively since the early nineties using modern techniques and collaborative approaches this overview summarizes some of the resulting advances that were made concerning the physical oceanography of the western mediterranean the water mass formation processes are now much better understood and have been quantified to a large extent the boundary conditions of the system in terms of surface fluxes and strait transports can be determined with improved accuracy thus enabling future investigation of interannual variability the dynamics of the surface and intermediate layers have revealed a variety of eddy and mesoscale processes that are important for the circulation and spreading of water masses the deep circulation is being investigated with lagrangian techniques tracers and floats first results show a large component of the deep water originating from the tyrrhenian sea and intense cyclonic and anticyclonic eddy flows

acoustic properties of carbonates effects of rock texture and implications for fluid substitution more than 250 plugs from outcrops and three nearby boreholes in an undisturbed reef of miocene tortonian age were quantitatively analyzed for texture mineralogy and acoustic properties we measured the p and s waves of carbonate rocks under dry humidified and brine saturated conditions at 10 mpa effective pressure with an ultrasonic pulse transmission technique 1 mhz the data set was compared with an extensive database of petrophysical measurements of a variety of rock types encountered in carbonate sedimentary sequences two major textural groups were distinguished on the basis of trends in plots of compressional wave velocity versus poisson s ratio a specific ratio of p wave over s wave velocity in granular rocks the framework of depositional grains is the main medium for acoustic wave propagation in crystalline rocks this medium is provided by a framework of interlocking crystals formed during diagenesis rock textures are connected to primary depositionalparameters and a diagenetic ov

climate at the equilibrium line of glaciers the relationships between temperature precipitation and radiation on glacier equilibrium lines are investigated using 70 glaciers for which the mass balance and meteorological observations have been carried out for sufficiently long periods it is found that the characteristic climate at glacier equilibrium lines can be described using the summer 3 months temperature in a free atmosphere annual total precipitation and the sum of global and long wave net radiation all of these are measured at or very near the equilibrium line altitudes then it is shown how the shift of the equilibriumline will occur as a result of a climatic change finally the effect of the shift of the equilibrium line on the annualmean specific mass balance is analytically derived and compared with observations the present results make it possible to identify the altitudes in climate models where glacierization should begin and to evaluate the mass balance changes as a result of possible future changes in the climate

particle dynamics as controlled by the flow field of the eastern equatorial pacific abstract profiles made during the jgofs egpac october 1992 time series cruise to the equator with a large aggregate profiling system laps recorded the concentration and size distribution of particles in the marine snow size range 0 5 mm diameter profiles were made routinely at local midnight during the twenty day occupation of the time series station on the equator the laps data set when combined with the ctd transmissometer data set from the egpac program s intensive profiling operation reveals a complex dynamic of particle production and aggregation driven by the equatorial flow field during the cruise the influence of the passage of a tropical instability wave tiw was observed in the transmissometer aggregate and temperature salinity data sets a peak in particle production rate occurred in conjunction with the maximum shoaling of the thermocline and a minimum in the aggregate volume concentration subsequently the particle load in the surface water increased followed by an increase in the aggregate abundance quasi oligotrophic conditions on the equator subsurface particle and aggregate maxima and decreased particle production rates toward the end of the time series corresponded to the appearance of stratified low salinity water characteristic of more northerly conditions the variations in temperature and salinity fields are ascribed to the passage of a tiw in sequence the trailing edge the northwestward flowing cool cusp water and the convergent front current meters at the equator recorded a rotational flow at 80 m with a shift from southeastward to northeastward flow during this sequence a simple conceptual model of meridional flow field is presented in which poleward transport of the upwelled water from the equator returns at shallow depths to mix with the euc the signature of this return flow is reflected in the decrease in the vertical gradients of nutrients and oxygen at the equator with respect to the poleward gradients and a subsurface particle maximum below the euc the flow field s effect on the distribution of particles is reflected in the meridional pattern of the sediment accumulation rate murray and leinen 1996 the return flow of particles to the equator is reflected in the maximum in accumulation rate near the equator the longterm influence of tiw s is reflected in the asymmetry of the accumulation rates across the equator as described by murray and leinen 1996 with the maximum accumulation rate found south of the equator a local minimum in accumulation rate at approximately 2 n and a local maximum near 4 n

in situ settling behavior of marine snow1 the settling velocities of undisturbed macroscopic aggregates known as marine snow were measured with scuba in surface waters off southern california and analyzed as a function of aggregate size mass and density the mean settling velocity was 74f39 m d l for aggregates ranging from 2 4 to 75 mm in maximum length sinking rates in the field varied exponentially with aggregate size and dry weight and were consistently up to four times slower than rates measured in the laboratory the excess densities of the 80 aggregates examined were calculated from volume and dry weight and ranged over four orders of magnitude with a median of 1 4 x 1o 4 g cm 3 aggregates of marine snow sank more slowly than predicted for either solid or porous spheres of equivalent volume and density although their velocities were within the range expected for equivalent sinking prolate ellipsoids no relationships between settling velocity and either excess density or particle shape were found drag coefficients of marine snow were also higher than predicted by theory for spheres of equivalent volume and density these deviations from theoretical expectations may be partially explained by errors in the estimation of the excess densities of aggregates variability in the densities of the heterogeneous primary particles comprising marine snow fecal pellets claymineral particlcs phytoplankton molts etc and the potential for buoyancy regulation by individual phytoplankton cells inhabiting aggregates make determination of excess density especially

seismic model of a progradational carbonate platform picco di vallandro the dolomites northern italy we present a seismic model of a well exposed carbonate platform to basin transition from the triassic succession at picco di vallandro the dolomites northern italy the core of this model is a detailed lithologic cross section that represents 1500 m of stratigraphic section over a distance of 3 5 km the cross section is based on detailed measured sections photo interpretation and careful projection of surface mapping into the plane of section samples from the measured sections provide control on velocity and density distribution and these data were used to generate an impedance model from the cross section the impedance model was convolved with a zero phase 25 hz peak frequency wavelet to produce the seismic model the results illustrate how the platform to basin transition might be imaged by conventional seismic reflection data how the reflections relate to the distribution of lithology and through going geological surfaces and how constructive and destructive interference and seismic resolution combine to produce individual reflections the picco di vallandro section displays several scales of progradation and retrogradation ranging from stacked buildups that are several hundreds of meters thick to the interfingering of individual beds the seismic model displays this complexity with varying degrees of success the individual buildups are well imaged but as expected smaller features are not as clearly reproduced by the model intermediate scale sections that represent platform retrogradation are shown as surfaces of onlap in the seismic model but field work shows that the platform was still shedding sediment while retreating and that no simple onlap surface exists in the outcropping section a similar lack of stratigraphic resolution is shown by the model at the downlapping clinoforms which do not correspond to iscrete stratal surfaces but to the toe of slope position where major bedding units thin below seismic resolution smaller features such as the position of downslope terminations of carbonate breccia wedges are not resolved by the model toe of slope packages of limestone turbidites are represented only by reflection segments of higher amplitude the decreasing seismic resolution is caused by limited seismic bandwidths and by the effects of constructive and destructive interference interference has the largest effects at the complexly interfingering buildup to basin transition or at the most detailed part of our model an understanding of these types of seismic limitations and their controls is important as we try to extract more and more stratigraphic information from seismic data p

topographic influences on the general circulation of the southern hemisphere a numerical experiment abstract a july integration of a gfdl spectral general circulation model is repeated after eliminating from the model the topographic elevations zonally averaged mean fields regions of frequent cyclogenesis and cyclone tracks and the standing waves in geopotential at 500 mb simulated for the southern hemisphere in both integrations are compared it is argued that the differences in the results support the interpretation advanced by mechoso 1980 concerning the influence of antarctica an the general circulation of the southern hemisphere

depositional bias and environmental change important factors in sequence stratigraphy abstract differences among depositional systems here called depositional bias strongly influence sequence patterns siliciclastics and shallow water carbonates for instance shed most of their sediment during opposite phases of a sea level cycle lowstand shedding and highstand shedding respectively furthermore the two systems generate their own system specific relief on the sea floor disperse their sediment load along different avenues and differ in the way they are deactivated reefs and carbonate platforms can be drowned whereas siliciclastic deposition can be shut off and renewed at any depth as a consequence of these differences pronounced unconformities drowning unconformities develop where carbonate platforms are terminated and buried by siliciclastics the siliciclastic to carbonate transition tends to be more gradual drowned platforms and drowning unconformities appeared world wide in great abundance in the miocene cretaceous valanginian turonian jurassic toarcian and devonian frasnian famennian examples of drowning unconformities interpreted as sequence boundaries include those of the early cretaceous platforms off new jersey and off morocco the mid cretaceous unconformity in the gulf of mexico and miocene unconformities on top of reefs in the far east the eustatic cycles postulated from sequence stratigraphy are a very unlikely cause for the mass drownings of reefs and platforms either the rates of rise are an order of magnitude lower than the growth potential of platforms or cycle amplitudes are too small or the cycles are too short and subsidence too slow to remove platforms permanently from the photic zone the critical element in the mass drowning seems to be environmental stress that reduces the growth potential of carbonate systems thus drowning unconformities demonstrate the importance of environmental change as a control on sequence development other examples of dominantly environment controlled sequences include slope deposits shaped by shifting currents e g florida and blake plateau and basin fills that were controlled by changes in sediment input related to tectonic alteration of the drainage in the hinterland e g gulf of mexico environmental change must be considered as a third independent factor that competes with eustatic and tectonically driven regional changes of sea level for control of sequences the definition of sequences and sequence boundaries should be broad enough to include the possibility of non sea level controls

seismic models of a carbonate foreslope to basin transition picco di vallandro dolomite alps northern italy detailed geologic cross sections augmented by laboratory calibration of lithology and acoustic impedance have been used to produce synthetic reflection seismic sections of a carbonate foreslope to basin transition two areas from the picco di vallandro region of the dolomite alps were modeled a progradational section and a retrogradational backstepping section the resulting models show how these complex areas of strata interfingering might be displayed on conventional reflection seismic lines in the area of progradation rapid stratigraphic thinning below seismic detectibility coupled with abrupt impedance changes produces a reflection discontinuity between steeply dipping reflections of the foreslope and gently dipping parallel reflections of the basin section this apparent downlap surface marks the toe of slope for successive clinoforms but does not correspond to a discrete stratigraphic surface in the backstepping example similar stratigraphic thinning and impedance changes create an apparent onlap surface wavelet interference causes complications in both examples these models indicate how stratigraphic complexity can be simplified by the seismic reflection process and suggest that caution should be exercised when using seismic data to construct general models in areas of complex depositional geometries and rapidly changing facies

use of the underwater video profiler for the study of aggregate dynamics in the north mediterranean abstract the underwater video profiler is a vertically deployed survey system designed for the quantification of particles 280 m and of large zooplankton in the 0 1000 m water column light reflected by undisturbed target objects forms a dark field image which is recorded at 25 hz frequency the recorded images are automatically digitized and analysed the results are expressed as abundance or size distributions and they can be converted to volume or mass units the system can be configured as a multi instrument array and can simultaneously acquire biological and physical data from may 1994 to april 1995 a monthly survey was performed across a frontal structure associated to the north ligurian geostrophic current in winter and spring 1995 the front as well as the offshore dispersion limit of particles were located near to the coast in contrast during autumn 1994 its position was open sea and the terrestrial matter was dispersed far from the coast the continuous presence of intermediate nepheloid layers along the continental slope indicates that different processes may supply and transport the particulate matter to deeper layers from where it can be diffused into the basin

the influence of continental ice sheets on the climate of an ice age the climate influence of the land ice that existed 18 000 years before present 18k b p is investigated by use of a general circulation model of the atmosphere coupled with a static mixed layer ocean simulated climates are obtained from two versions of the model one with the land ice distribution of the present and the other with that of 18k b p in the northern hemisphere the tropospheric flow field is strongly influenced by the laurentide ice sheet and features a split flow straddling the ice sheet with a strong jet stream forming the southern branch the northern branch of the flow brings very cold air over the north atlantic ocean where thick sea ice is maintained the distribution of sea surface temperature sst difference between the two experiments in the northern hemisphere resembles the difference between the sst at 18k b p and at present as estimated by the climap project 1981 the 18k b p ice sheets have very little influence upon atmospheric temperature and sst in the southern hemisphere this is because the interhemispheric heat transport hardly changes as the loss of heat energy due to the reflection of solar radiation by continental ice sheets in the northern hemisphere is almost completely counterbalanced by the in situ reduction of upward terrestrial radiation hydrologic changes in the model climate are also found with statistically significant decreases in soil moisture occurring in a zone located to the south of the ice sheets in north america and eurasia these findings are consistent with some geological evidence of regionally drier climates from the last glacial maximum

seismic models of a mixed carbonate siliciclastic shelf margin permian upper san andres formation last chance canyon new mexico seismic models based on randomly distributed samples from the permian upper san andres formation last chance canyon new mexico verify that the most prominent seismic reflections are related to stratal geometry however at least some reflections arise from lateral facies transitions that are commonly associated with highly prograding mixed carbonate siliciclastic sediments understanding seismic reflections and reflection terminations in sedimentary rocks requires simulation of at least 2 d cross sections of the impedance distribution to investigate the cause of reflections in strongly prograding mixed carbonate siliciclastic shelf margins acoustic velocity and bulk density were measured on more than 60 plugs that spatially cover one higher order genetic sequence the resulting impedance values were gridded and contoured and the genetic sequence was repeated to create a cross section averaging impedance values for each lithofacies zone generated a second impedance cross section seismic models of both impedance cross sections revealed the following observations 1 reflections associated with the sequence boundaries are subject to amplitude and polarity phase changes and 2 at least one reflection within the high order sequences is related to subhorizontal facies changes and is associated with two pseudounconformities the contoured impedance model is suggested to closer resemble the true impedance function in outcrop and shows subtle vertical shifts and significantly higher impedance contrasts

pseudo unconformities in seismic models of large outcrops traditionally seismic modeling has concentrated on one dimensional borehole modeling and two dimensional forward modeling of basic structural stratigraphic schemes which are directly compared with real seismic data two dimensional seismic models based on outcrop observations may aid in bridging the gap between the detail of the outcrop and the low resolution of seismic lines examples include the dolomites north italy the vercors se france and the high atlas morocco the seismic models are generally constructed using the following procedure a construction of a detailed lithologic model based on direct outcrop observations b division of the lithologic model into lithostratigraphic units c assignment of petrophysical properties to these lithostratigraphic units d ray tracing to compute time or depth sections of reflectivity e convolution of the reflectivity sections with source wavelets of different frequencies the lithologic detail modeled in the case studies led to some striking results particularly the discovery of pseudo unconformities pseudo unconformities are unconformities in seismics but correspond to rapid changes of dip and facies in outcrop none of the outcrop geometries studied were correctly portrayed seismically at 25 hz peak frequency however in some instances the true relationship would gradually emerge at peak frequencies of 50 100 hz the examples given in this study demonstrate that detailed outcrop derived seismic models can reveal what stratigraphic relationships and features are likely to be resolved under ideal or less ideal conditions and what pitfalls may befall the interpreter of real seismic data