the carbonate factory revisited a reexamination of sediment production functions used to model deposition on carbonate platforms there are two widely used models of sediment production on carbonate shelves 1 production varying with depth and 2 production as a function of depth and distance from the platform margin here we recalculate carbonate production on great bahama bank west and northwest of andros island using broecker and takahashi s 1966 pioneering geochemical study in the calculations residence time and depth are used to calculate yearly carbonate production based on equilibrium thermodynamics of caco3 loss from water over the banks whereas broecker and takahashi assumed a uniform depth of 4 5 m in their work here we use realistic depths and residence times interpolated over the bank our results suggest that sediment production is on the order of 4 5 kg m2 y on the bank margins and that production rapidly drops off into the platform approaching zero in the shallowest innermost parts of the platform thus for a large platform such as great bahama bank with long residence times the shallowest areas of the platform may become sinks not sources of sediment

organic walled microfossil assemblages from glacial and interglacial neoproterozoic units of australia and svalbard before the onset of the neoproterozoic snowball earth glaciations eukaryotes had begun diversifying and in their aftermath macroscopic life including both animals and macroalgae became abundant and widespread although glacially driven mass extinctions have been hypothesized little is known about the biosphere during and between these glaciations here we present new data from organic walled microfossil assemblages from five successions in australia and svalbard that collectively span the first sturtian glaciation and interglacial interval and integrate them with data derived from a critical evaluation of the literature to produce a new estimate of eukaryotic diversity from 850 to 650 ma these new glacial and interglacial assemblages consist of only smooth walled spheroids leiosphaerids aggregates of cells and filaments in contrast to the much more diverse organic walled microfossil assemblages found in early neoproterozoic rocks this contrast is not attributed to biases in deposition or preservation but is instead interpreted as reflecting an interval of lowered eukaryotic diversity that spanned the glaciations and that may have begun millions of years prior to their onset

impact of sedimentology and diagenesis on the petrophysical properties of a tight oolitic carbonate reservoir the case of the oolithe blanche formation bathonian paris basin france the oolithe blanche formation was studied in three quarries located at the south eastern edge of the paris basin france heterogeneities in reservoir properties were assessed through a sedimentological diagenetic and petrophysical study the relationships between depositional settings diagenesis and petrophysical properties were analysed using detailed petrographic studies image analysis nano ctscans and petrophysical measurements the carbonate reservoir pore network is mainly controlled by intraparticle microporosity which ensures the connectivity with interparticle meso and macroporosity early cementation vs early compaction processes mainly grain interpenetration may have considerable influence on fluid flow properties and parameters such as permeability acoustic velocities and tortuosity better reservoir properties are found when compaction processes begin before cementation from statistical analyses e g principal component analysis and linear discriminant analysis a sedimentological diagenetic and petrophysical model is proposed that is in a good agreement with the geological model developed from field work

temporal and spatial distribution of whitings on great bahama bank and a new lime mud budget analysis of 69 shuttle and satellite images of the northern bahamas taken from 1965 to 1993 allowed us to calculate the regional and temporal distribution and longevity of 888 whitings on the great bahama bank at any time whitings cover between 35 and 200 km 2 in the area centered at lat 25 n and 78 50 w the highest occurrences of whitings are in april and october suggesting a seasonal component using a measured average of 10 6 mg l suspended sediment in typical whitings we calculated that 1 35 10 6 metric tons of lime mud are suspended every year if the suspended carbonate is precipitated in the water column these phenomena account for 280 of all of the holocene accumulated bank top mud and more than 40 of the total bank top mud and periplatform mud that have accumulated on the west side of great bahama bank

high energy carbonate sand accumulation the quicksands southwest florida keys abstract high resolution seismic reflection profiles of the quicksands located along a broad ridge on the platform shelf west of key west florida indicate a significant deposit of non oolitic carbonate sand occurs in a belt 47 km long by 28 km wide the surface of the belt is ornamented by large 5 m migrating tidal bars oriented in a north south direction on which sand waves oriented in an east west direction are superimposed some of the sand waves are awash at low tide the sand waves are formed by strong reversing tidal currents flowing between the gulf of mexico and the straits of florida the waves migrate directly over pleistocene bedrock to the east but the deposit thickens to the west and sand waves there overlie non oolitic holocene accumulations as thick as 12 m westward ipping accretionary bedding indicates that net migration of the sands is to the west despite north south movement of tidal currents the westward edge of the accumulation has accreted over deeper muddier deposits although tidal currents and resultant bedforms appear identical to those of active ooid deposits in the bahamas and elsewhere no oolitically coated grains were found in this study thin section analyses show the principal component average 48 of the sands is fragmented plates of species of the green alga halimeda followed by particulate coral average 17 which increases off the flanks of the main sand body short vibracores confirm the presence of cross bedding

solution of shallow water carbonates an insignificant buffer against rising atmospheric co2 model predictions suggest that the saturation state of surface ocean waters with respect to carbonate minerals will decline during the twenty first century owing to increased invasion of atmospheric co 2 as a result calcareous organisms may have difficulty calcifying leading to production of weaker skeletons and greater vulnerability to erosion alternatively it has been suggested that there will be no significant impact on coral reef ecosystems because any changes in saturation state and ph will be restored by dissolution of metastable carbonate minerals to resolve this controversy we employ a physical biogeochemical box model representative of the shallow water ocean environment numerical simulations demonstrate that the carbonate saturation state of surface waters could significantly decrease and hamper the biogenic production of caco 3 during the twenty first century similarly the average saturation state of marine pore waters could decline significantly inducing dissolution of metastable carbonate phases within the pore water sediment system such dissolution could buffer the carbon chemistry of the pore waters but overlying surface waters of reefs and other shallow water carbonate environments will not accumulate sufficient alkalinity to buffer ph or carbonate saturation state changes owing to invasion of atmospheric co 2

lack of impact of hurricane michelle on tidal flats andros island bahamas integrated remote sensing and field observations abstract major storms commonly are cited as important geomorphic and sedimentologic agents and their significance is perhaps most emphatically noted in interpretations of modern and ancient tidal flats the purpose of this study was to assess the impact of hurricane michelle on the carbonate tidal flats of andros island bahamas by coupling remote sensing data and field observations although regional modis remote sensing data reveal that the passage of hurricane michelle suspended sediments across the shallow bank integrating ultra high resolution remote sensing data with field observations demonstrates that the hurricane had almost no impact on the andros island tidal flats this lack of impact is surprising because the storm still had hurricane force winds as it passed immediately over some of the focal areas with this hurricane storm strength and intra storm interval were probably not limiting factors instead several influences combined to minimize the storm s impact on both sedimentation and erosion including the low level of water on the bank resulting from ebb tides and in the northwestern area from offshore storm winds and the short duration of the storm resulting from its high forward velocity given the apparent prevalence of widespread storm effects on ancient tidal flats but the spatially limited major geomorphic impacts of storms on many modern systems we suggest that the geologic evolution and preservation of ancient tidal flat successions may have been shaped by mega storms that simply cannot form in the present global climate nonetheless understanding tidal flat dynamics and changes on human timescales will require continued monitoring and assessment of low impact storms such as michelle as well the more sensational hurricanes that produce widespread death destruction and deposition

controls on morphology and sedimentology of carbonate tidal deltas abacos bahamas abstract although the physical processes controlling siliciclastic shoal bodies are reasonably well understood the sedimentary processes that shape carbonate tidal shoal systems are poorly constrained the tidal deltas of the northwestern abaco islands bahamas provide a unique opportunity to explore the relationships between physical and sedimentary processes in a variety of sizes and shapes of tidal deltas the length of tidal deltas range in size from 350 m to 2 km from the inlet to the termination of the delta lobe and shoal crests consist primarily of well sorted ooid sands mean grain size between 400 and 500 m mcmanus sorting factor 0 70 coated grains are rare off the shoals and sorting decreases with distance from the shoal crests velocities in the rocky inlets can exceed 2 m s in the daily tidal cycles and the systematic distribution of grain types reflects tidal velocity variations areas with the highest velocities 0 80 m s correspond to hard rocky bottoms with some skeletal grains these regions of elevated velocities are focused within the inlets where jets of high velocity currents form during each tidal cycle these current jets extend outward from the inlet throat and velocities decrease outwards as the flow disperses velocities between 40 and 80 cm s occur in the regions with abundant ooids and peloids are most abundant in the lowest velocity regions the results of this study illustrate a clear relationship between quotidian physical processes and their geomorphic and sedimentologic products in a carbonate system in this context carbonates appear to share many geomorphic similarities with siliciclastic analogs the results represent an important step beyond qualitative facies interpretations and towards quantitative specified environmental and sedimentologic characterizations for carbonate systems

circulation in enewetak atoll lagoon1 currents at enewetak atoll marshall islands were measured on the reef margins in the channels and in the lagoon lagoon circulation is dominated by wind driven downwind surface flow and an upwind middepth return flow this wind driven flow has the characteristics of an ekman spiral in an enclosed sea lagoon flushing is accomplished primarily by surf driven water input over the windward eastern reefs and southerly drift out the south channel mean water residence time is 1 month while water entering the northern portion of the atoll takes about 4 months to exit

holocene millennial to centennial carbonate cyclicity recorded in slope sediments of the great bahama bank and its climatic implications a 38 m long sediment core md992201 retrieved from a water depth of 290 m from the leeward margin of the great bahama bank gbb 25 53 49 n 79 16 34 w has been investigated for changes in aragonite content the core covers the mid to late holocene the past 7230 yr sediment lightness l values was used as a proxy for aragonite content based on a high linear correlation r 0 93 between the x ray diffraction derived aragonite content and l values the resulting time resolution of the l values derived aragonite content ranges from 1 yr at the base of the core to 4 yr at the top detailed time series analysis using monte carlo singular spectrum analysis and spectral analysis lomb scargle fourier transform identifies the presence of seven signals with varying amplitudes and wavelengths that could be traced throughout the past 5500 yr during the first 1600 yr of sedimentation the aragonite record is dominated by the initial flooding of the flat topped gbb superimposed on a multimillennial signal related to holocene sea level changes a millennial scale fluctuation and five quasi periodic oscillations were detected 1 3 2 kyr 500 600 yr 380 yr 260 yr 200 yr and 100 yr period comparisons with other proxies e g tree ring 14c 10be and 18o in ice cores provides information on the origin and dynamics of the individual signals the analysis shows that the 200 yr and 100 yr signals can be attributed to solar forcing the 260 yr 380 yr and the 500 600 yr quasi periodic signals are found to be of climatic origin whereas the millennial scale fluctuations remain enigmatic although solar forcing mechanisms seem likely the data show that variability of solar output as well as past oceanographic and atmospheric changes have modulated the mid to late holocene climate which in turn controlled sediment input variations found in the holocene wedge leeward of the gbb although these periplatform sediments have a rather uniform appearance they still contain a large variety of subtle sedimentary variations

form function and feedbacks in a tidally dominated ooid shoal bahamas although the general aspects of oolitic depositional systems are well documented seascape scale 10 3 10 6 m 2 patterns of oolitic shoals and the details of processes acting on them are not well understood or quantified to begin to fill this basic gap in understanding this paper describes the morphology and hydrodynamics of lily bank a modern tidally dominated bahamian ooid shoal in this study integrating remote sensing imagery with quantitative geo located bathymetrical hydrological and granulometric data in a geographic information system documents geomorphic and sedimentological patterns and facilitates interpreting these patterns in the context of the processes operating in this system the results of these analyses reveal that parabolic bars up to several kilometres in wavelength and several metres in height form a common morphologic motif although there is considerable variation on that general theme the seascape scale configuration of bars and superimposed sedimentary structures is closely linked to spatial patterns of tidal movements and includes the presence of mutually evasive flood and ebb channels sedimentologically bars are neither homogenous nor random bodies instead granulometric parameters such as sorting and percentage mud vary systematically as shaped by hydro geomorphic controls the best sorted coarsest ooids are on bar crests whereas the finest grains are found in the lower energy deeper interior and flanking regions in short results clearly document hydrodynamic bathymetrical influences on these ooid shoals and their granulometry linkages akin to siliciclastic analogues sedimentological hydrodynamic and geomorphic observations are consistent with a conceptual model for the formation of parabolic bars in which initial irregularities in nonparabolic bars are enhanced through their effect of focusing flow constricted flow leads to higher flow velocities tidal flow velocity asymmetries differential net sediment transport and growth of bathymetrical highs this bathymetrical divergence creates separate paths for flood and ebb tides facilitating emergence of better developed parabolic forms the resultant parabolic geometries and component bedforms appear to be either in dynamic equilibrium with both ebb and flood tide flows or evolving toward that state in exploring patterns and processes within carbonate shoals this study illustrates some of the first documented insights on quantitative details of morphology and dynamics and in the links between geomorphic framework and grain size and sorting in an oolitic carbonate system assuming a

holocene atlantic climate variations deduced from carbonate periplatform sediments leeward margin great bahama bank a marine sediment core from the leeward margin of great bahama bank gbb was subjected to a multiproxy study the aragonite dominated core md992201 comprises the past 7230 years in a decadal time resolution and shows sedimentation rates of up to 13 8 m kyr aragonite mass accumulation rates age differences between planktonic foraminifera and aragonite sediments and temperature distribution are used to deduce changes in aragonite production rates and paleocurrent strengths aragonite precipitation rates on gbb are controlled by exchange of carbonate ions and co2 loss due to temperature salinity conditions and biological activity and these are dependent on the current strength paleocurrent strengths on gbb show high current velocities during the periods 6000 5100 years bp 3500 2700 years bp and 1600 700 years bp lower current speeds existed during the time intervals 5100 3500 years bp 2700 1600 years bp and 700 100 years bp bahamian surface currents are directly linked to the north atlantic atmospheric circulation and thus periods with high low current speeds are proposed to be phases of strong weak atmospheric circulation

biological markers and their carbon isotopes as an approach to the paleoenvironmental reconstruction of nansha area south china sea during the last 30 ka paleoenvironmental changes over the last 30 ka have been documented for the nansha area south china sea from the biomarkers and their carbon isotopic composition of 314 sediment samples from core 17962 the concentrations of marine lipids indicate that the paleoceanographic and paleoclimatic conditions were not stable in the nansha area during the last glacial period the accumulation rates of c37 alkenones and c30 alkyl diols show that marine paleoproductivity was higher during the last glaciation than in the holocene and correlate well with those of terrestrial biomarkers simultaneous variations in accumulation rates of marine biomarkers and terrigenous long chain compounds during the last glacial period suggest that this enhancement of marine productivity was caused by an increase in terrestrial nutrient supply from the sunda shelf the stable carbon isotopic composition of terrigenous long chain compounds such as n alkanes 13 c 28 6 34 2 n alkanoic acids 13 c 27 1 31 8 and n alkanols 13 c 26 6 35 3 indicates that these compounds were derived from c3 plants and the surrounding area of southern south china sea was not dry during the last glacial 2002 elsevier science ltd all rights reserved

geomorphic and sedimentologic heterogeneity along a holocene shelf margin caicos platform abstract reef rimmed margins on isolated carbonate platforms are heterogeneous and studying holocene platforms provides a valuable means for characterizing the spectrum of facies variability and understanding the processes behind facies changes in this study insights from petrographic data granulometric measurements bottom observations and remote sensing and subbottom profiler data from the northwestern part of caicos platform make this one of the more intensely studied shallow marine carbonate systems in the caribbean collectively analyses of the thicknesses and heterogeneity of shallow water holocene carbonates and the platform margin reveal details of the considerable geomorphic variability present on this part of the margin on which the orientation changes markedly in this area the shelf margin arcs from northwest to north to northeast facing accompanied by changes in morphology as well as the distribution thickness and type of sediments the nw facing margin has discontinuous reefs near the margin flanked platformward by coarse sands in poorly developed aprons that transition to a deeper rocky bottom interpreted as pleistocene with patches of sand to fine gravel sized sediment locally lobate to arcuate shoals with medium sands 500 m mean size form a reef apron that is continuous along strike and over 1000 m wide because the interpreted top pleistocene surface is gently dipping seaward holocene sediment thicknesses vary with changes in bathymetry and generally thin away from the reefs sediments in most areas are between 1 5 and 3 m thick but can reach over 6 m locally although the interpretation of the base of the holocene can be ambiguous under shelf margin reefs subbottom profiles illustrate that backreef shelf patch reefs do not necessarily nucleate on pleistocene bedrock highs facies trends are interpreted to reflect a combination of physical oceanographic sedimentologic and biologic processes acting within the framework of pleistocene bedrock configuration the ne facing margin with the continuous reef and expansive sand apron is interpreted to be more wave dominated influenced by swells from the open atlantic and by locally derived wind waves comparison with bahamian platforms suggests that this trend is not unique margins facing open atlantic swells from the north northeast have the best developed reefs not east facing windward margins in contrast the beaches and tidal deltas on the nw facing margin suggest tidal currents and longshore transport are more pronounced here consistent with the nature of geomorphic changes through time the enhanced tidal influence reflects the inlets open to the open bank interior a factor which also may discourage growth of reefs on this part of the margin nonetheless other bahamian platforms with fewer tidal passes have comparable discontinuous reefs suggesting that island continuity is not the most influential parameter on leeward reef development collectively these results illustrate the nature scale and causes of along strike heterogeneity and provide a conceptual model for ancient carbonate platform margins some of which may be equally complex in their facies architecture

recent sedimentary facies of isolated carbonate platforms belize yucatan system central america abstract distribution of recent sedimentary facies is significantly different in four closely spaced isolated carbonate platforms off the coasts of belize and yucatan se mexico these differences are largely controlled by variations in submarine topography which are likely the result of differential subsidence and karstification from platform to platform a variety of depositional styles and patterns is found in platform interiors that developed during the same holocene sea level rise composition and texture of surface sediment samples were used to define four major sediment types and to distinguish nine depositional environments within the platforms facies maps are based on analysis of 454 sediment samples and high resolution satellite imagery glovers reef is characterized by a circular facies distribution the nearly continuous reef margin is composed of a coral red coralline algae halimeda grainstone mixed nonskeletal peloidal skeletal wackestones and packstones are present in shallow lagoon parts 5 m water depth behind the margin the deep interior from 5 to 18 m water depth is characterized by mollusk foram halimeda wackestones with over 860 patch reefs of coral red algae halimeda mollusk packstones lighthouse reef also has a coral grainstone and coral red algae halimeda grainstone rim but the facies distribution in the interior is strongly asymmetrical the 8 m deep eastern lagoon is floored by mollusk foram halimeda wackestones and packstones the 3 m deep western lagoon has the mixed peloidal skeletal wackestone to grainstone facies a linear trend of coalescing patch reefs and skeletal grainstone facies separates the two lagoons banco chinchorro is similar to lighthouse reef inasmuch as facies distribution patterns of the interior are strongly asymmetrical mollusk foram halimeda wackestones grainstones are present in eastern lagoon areas whereas shallow 5 m western parts of the lagoon are composed of mixed peloidal skeletal grainstones packstones and wackestones most of turneffe islands is protected on the east by lighthouse reef accordingly turneffe has a narrower reef and skeletal grainstone rim than the other two platforms restricted interior lagoons are up to 8 m deep and are enclosed by wide mangrove rims and surface sediments dominated by halimeda rich wackestone rich in organic matter coral patch reefs are absent in contrast the unprotected northernmost part of turneffe has a wide reef and grainstone rim and the open lagoon area consists of mollusk foram halimeda wackestones packstones with abundant patch reefs

interactions between tidal flows and ooid shoals northern bahamas abstract oolitic strata are common through the geologic record ooids generally form in high energy environments but it remains unclear how ooids remain in such systems and form geomorphic features such as bars and shoals integrating remote sensing hydrodynamic bathymetric granulometric and field observations of modern tidal systems in the bahamas provides insight into this fundamental question oolitic tidal sands in the northern abacos display a geomorphic pattern in which bedrock islands restrict and focus tidal flow down a main channel within the channel a shallow shoal separates an ebb dominated subchannel from a flood dominated subchannel tidal velocities in these subchannels can exceed 1 m s enough to transport the oolitic sediments hydrodynamics and bathymetry in these subchannels produce a net circular hydrodynamic pattern around the shoal the spin cycle allowing the sands to remain in motion without being transported out of the ooid factory this general pattern is apparent in several other ooid shoal complexes this concept provides integrated insights into the physical influences impacting the formation suspension transport and deposition of ooids and the resulting geomorphic forms these results represent first steps toward developing more comprehensive and predictive analogs of spatial heterogeneity in ancient tidally dominated oolitic shoals

holocene ooids of aitutaki atoll cook islands south pacific although oolitic sands are widespread throughout the phanerozoic carbonate rock record and they are abundant in some modern shallow marine carbonate systems recent ooids have not been recognized from any locality in the pacific basin on aitutaki atoll cook islands south pacific oolitic sands occur in water depths of 2 1 6 3 m within the lagoon and less abundantly on shallower reef aprons most ooids are between 150 and 300 m in diameter have between 1 and 25 concentric to irregular laminae and occur with peloids and skeletal grains cortical laminae include mg calcite crystals with radial tangential and random orientations the occurrence and distribution of oolitic sands on aitutaki are facilitated by a convergence of hydrodynamic and chemical factors wave driven currents combined either with flow separation over a sharp increase in depth at the reef apron lagoon interface or with oceanward flow in the lee of islands serve to transport ooids without flushing them from the system here ph alkalinity and carbonate supersaturation are at some of the highest levels in the tropical pacific the restricted occurrence of holocene oolitic sands in areas of the pacific and atlantic basins with elevated ph and total alkalinity reflects the importance of carbonate saturation thresholds in limiting the spatial distribution of these sands in modern shallow marine settings these recent examples are consistent with interpretations of the importance of these variables in determining the distribution abundance and cortical mineralogy of oolitic sands throughout the phanerozoic stratigraphic record of carbonate accumulation

a currents of removal approach for interpreting carbonate sedimentary processes abstract this study develops a currents of removal methodology to examine and quantify the nature of physical transport processes affecting the formation of bioclastic deposits in the cocos keeling islands an indian ocean atoll this approach is based on the hydraulic settling and threshold of entrainment characteristics of bioclastic deposits and measured current energy potential mobility pm analysis of 157 samples quantifies the proportion of deposits able to be transported in each geomorphic zone results show that under mean energy conditions wave induced currents entrain and transport sediment within the atoll and show that transport of sediment is not solely reliant on storm energy conditions gradients of pm values are used to reconstruct sediment transport pathways from the reef flat 85 100 pm through shallow passages 75 100 pm to sand aprons 50 75 pm and shallow 0 20 pm and deep lagoon 0 pm comparison of settling velocity distributions of bed material and sediment retained in sediment trap experiments show that actual mobility levels correspond with pm estimates potential mobility analysis also identifies the immobile portions of deposits which increase lagoonward constituent analysis of immobile fractions and analysis of settling frequency distributions are used to differentiate the importance of physical and biological processes in the formation of deposits throughout cocos and identify the role of each geomorphic zone in the transport system the pattern of sediment is controlled by physical processes between the reef flat the primary sediment production zone with sediment transported through the shallow passage conduits to the sand aprons the formation of shallow and deep lagoon deposits is controlled by autochthonous sediment production and storm deposition potential mobility analysis is a powerful tool enabling physical transport processes within bioclastic sedimentary environments to be quantified the ability to examine an individual deposit s hydraulic behaviour and development also enables depositional processes to be examined at a much finer resolution than previously attempted using conventional textural approaches

effects of natural and artificial thalassia on rates of sedimentation abstract sediment traps were used to measure the rate of deposition in a thalassia seagrass meadow and in an adjacent sandy grass free area the average depositional rate of mud in the thalassia bed was 4 96 g trap 1 week 1 and in the sandy area it was 3 04 g trap 1 week 1 in the summer in winter these rates were 1 60 and 1 50 g mud trap 1 week 1 respectively electromagnetic current meter measurements showed that the mean flow velocity in the grass free area is higher than in the seagrass area therefore the increased depositional rate of mud within the seagrass is due to slowing of water currents by the grass blades confirmation of the blade baffle effect was obtained by placing a 1 m2 plot of artificial thalassia in a grass free area sediment traps in the artificial thalassia contained 5 45 g mud trap 1 week 1 as compared to 3 04 g mud trap 1 week 1 in the adjacent grassfree area in summer in winter these rates were 1 82 and 1 50 g mud trap 1 week 1 respectively epiphytes in the indian river lagoon had no detectable effect on the amount of mud deposited in thalassia beds

controls on platform scale patterns of surface sediments shallow holocene platforms bahamas isolated carbonate platforms occur throughout geological history and commonly exhibit considerable spatial variability to evaluate the controls on the nature of sediment accumulation across the expansive shallow platform tops this study systematically compares and contrasts patterns in surface sediments from several shallow 10 m holocene bahamian examples remote sensing data field observations petrographic characterization and quantitative grain size analyses reveal the spatial patterns of sediment accumulation on crooked acklins platform and the berry islands bank integration of these data with synoptic observations of waves tides and currents along with regional geochemical data provides a means to explore the factors that influence platform scale sedimentary patterns these data illustrate that the platform interiors of both crooked acklins platform and berry islands bank are blanketed with medium to coarse sand size sediment peloids are most common in the interior of crooked acklins platform whereas the berry islands bank includes more abundant composite grains in both areas very little mud is present with surface sediments averaging 2 mud comparison of these results with published data from little bahama bank great bahama bank and caicos platform suggest that contrary to previous interpretations the presence of open margins and or brisk winds are not necessary for the occurrence of a platform top with little mud although the muddy sediment fraction of the interior can be suspended by elevated wave energy wind generated current speeds in protected platform interiors are relatively low instead in parts of the platform interiors transport and winnowing of fines is enhanced greatly by tidal currents which carry suspended sediments off the shallow platforms even if shielded by islands beyond physical influences however regional geochemical compilations suggest that the bahamian tides supply highly supersaturated waters rich in dissolved oxygen to these platform interiors this exchange is interpreted to facilitate favourable conditions for calcium carbonate precipitation in the form of ooids marine cements and hardened peloids across vast expanses of the platform interiors such fundamental controls on holocene platform scale sediment dynamics are likely to have influenced carbonate systems through the geological record

epiphytic calcium carbonate production and facies development within sub tropical seagrass beds inhaca island mozambique seagrass beds have been widely recognised as playing an important role in influencing carbonate sediment facies development this reflects their role not only as facilitators of fine sediment settling and stabilisation but also as substrates for epiphytic organisms that after death contribute skeletal carbonate to the sediment substrate in low latitude reef related settings epiphytic carbonate production rates are often high and this in combination with the trapping of carbonate mud produced by a range of associated calcareous algal species typically results in the development of carbonate mud rich facies whilst such environments and their associated sediment substrates have been widely documented studies of seagrass facies in marginal sub tropical warm temperate marine settings have not been conducted from a sedimentological perspective this study determines rates of epiphytic carbonate production on two seagrass species thalassodendron ciliatum and thalassia hemprichii and examines seagrass sediment facies from a sub tropical reef related environment in southern mozambique dense seagrass beds colonise primarily siliciclastic sediment substrates and are characterised by low rates of epiphytic carbonate production mean 43 9 g caco3 m 2 year 1 for t ciliatum and 33 4 g caco3 m 2 year 1 for t hemprichii epiphytic encrusters are dominated by thin monostromatic layers of the crustose coralline red algae hydrolithon farinosum along with rotaliid smaller benthic foraminifera including asterorotalia cf gaimardi and spirillina sp and the soritid peneroplis sp as well as rare encrusting acervulinid foraminifera serpulids and bryozoans epiphytic calcium carbonate production rates are therefore low and this is reflected in the low 15 carbonate content of the seagrass sediments as well as the low 1 sediment fine 63 m size fraction content this study suggests that mud rich sediment facies do not necessarily develop in conjunction with seagrass beds and that mud poor facies may develop in some seagrass colonised environments at these sites sub tropical and siliciclastic sediment dominated this is suggested to reflect both the low levels of epiphytic and benthic algal carbonate mud production as well as a net outflux of fine grained sediment due to tidal current induced sediment resuspension in keeping with carbonate facies generally there are thus likely to be latitude and environment related shifts in the composition and character of seagrass related sediment facies

density cascading off shelf sediment transport evidence and implications bahama banks abstract suspended sediment concentrations in shelf waters are often appreciable but sediments on their own rarely contribute enough density to cause suspension currents to transport them offshelf however sediment charged hyperpycnal high density waters are often generated over continental shelves because of the same set of air sea interactions intense heat and moisture flux responsible for thermohaline production and circulation of deep ocean water masses we propose the term density cascading to describe the processes by which hyperpycnal waters are released from shelf areas and together with entrained sediments sink to their compensation level in the adjacent ocean density cascading probably involves a range of physical processes from pelagic draping of slopes and basins with shelf muds to initiating turbidity currents capable of scouring canyons and plunge pools sediment bypassing of slopes and deposition of sands in deep basins thermodynamic air sea interactions can generate hyperpycnal water at all latitudes therefore density cascading has important implications not only to the sedimentology of modern carbonate platforms such as the florida shelf and bahama banks and stratigraphy in ancient limestones but also to offshelf sediment flux in siliciclastic environments

on long term net flow over great bahama bank abstract a 398 day time series of middepth current measurements is combined with available wind and bottom pressure measurements and historical salinity data to characterize long term net flow patterns over great bahama bank between the tongue of the ocean and exuma sound the progressive vector diagram constructed from current measurements indicates a resultant flow of just over 2 cm s 1 toward 002 semidiurnal tides dominate the instantaneous current a one dimensional numerical model is used to investigate the local response to regional forcing results suggest that the long term net flow is a response to the combined effect of wind driven and density driven flow toward the northwest and a tide induced residual current toward the east southeast aided by a quasi steady barotropic pressure gradient toward the north northeast the slope of the sea surface is specified according to its ability to reproduce measurements under steady state conditions the model reproduces the middepth flow and indicates

the carbonate chemistry of grand bahama bank waters after 18 years another look major features of the chemistry of northern grand bahama bank waters found in 1981 were similar to those found by broecker and takahashi in 1962 and 1963 measured variations in salinity and extent of caco3 removal are close to the same our findings indicate a more complex salinity distribution on the northern banks probably resulting from transport of high salinity waters from the south and higher pco2 values in the high salinity waters inorganic 14co2 measurements made in this study can be used to calculate very approximate residence times of water on the bank there is general agreement with the values previously determined by broecker and takahashi ship aerial and satellite observations indicate that whitings are not randomly distributed over the bank they form a halo around the northern and western flanks of the high salinity waters s greater than 40 the rate of caco3 removal is about 1 5 times faster in the waters where the whitings are common than in the high salinity waters even though our analytic precision was over 80 times greater than the change predicted in alkalinity if whitings precipitate directly from seawater no differences could be found between whiting and adjacent waters in our opinion the major features responsible for creating whitings controlling their distribution and determining caco3 removal rates on the grand bahama bank remain largely unresolved

great bahama bank aragonitic muds mostly inorganically precipitated mostly exported sr concentrations in aragonitic muds on the banktop and leeward slope of great bahama bank gbb are greater than in the needles produced by codiacean green algae but the same as in precipitated sand size ooids and other nonskeletal grains results of two part and three part mixing models suggest that chemically precipitated aragonite contributes an average of 55 78 of the clay size aragonite individual samples contain as much as 95 chemically precipitated aragonite nonskeletal aragonitic needles apparently are precipitated ubiquitously over gbb but accumulate on the banktop only leeward of andros island much of the mud produced on the bank is exported to deeper waters the thickness of mud dominated holocene sediments on the leeward slope can reach 90 m and the aragonitic lysocline around the bahamas is depressed by nearly 1500 m

whitings a sedimentologic dilemma whitings drifting clouds of water milky because of suspended carbonate have been claimed to originate from either the action of bottom feeding fish or direct precipitation of calcium carbonate five cruises during different seasons were made to the great bahama bank to collect data pertinent to the controversy measurements of particulate concentrations average 10 mg liter with a maximum of 20 mg liter of carbonate sediment suspended in whiting water compared with an average of 1 5 mg liter for clear water outside the whitings the particles are dominantly acicular aragonite but mg calcite composes as much as 20 percent of some whitings sedimentation rates measured with fixed and drifting sediment traps were as great as 34 g m 2 hr sediment suspended in whitings aggregated into silt and sand size fioccules and settled to the bottom of settling tanks within six hours even on a rocking ship sediment in artificial whitings created by stirring sediment from the bottom with a shrimp trawl settled to the bottom in about the same time natural whitings on the other hand were never observed to dissipate because sedimentation from whitings occurs at rates sufficient to cause dissipation of the whitings within six hours we conclude that the natural whitings are continually replenished by direct precipitation the search for fish in whitings utilized sidescan sonar and fathometer imaging shrimp trawls rotenone remote video and direct scuba observation these methods and 25 years of casual observations leading to this study indicate that fish are not involved in the formation of most bahamian whitings several whitings were found over rocky or sandy bottoms where there was no mud available for fish to suspend the distance of these whitings from areas of muddy bottom precluded their having been made elsewhere by fish stable carbon and oxygen isotopic analyses and delta 14 c activity are interpreted to indicate that the suspended sediment in whitings contains some precipitated calcium carbonate and is not merely bottom sediment stirred into suspension estimates indicate that the amount of new carbonate produced in whitings on the great bahama bank is substantially higher than that arising from algal production consequently the amount of sediment transported to deep water may be much greater than previously thought