the fertilization of the bahamas by saharan dust a trigger for carbonate precipitation the enigma of the bahamas is that this highly productive carbonate system has existed for at least 100 m y building a vast edifice of carbonates thousands of meters thick in an essentially nutrientpoor environment based on measurements of the insoluble material the fe and mn in the carbonate fraction and the d 15 n of the sedimentary organic matter we suggest a paradigm shift in order to explain the formation of the bahamas and possibly other similar platforms we propose that the great bahama bank is currently and may in the past have been fertilized by atmospheric dust promoting the fixation of atmospheric n 2 by cyanobacteria these cyanobacteria provided a source of nitrogen to the rest of the community in this nutrient poor environment the fixation of n has imparted a characteristic d 15 n signal and has been responsible through the drawdown of co 2 for initiating the precipitation of carbonate in the shallow waters this phenomenon might be responsible for the formation of vast amounts of sediments in the oceans not only within recent times but throughout geological history particularly in the early history of the earth prior to the existence of calcium carbonate secr eting organisms

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

massive recrystallization of low mg calcite at high temperatures in hydrocarbon source rocks implications for organic acids as factors in diagenesis we document massive recrystallization of low mg calcite lime mudstone source rocks at moderately high temperatures in the smackover formation of mississippi this process was driven by organic acids produced during kerogen maturation and holds the implications for studies of primary migration of hydrocarbons secondary porosity generation in reservoirs and global environmental change this interpretation is based on contrasting the following petrographic and bulk rock geochemical characteristics of organic poor vs organic rich lime mudstones

calcification of cyanobacterial filaments girvanella and the origin of lower paleozoic lime mud the origin of lime mud in ancient shallow marine carbonate platforms is one of the most vexing mysteries in sedimentary geology possible sources previously suggested include the physical and biological disintegration of animal and algal bioclasts mineralizing pelagic organisms and spontaneous whitings from either abiotic precipitation or calcification of suspended picoplankton and organic matter the styles of occurrence of micrite walled tubes referred to girvanella in lower paleozoic limestones argue that occasional permineralization of mucilaginous sheaths of the benthic filamentous cyanobacteria precursor could have produced lime mud when the tubes fell apart estimated volumes are comparable to those calculated for the modern bahama platform the copious growth of cyanobacteria typical in shallow marine settings indicates that this mechanism could account for a considerable amount of lime mud especially in the early paleozoic

whiting events and the formation of aragonite in mediterranean karstic marine lakes new evidence on its biologically induced inorganic origin this paper discusses the formation of whiting events drifting milky clouds of water and their role in the formation of recent aragonite sediments in a semi enclosed karstic marine lake on the island of mljet adriatic sea this study is based on detailed structural morphological and sedimentological characterization of and strontium distribution in particles originating from suspended matter and sediments the particles were examined by x ray diffraction and electron diffraction analyses scanning and transmission electron microscopy as well as electron microprobe energy dispersive x ray analyses and atomic absorption spectroscopy morphological features granulometric characteristics strontium enrichment and characteristic details of the aragonite structure in the needle like particles were identical in both the suspended matter collected during whiting events and in the bottom sediments the whiting events which occasionally occurred in surface waters were found to be sites of short term active authigenic aragonite precipitation and the main source of fine grained aragonite mud this study exemplifies the role of biologically induced inorganic precipitation processes in the formation of recent aragonite mud in a restricted mediterranean environment

depositional sequences in deep shelf environments a response to sea level changes and shallow platform carbonate productivity oxfordian germany and spain abstract oxfordian deep shelf deposits of southern germany and southern spain are characterized by marl limestone alternations that are stacked into small scale medium scale and large scale depositional sequences the german sections contain autochthonous sponge reefs and associated fragments of microbialites whereas the sections studied in spain display tempestites composed of autochthonous deeper water and allochthonous shallow water particles the facies of the german limestones and marl layers has been analyzed in detail condensed intervals implied by glauconitization of particles abundant cephalopods intense bioturbation and generally more marls are also enriched in sponge reefs and associated particles and in nannofossils limestone rich intervals however contain fewer sponges and fewer nannofossils neither bioerosion of sponge reefs nor nannofossil blooms can thus explain the abundance of carbonate mud that forms the limestones consequently it is suggested that most of the carbonate mud is exported from shallow platform areas where carbonate productivity is high the clay fraction was derived from weathering of massifs in the hinterland the observed depositional sequences can be correlated between the studied sections in germany and spain situated in different paleotectonic and paleoclimatic domains and also between deeper water sections and platform sections this suggests that they formed through allocyclical processes comparision with published time scales implies that the small scale and medium scale sequences formed in tune with the 100 kyr and 400 kyr orbital eccentricity cycles respectively however the number of marl limestone alternations is not always consistent with the expected number of 20 kyr precessional cycles 5 per 100 kyr cycle the large scale sequences reflect long term third order sea level changes the observed marl limestone alternations are interpreted to have formed through cyclically varying export of carbonate mud from the platform towards the deep shelf the variations being controlled by climatically induced high frequency sea level fluctuations enhanced marl deposition on the deep shelf can be related to sea level fall causing exposure of the shallow platform and reducing the area of carbonate production or by rapid sea level rise maximum flooding leading to partial or total drowning of the platform and or to retrogradation of facies belts enhanced carbonate deposition occurs during transgression when large production areas are created on the platform or during late highstands when progradation is forced and carbonate mud export enhanced depending on the long term trend of sea level change on which the high frequency fluctuations are superimposed one high frequency 20 kyr sea level cycle can thus create one or two marl limestone alternations or only marly deposits consequently one 100 kyr eccentricity cycle may be formed of a variable number of marl limestone alternations commonly 2 to 8 the studied deeper water depositional sequences thus are strongly linked to the history of the adjacent shallow carbonate platforms

caco3 precipitation kinetics in waters from the great bahama bank implications for the relationship between bank hydrochemistry and whitings the source of whitings on the great bahama bank and their relationship to major changes in the chemistry of bank waters have been among the longest and most hotly debated topics in carbonate geochemistry in this paper we demonstrate that the reaction kinetics of calcite with bank waters for a given saturation state are similar to but somewhat slower 2 to 3 times than with gulf stream water the interpretation of the reaction kinetics of suspended bank sediment with bank water requires that the precipitating phase be about twice as soluble as aragonite good agreement at equivalent saturation states was found between experimental precipitation rates and those calculated for the rate of change of bank water chemistry in the region of whitings these results indicate that the dominant mode of carbonate removal is via precipitation on resuspended sediments rather than the rapid pseudo homogeneous precipitation of calcium carbonate in the water column resulting in the formation of a whiting estimates indicate that single aragonite needles may be resuspended many times over a period of decades during which they experience repeated overgrowth a major portion 98 of suspended calcium carbonate is outside the visually dramatic whitings thus as visually spectacular as they are whitings do not represent a short term locally massive precipitation of carbonate on the great bahama bank nor are they even likely to be the dominant sites of carbonate removal in this region although future refinements are needed that include seafloor processes we have at this point arrived at a mechanistic kinetic model that provides a reasonably quantitative explanation for the hydrochemistry of the carbonate system on the northern great bahama bank

the uncertainties involved in extracting amplitude and frequency of orbitally driven sea level fluctuations from shallow water carbonate cycles in outcrops shallow water carbonates often form thick vertically stacked metre scale exposure bounded depositional packages here called carbonate cycles because the facies within carbonate cycles can be tied precisely to water depths they are considered ideal for reconstructing past sea level changes there is however increasing evidence that these depositional cycles are unreliable recorders of both the frequency and the amplitude of orbitally driven sea level fluctuations carbonate cycles record only a fraction of the amplitude of a sea level cycle a portion of the rise and nothing of the fall cores through the pleistocene cycles on great bahama bank illustrate this shortcoming although the amplitudes of the last nine sea level changes are each a hundred metres or more the thickness of the cycles varies from a few metres to ca 15 m the lack of correlation between sea level amplitude and cycle thickness is not eliminated for cycles deposited during times of lower sea level amplitudes for example the cretaceous upper cretaceous cycles on the maiella platform margin document the irregularly filled accommodation space and the resultant variability in cycle thickness and frequency uncertainties in assessing the frequencies of sea level changes from shallow water carbonate cycles are caused by missed beats and metre scale oscillations of sea level within highstands that potentially produce cycles of very short duration the random amplitude variability during the last 57 glacio eustatic sea level changes illustrates the difficulty of assessing missed beats where a sea level fluctuation is not recorded because the sea level rise does not reach the platform top missed beats are also produced by the depositional topography that is created by irregularly filled accommodation space as a result variable numbers of cycles are deposited across the platform further complicating orbital frequency analyses are decametre scale oscillations of the sea level during highstands the amplitude of these sub orbital sea level oscillations up to 17 m within the last interglacial marine isotope stage 5e are sufficient to expose shallow platforms like great bahama bank and subsequently produce an additional depositional cycle with similar facies successions the combined effects of missed beats and oscillations within highstands are likely to produce cycles and hiatuses of variable duration that are difficult to extract from the rock record consequently estimates of the orbital forcing mechanisms and frequencies from ancient shallow water carbonate cycles carry large uncertainties

cyanobacterial precipitation of gypsum calcite and magnesite from natural alkaline lake water results from transmission electron microscopy provide direct evidence for cyanobacterial biomineralization of gypsum and calcite in aquatic environments laboratory simulations using filter sterilized natural lake water inoculated with synechococcus sp isolated from fayette ville green lake new york revealed epicellular biomineralization of gypsum calcite and magnesite experimental electron microscopical and sedimentological evidence indicates that synechococcus is responsible for a major proportion of the marl sediment and carbonate bioherms in green lake the elucidated role of synechococcus in biomineralization and its ubiquitous distribution in nature have widespread implications for cyanobacterial mineralization in marine and freshwater environments since late archean time

the extent and patterns of sediment filling of accommodation space on great bahama bank abstract great bahama bank gbb is the modern example of a flat topped isolated carbonate platform it is a major modern location of carbonate deposition that stands behind much of the understanding of modern processes of carbonate sedimentation serves as a training venue for academia and industry is the basis for numerous geological models and is commonly used as a reservoir analog gbb also provides valuable insight into the extent and patterns of sediment fill of accommodation space atop an isolated carbonate platform satellite imagery landsat tm and etm and an extensive set of water depth measurements n 5 723 were used to map bathymetry across gbb and derive a digital terrain model dtm analyzing the extent patterns and nature of sediment fill of accommodation space was facilitated by partitioning a depositional facies map on the basis of the dtm to show that 18 of accommodation on the gbb is overfilled and filled emergent to 1 5 m water depth 52 is underfilled 1 5 to 6 0 m and 30 is unfilled 1 5 m water depth and therefore the greater part of available accommodation 88 000 km2 remains incompletely filled with sediment areas of filled accommodation mostly extend platformward from the western coastlines of islands which in turn are preferentially distributed along the eastern windward margin of the gbb seventy percent of sediment in the areas of filled accommodation is rudstone high energy grainstone grainstone and mud poor packstone although dominated by grainstones 45 since it occupies so much space 55 000 km2 the underfilled sector also contains the most heterogeneous facies mosaic and by definition therefore the greatest and most complex lateral facies variations although islands are numerous n 1 430 they occupy only 8 or 8 700 km2 of the platform top despite their small proportional occupancy of the platform islands play a direct role in the accumulation of muddy fabrics and exert a sphere of influence over the character of sedimentation for many tens of kilometers from their coastlines further since islands represent the portion of the bank where accommodation has been overfilled their rarity emphasizes the challenges that such a large platform faces in filling accommodation space even given the diverse grain factories producing carbonate sediment mud precipitation through whitings ooids and reefs along the platform margins skeletal sediments and nonskeletal grains such as fecal pellets peloids and pelletoids two factors can be evoked to explain the inability of gbb to fill accommodation space first falling sea level during the pleistocene appears to have repeatedly aborted the filling process by analogy it will not be able to do so in the holocene either a situation exasperated by the second factor which is that most of the holocene gbb particularly areas away from islands presently lack any appreciable filling instead facies analysis suggest reduced sedimentation and enhanced cementation through regional mapping the work delivers a platform wide assessment of how accommodation is filled and the facies responsible for the filling amongst other trends by highlighting how the thickest accumulations of holocene sediment are skewed to the windward margin of the gbb and related to the presence of islands whereas the thinner accumulations which dominate the platform interior and are more grainy but also more heterogeneous than the thicker deposits these data might contain useful guidelines as to how depositional cycles might vary laterally in ancient systems

whiting related sediment export along the middle miocene carbonate ramp of great bahama bank modern aragonite needles are present all along the modern leeward margin of great bahama bank odp leg 166 while middle miocene sediments contain needles only in more distal areas sites 1006 and 1007 in contrast to the rimmed flat topped platform topography during the plio pleistocene the miocene great bahama bank morphology is a carbonate ramp profile this might imply a different location and precipitation type for miocene aragonite needles in this study aragonite needles in miocene sediments were isolated using a granulometric separation method furthermore the isolation of the various carbonate components enables the identification of primary versus diagenetic components the miocene aragonite needles are concentrated in the finest granulometric sediment fractions 12 m the fraction specific geochemical analyses 13c 18o and sr elemental abundance represent useful tools to assess the possible sources of the aragonite mud the geochemical variation of the fractions rich in pristine aragonite needles and the characteristics of the needle morphology point to whiting phenomena as the main sediment source and algal fragmentation as a minor component both components indicate shallow water environments as the main sediment source area ramp top related fine grained particles now present at distal sites were likely exported as suspended material similar to present day transport mechanisms the scarcity of needles at proximal sites is probably linked to hydrodynamic processes but dissolution and recrystallization processes cannot be excluded the granulometric separation approach applied here enables a better characterization of the finest carbonate particles representing an important step towards the discrimination between primary and diagenetic fine grained components

oxygen isotope composition of natural waters in the mt etna area oxygen isotopes of both rainwater and groundwater samples from mt etna were used to obtain information on recharge areas flow paths and the origin of wet air masses oxygen isotope composition was determined in rainwater samples collected for a period of 3 years october 1997 october 2000 in 11 rain gauges distributed along the flanks of mt etna from sea level to 2900 m of altitude values ranged from 213 8 to 1 9 the lowest values being measured at higher altitudes and or during cooler periods for rain gauges located from sea level up to 1000 m altitude volume weighted values defined an isotopic gradient of 22 7 km which is in the range observed in the mediterranean area higher altitude gauges yielded a much lower gradient 2 0 6 km probably due to the fact that vapour condensing at higher altitudes was mixed with an 18 o rich volcanic component deriving from the huge vapour output of the summit craters the oxygen isotope composition of about 210 groundwater samples collected all around the volcano ranged from 2 9 3 to 2 5 0 the higher values measured on the eastern flank indicated that recharge occurs at lower altitudes on this flank the low variability 0 30 0 65 of the monthly values gathered from 14 groundwater sampling points over a period of 2 years indicates that the groundwater system is isotopically well mixed some long term trends may be explained by variations in annual recharge due to the prevalent isotope composition of wet air masses q 2004 elsevier b v all rights reserved

evidence for major environmental perturbation prior to and during the toarcian early jurassic oceanic anoxic event from the lusitanian basin portugal the timing and causal relationships between the pronounced negative c isotope excursion and paleoenvironmental perturbations associated with the toarcian oceanic anoxic event early jurassic remain unclear particularly because biotic crises and carbonate production decrease appear to have been initiated earlier than the main c isotope anomaly here we present a new quantification of late pliensbachian early toarcian calcareous nannofossils abundance and size from the peniche reference section portugal together with o and c isotope records of well preserved brachiopod shells from the same section the brachiopod shell 13c curve parallels that of bulk carbonate and records two pronounced negative isotopic excursions close to the pliensbachian toarcian boundary 2 and during the toarcian oceanic anoxic event 3 5 our results indicate that both c isotope negative excursions were characteristic of benthic and shallow water environments suggesting that these two carbon cycle perturbations affected all epioceanic reservoirs coeval shifts toward lower values of brachiopod oxygen isotope compositions and closely correlated northward migrations of mediterranean ammonite fauna suggest that both events coincided with major rises in seawater temperatures probably as a result of increased co2 levels and enhanced greenhouse conditions co2 induced changes in seawater chemistry likely affected the calcification potential of both neritic and pelagic systems as evidenced by synchronous drops of platform derived carbonate accumulation and drastic reductions in size 3 m of the main pelagic carbonate producer schizosphaerella we suggest that the early toarcian paleoenvironmental crisis occurred in two distinct episodes that were most likely related to two successive phases of intense volcanic degassing in the karoo ferrar province

a paleoproterozoic drowned carbonate platform on the southeastern margin of the wyoming craton a record of the kenorland breakup abstract the nash fork formation in the upper part of the early paleoproterozoic snowy pass supergroup medicine bow mountains of wyoming was deposited on a mature passive margin along the southern flank of the wyoming craton and straddles the end of the ca 2 2 2 1 ga carbon isotope excursion two drowning events marked by black shales subdivide the carbonate platform into three parts the lower nash fork formation consists of outer shelf to supratidal deposits represented by massive and stromatolitic dolomites heterolithic siliciclastics carbonates large silicified domal digitate stromatolites nodular dolomites and stromatolitic dolomites molds after evaporite crystals are pervasive in the heterolithic siliciclastics carbonates large silicified domal digitate stromatolites formed biostromes and bioherms following flooding events the middle nash fork formation comprises two intervals of black shale separated by inner shelf heterolithic siliciclastics nodular carbonates black shales are organic and pyrite rich contain turbidites and developed in response to drowning of the platform overlying massive dolomite of the upper nash fork formation was deposited in an unprotected intertidal setting and displays an upward shallowing trend terminated by a prominent karstic surface in the middle of the unit the nash fork formation is open marine with no evidence for restricted circulation on the carbonate platform the two drowning events on the carbonate platform are likely related to dissection of the mature passive margin associated with the breakup of kenorland the younger drowning event is associated with the end of the carbon isotope excursion the main building elements of the lower and middle nash fork carbonate platform are dolomitic mudstones and stromatolites macroscopic seafloor precipitates are volumetrically negligible with the exception of tufa deposits and domes in the massive and stromatolitic dolomites and possibly digitate stromatolites within domal digitate stromatolites the upper nash fork formation comprises dolomitic mudstones relatively rare stromatolites and inorganic precipitates that are more common than in the underlying carbonates styles of carbonate deposition on this early paleoproterozoic platform differ from those documented on late archean carbonate platforms there are fewer macroscopic seafloor precipitates and more dolomitic mudstones this pattern is considered to be related to a rise of the atmospheric oxygen level that led to a decrease in bicarbonate saturation in the ocean

a comparative study of modern carbonate mud in reefs and carbonate platforms mostly biogenic some precipitated abstract carbonate mud from reefs and carbonate platforms in six locations of the atlantic indian and pacific oceans belize bahamas florida the maldives french polynesia great barrier reef was systematically and quantitatively analyzed with regard to texture composition mineralogy and geochemistry mud composition shows considerable variability however the data supports the contention that these muds are largely derived from the breakdown of skeletal grains and codiacean algae only mud from the bahamas and northern belize areas which are characterized by common whitings is interpreted to be mainly inorganically precipitated three grain size fractions 63 20 m 20 4 m

paleoenvironmental reconstruction of a saline lake in the tertiary evidence from aragonite laminae in the northern tibet plateau abstract the origin of aragonite has long been debated because it is precipitated and preserved under specific conditions aragonite laminae first found from eocene to miocene strata in the western qaidam basin northern tibet plateau contain much information on paleolake signatures mineralogical and geochemical analyses were conducted on alternating yellowish and grayish aragonite layers the yellowish layers are mainly composed of aragonite crystals while the grayish layers contain less aragonite and fewer organic remnants that accumulate among debris with sporadic framboidal pyrite the 13c values of yellowish layers are remarkably positive by approximately 4 01 vpdb and the 18o values are slightly negative compared with base data of the qaidam basin considering the 12co2 absorption of algal blooms positive excursions of 13c shown in aragonite indicate high 13c values in depositional water therefore a seasonal algal influenced inorganic origin is proposed to explain the formation of aragonite laminae during warm seasons mg ca ratios are elevated because of evaporation effects the algal blooms decrease the co2 content leading to high ph values these conditions promote the rapid crystal growth of aragonite instead of other carbonate minerals slightly negative 18o values in yellowish layers are interpreted as the result of intense inflow during warm seasons which leads to less precipitation of organic matter and debris the grayish layers in cold seasons are the opposite from the eocene to oligocene the progressively decreasing 18o values of aragonite reflect global cooling during this time a conspicuously positive step in 18o values indicates an arid environment coinciding with the uplift of the himalaya from the oligocene to lower miocene the results from this study show that understanding of aragonite in the qaidam basin is essential to reconstruct the high resolution paleoenvironment and to reveal the tertiary evolution of paleoclimates in the northern tibet plateau

very high energy sedimentation supratidal hurricane deposits and mid holocene highstand on carbonate platforms andros bahamas an alternative view abstract twenty eight 14 c dates were obtained from 90 cores drilled through the tidal flats of western andros island a large area 2000 km 2 mostly covered by mangroves and stromatolitic deposits these dates are correlated with location of the samples sedimentological description position of the samples at the time of deposition and their present day sedimentological position measurements were made on bulk algal sediments and location of the relative paleo sea level is inferred from paleobathymetric reconstructions on the basis of 14 c dates i conclude that the whole area was flooded about 5000 years bp and then was covered by the subtidal greyish mud facies rich in peneroplids in the area of deep creek north of the study area pelican creek and wide opening southeast of the study area the level of this paleo great bahama bank sea continued to rise until 3500 years bp and then slowly fell asymptotically toward the present day level thick hurricane marks aragonitic lime mud with didemnids spicules several cm thick previously defined as hurricane trails can be observed throughout the cores from 2000 years bp onwards from 2000 years bp onwards it seems that the modern physiography was already established including tidal estuaries channels and stromatolite bearing swamps paleogeographic maps and sections have been drawn and accumulation rates calculated in the various sedimentological zones of the tidal flats of western andros these vary between 0 5 to 0 04 mm yr the modern topography of the tidal flats was established by 2000 years bp similarly the modern geological features controlled by a tropical climate strictly dependent on hurricanes were already established by 2000 years bp and seem to have been initiated between 2500 and 2000 years bp

accumulation of bank top sediment on the western slope of great bahama bank rapid progradation of a carbonate megabank high resolution seismic profiles and submersible observations along the leeward slope of western great bahama bank show large scale export of bank top sediment and rapid progradation of the slope during the holocene a wedge shaped sequence up to 90 m thick is present along most of the slope and consists of predominantly aragonite mud derived from the bank since flooding of the platform 6 8 ka total sediment volume of the slope sequence is 40 80 that of holocene sediment currently retained on the bank maximum rates of vertical accumulation and lateral progradation are 11 15 m ka and 80 110 m ka respectively 10 to 100 times greater than previously known for periplatform muds slope deposition of exported mud during sea level highs appears to have been a major mechanism for the westward progradation of great bahama bank throughout the quaternary this may provide a critical modern analogue for ancient progradational margins

high production and highstand shedding from deeply submerged carbonate banks northern nicaragua rise abstract because the shallow isolated carbonate banks on the northern nicaragua rise the nicaragua honduras and southern jamaica carbonate shelves and many other modern carbonate banks worldwide are covered by an average of 20 to 30 m of water and by a thin blanket of coarse carbonate sediments other carbonate sedimentologists have considered these banks good examples of incipiently drowned or even drowned carbonate banks however based on recent research on the northern nicaragua rise we can demonstrate that these banks currently are healthy producers of large volumes of periplatform sediments fine aragonite and magnesian calcite which are exported almost entirely to the deep surrounding slopes these sediments which were deposited during the past 9000 10 000 years form peripl tform wedges on the middle and upper slopes which are seen clearly on 3 5 khz seismic profiles radiocarbon ages of the wedge surface sediment range between 230 and 610 ybp and provide clear evidence for the contemporaneous production of sediments on the shallow bank and shelf and their instantaneous export to the upper slopes for the last 5000 years sedimentation rates ranged from 2000 mm ky off pedro bank to 1300 mm ky off jamaica these rates are somewhat lower than but of the same order of magnitude as sedimentation rates on the western leeward slopes of great bahama bank deposition of the metastable carbonate sediments discussed in this paper occurred as soon as sea level rose to flood the bank and shelf which flank walton basin following the last glacial lowstand they com rise a wedge shaped package of transgressive and highstand sediments on the upper and middle slopes of pedro bank and the southern jamaica shelf

shear wave splitting study of crustal anisotropy in the new madrid seismic zone abstract this study investigates crustal anisotropy in the new madrid seismic zone nmsz by analyzing shear wave splitting measurements from local earthquake data in addition to the waveforms provided by the center for earthquake research and information ceri for over 3000 events seismograms recorded by the portable array for numerical data acquisition panda network were obtained for over 800 events data reduction led to a final data set of 168 and 43 events from the ceri and panda data respectively one hundred and eighty six pairs of measurements were produced from the ceri data set by means of the automated shear wave splitting measurement program mfast and 49 from the panda data set two dominant directions respectively striking northeast southwest and west northwest east southeast are identified and interpreted to be due to stress aligned microcracks the northeast southwest polarization direction is consistent with the maximum horizontal stress orientation of the region and has previously been observed in the nmsz whereas the west northwest east southeast polarization direction has not path normalized time delays range from 1 to 33 ms km for the ceri network data and 2 to 31 ms km for the panda data these results produce a range of estimated differential shear wave anisotropy between 1 and 8 these values are higher than those previously determined in the region the majority of large path normalized time delays 20 ms km are located along the reelfoot fault segment these high values are believed to be indicative of high crack densities and high pore fluid pressures which agrees with previous results from local earthquake tomography and microseismic swarm analysis online material tables of stations events and associated shear wave splitting measurements

benthic carbonate factories of the phanerozoic marine carbonate precipitation occurs in three basic modes abiotic or quasi abiotic biotically induced and biotically controlled on a geologic scale these precipitation modes combine to form three carbonate production systems or factories in the benthic environment 1 tropical shallow water factory dominated by biotically controlled mainly photo autotrophic and abiotic precipitates 2 cool water factory dominated by biotically controlled mainly heterotrophic precipitates and 3 mud mound factory dominated by biotically induced mainly microbial and abiotic precipitates sediment accumulations of the factories differ in composition geometry and facies patterns and some of these differences appear prominently in seismic data thus facilitating subsurface prediction the characteristic accumulation of the tropical factory is the flat topped often reef rimmed platform in cool water systems reefs in high energy settings are scarce and hydrodynamic influence dominates producing seaward sloping shelves and deep water sediment drifts often armored by skeletal framework the typical accumulation of the mud mound factory is groups of mounds in deeper water where the mud mound factory expands into shallow water it forms rimmed platforms similar to the tropical factory the tropical factory is most productive the mud mound factory reaches 80 90 and the cool water factory 20 30 of the tropical growth rate the three factories represent end members connected by transitions in space transitions in time are linked to biotic evolution

biochemical and ultrastructural evidence for the origin of whitings a biologically induced calcium carbonate precipitation mechanism we propose that bahamian whitings floating patches of lime mud are in part the result of biological precipitation of calcium carbonate induced by picoplankton and cellular components the cells may act as nucleation sites for crystallization from seawater supersaturated with caco3 our model represents a new hypothesis concerning bahamian whiting formation and is based on several lines of evidence biochemical data suggest that the macromolecules from carbonate suspended in bahama banks whitings are distinct from those found in the lime mud sediment producer penicillus and from bottom sediment direct ultrastructural evidence indicates that mineralization occurs on the surfaces of picoplankton cells and degrading organic cellular components the organic constituents include but are not restricted to whole and fragmented algal cells and composite structures 20 30 m in diameter the observed calcium carbonate crystals are distinct from skeletal debris because of the magnitude of the whiting phenomenon calculations of carbonate sediment production budgets should include the potential contribution of calcium carbonate produced by epicefular precipitation knowledge of carbonate mud genesis is critical to interpretation of ancient occurrences of lime mud as well as to understanding ancient and global carbon cycles

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

accumulation and intense bioturbation of bioclastic muds along a carbonate platform margin dry tortugas florida abstract a sedimentological study was undertaken in the dry tortugas lowermost florida keys to examine processes influencing the accumulation and sedimentary fabric characteristics of carbonate muds along the shallow margin of a carbonate platform sedimentological studies physical properties 234th and 210pb geochronology were coordinated with observations of benthic biology and benthic boundary layer dynamics sampling was concentrated around a mud depocenter in southeast channel a reentrant 20 30 m deep in the southern edge of the dry tortugas platform in the depocenter of southeast channel accumulation rates of 0 10 0 32 cm y 1 are estimated from 210pb profiles likely sources of sediment are in shallower water and include reefs and beds of calcareous green algae fine sediment silt and clay sized is advected to the seabed of southeast channel by a combination of tidal and storm driven currents flows from shallow water decelerate upon entering deeper water in southeast channel producing conditions favorable to long term accumulation once deposited sediment in southeast channel is subjected to bioturbation by a two tiered benthic community the surface zone 0 5 cm is mixed biodiffusively by a surface community of small bivalves and polychaetes db 9 cm2 yr 1 over periods of weeks to months and is characterized by mm scale mottling other near surface structures include biodeposited fecal mounds of deep deposit feeders and ephemeral physical stratification as thin 1 5 cm layers of fine sediment below 5 cm the dominant post depositional process is bioadvective mixing by a deep deposit feeding community of callianassid shrimp and notomastid polychaetes the upper 10 15 cm is mixed completely every 6 21 years and bioturbation below this depth is slower even though the study area has been hit by 10 major hurricanes in the last 120 years winds 190 km h 1 sediments preserved i e below 5 cm depth in southeast channel during this period reveal wholly biogenic sedimentary fabric produced by the deep callianassid notomastid community bioturbation is thus capable of erasing the stratigraphic signature of the most extreme sediment transport events over decadal time scales

factors controlling the rate of caco3 precipitation on great bahama bank measurements by langdon et al 2000 in the man made mesocosm coral reef at biosphere 2 s ocean reveal a strong dependence of calcification rate on the degree of supersaturation of caco 3 in seawater a similar trend was previously encountered on the bahama banks where halimeda and other calcifiers are likely responsible for aragonite precipitation broecker and takahashi 1966 in this paper we compare these two sets of results and conclude that the dependence on saturation state is significant but less strong in the bahamas however it must be kept in mind that to some extent the reduction in caco 3 precipitation on the bahama banks may be due to impact of higher salinity on the growth of the calcifying algae however if as many sedimentologists are convinced the precipitation of caco 3 on the bahama banks is inorganic macintyre and reid 1992 milliman et al 1993 then the comparison of the bahamas and biosphere 2 results for dependence of calcification rate on saturation state is telling us something quite different

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

investigating controls on boron isotope ratios in shallow marine carbonates abstract the boron isotope ph proxy has been widely used to reconstruct past ocean ph values in both planktic foraminifera and corals species specific calibrations are required in order to reconstruct absolute values of ph due to the prevalence of so called vital effects physiological modification of the primary environmental signals by the calcifying organisms shallow marine abiotic carbonate e g ooids and cements could conceivably avoid any such calibration requirement and therefore provide a potentially useful archive for reconstructions in deep pre cenozoic time however shallow marine abiotic carbonates could also be affected by local shifts in ph caused by microbial photosynthesis and respiration something that has up to now not been fully tested in this study we present boron isotope measurements from shallow modern marine carbonates from the bahama bank and belize to investigate the potential of using shallow water carbonates as ph archives and to explore the role of microbial processes in driving nominally abiogenic carbonate deposition for bahama bank samples our boron based ph estimates derived from a range of carbonate types i e ooids peloids hardground cements carbonate mud stromatolitic micrite and calcified filament micrite are higher than the estimated modern mean annual seawater ph values for this region furthermore the majority 73 of our marine carbonate based ph estimates fall out of the range of the estimated pre industrial seawater ph values for this region in shallow sediment cores we did not observe a correlation between measured pore water ph and boron derived ph estimates suggesting boron isotope variability is a depositional rather than early diagenetic signal for belize reef cements conversely the ph estimates are lower than likely in situ seawater ph at the time of cement formation this study indicates the potential for complications when using shallow marine non skeletal carbonates as marine ph archives in addition variability in 11b based ph estimates provides additional support for the idea that photosynthetic co2 uptake plays a significant role in driving carbonate precipitation in a wide range of shallow water carbonates