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

erosion rates on mars and implications for climate change constraints from the pathfinder landing site the observation that the mars pathfinder landing site looks very similar to its appearance after it was deposited by catastrophic floods around 1 8 3 5 ga allows quantitative constraints to be placed on the rate of change of the site since that time the abundance of erosional features such as an exhumed former soil horizon sculpted wind tails ripplelike and other tag deposits and ventifacts fluted and grooved rocks all suggest the site has undergone net deflation or loss of 3 7 cm of material the presence of barchan dunes and ventifacts argues for erosion by saltating crystalline sand size particles entrained in the wind most ventifacts probably formed soon after the catastrophic flood which likely introduced a large fresh supply of sand size particles distributed across the rocky plain the strongest winds blew toward the northwest during this time resulting in the sculpting of ventifacts deflation of the surface collections of dunes within big crater and other lows and possibly preferentially eroding small crater rims the predominant wind direction changed to blow toward the southwest similar to today these winds further deflated the surface completed the deposition of sand size material in dunes and ultimately trapped these dunes in lows the erosional features observed by pathfinder indicate extremely low long term deflation rates of 0 01 0 04 nm yr since the end of the hesperian 1 8 3 5 ga similar to less precise rates of 1 nm yr based on the preservation of craters at the viking 1 and pathfinder landing sites short term redistribution rates deposition and removal of atmospheric dust at the pathfinder landing site and preexisting dust and sand at other locations on mars are up to 10 5 nm yr estimates of erosion rates on mars show a rapid decrease by 3 6 orders of magnitude from 10 2 10 4 nm yr in noachian terrains characterized by rimless flat floored craters and valley networks to the exceedingly slow rates 10 2 10 nm yr operating during the hesperian and amazonian noachian erosion rates comparable to low continental denudation rates on earth are consistent with erosion by running water and perhaps a more clement climate the rapid decrease in erosion rates is consistent with a major climatic change during the noachian at the tail end of heavy bombardment and a cold dry desiccating climate similar to today s since that time

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

coupling between 210pbex and organic matter in sediments of a nutrient enriched lake an example from lake chenghai china abstract sediment cores were collected from deep water areas of lake chenghai china in june 1997 the vertical profile of 137cs activity gives reliable geochronological results the results also indicate that sediment accumulation rates in deep water areas of lake chenghai were relatively constant in recent decades averaging 0 43 g cm 2 y 1 despite a variable organic carbon influx 210pbeq 226ra activity was relatively constant also with an average value of 54 3 3 2 bq kg 1 vertical profiles of 210pbex 210pbtotal 226ra decreased exponentially resulting in somewhat lower sediment accumulation rates 0 3 g cm 2 y 1 these lower rates are likely less reliable as the relatively large fluctuations in 210pbex activities correlate closely to the organic carbon corg content of the sediments for example the vertical profile of 210pbex activity displays peaks at mass depths of 3 7 4 7 g cm 2 10 12 cm and 10 11 g cm 2 25 28 cm similar to the maxima in the vertical profile of corg this phenomenon must be related to the delivery of particulate organic matter pom from the water to the sediments or to watershed soil erosion since the mean atomic ratios of horg corg and corg norg in lake chenghai sediments are 5 5 and 7 0 respectively indicating that pom was predominantly derived from the remains of authigenic algae this eliminates watershed erosion rates as a primary control on lake sedimentation rates as resolved by 210pbex sedimentation fluxes f corg of particulate organic carbon since 1970 varied between 60 to 160 g m 2 y 1 and appeared to closely influence variations in 210pbex concentrations for example sedimentation fluxes of 210pbex f 210pbex showed maxima in the years 1972 1974 and 1986 1989 likely reflecting historical variations of lake biological productivity or carbon preservation

mapping bathymetry and depositional facies on great bahama bank satellite imagery and an extensive set of water depth measurements have been used to map and critically evaluate the magnitude and patterns of bathymetry across great bahama bank descriptions of previously collected sediment samples were combined with satellite imagery to map and refine the interpreted distribution of surficial carbonate sediments depositional facies data reveal that 60 of great bahama bank lies in 5 m or less of water the deep portion occurs mainly in a generally east west trending area in the southern portion of the platform the re evaluation of the facies reveals that great bahama bank is essentially a very grainy platform with muddier accumulations primarily in the lee of andros island this area of great bahama bank also experiences currents related to an excursion of the florida current onto the platform top possibly enhancing sediment mud production through the generation of whitings sediment equivalents to mudstones wackestones and mud rich packstones cover 8 5 and 14 respectively of the platform top whereas sediment equivalents to mud poor packstones grainstones and rudstones account for 20 45 and 3 of the surface area boundstones reefs were not specifically mapped in this study due to the resolution of the mapping there is a poor relationship between the occurrence of the depositional texture and water depth in that the grainier sediment types are abundant across the full range of water depths the most abrupt lateral facies changes portrayed on the facies maps are observed leeward of islands areas which also hold the highest diversity in facies type the majority of the islands on the platform align with the north west south east strike of the platform margin and these islands in turn exert control on the shape and orientation of facies belts that develop in proximity to them for this reason regions of the platform that contain principal islands host facies belts that align with the principal axis of the platform whereas for regions lacking islands the facies belts adopt an east west trend consistent with prevailing winds and currents there is a clear trend that the wide southern portion of the platform hosts the most continuous expanses of grain rich sediments

microbially mediated carbonates in the holocene deposits from sarlieve a small ancient lake of the french massif central testify to the evolution of a restricted environment both the mineralogy and facies of lacustrine bio induced carbonates are controlled largely by hydrological factors that are highly dependent upon climatic influence as such they are useful tools in characterizing ancient lake environments in this way the study of the sedimentary record from the small ancient sarlieve lake limagne massif central france aims to reconstruct the hydrological evolution during the holocene using petrographical mineralogical and geochemical analyses the fine grained marls mainly calcitic display numerous layers rich in pristine ca dolomite with small amounts of aragonite which are clearly autochthonous as these minerals are rather unusual in the temperate climatic context of western europe the question arises about their forming conditions and therefore that of the lacustrine environment ca dolomite prevails at the base of the sequence as a massive dolomicrite layer and in the middle part it builds up most of the numerous laminae closely associated with organic matter scanning electron microscope observations reveal the abundance of tiny crystals tens to hundreds of nanometres mainly organized as microspheres looking like cocci or bacilli such a facies is interpreted as resulting from the fossilization of benthic microbial communities by dolomite precipitation following organic matter consumption and extracellular polymeric substance degradation these microbial dolomites were precipitated in a saline environment as a consequence of excess evaporation from the system as is also suggested by their positive 18o values the facies sequence expresses the following evolution i saline pan i e endorheic stage with a perennial lowstand in lake level boreal to early atlantic periods ii large fluctuations in lake level with sporadic freshening of the system atlantic iii open lake stage sub boreal and iv anthropogenic drainage sub atlantic

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

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

microbes and ooids abstract the bahamian archipelago is one of the few locations in the world where ooid formation is actively occurring ooid cortices from six locations in the region were incrementally dissolved and analyzed for 14c 18o 13c mg ca and sr ca ratios ooids were examined under sem after each step in the incremental analyses to characterize the nature of dissolution radiocarbon dating indicates that ooids presently at the sediment water interface began forming across the archipelago between 1000 and 2800 yr bp and continue to form today the ooids have the same pattern of microboring alteration across the region the surface and outer cortex of the ooids are punctuated with unfilled microborings whereas the inner cortex contains two morphologies of aragonite cement filling the microborings the two morphologies of cement form in association with two different species of cyanobacteria one is solentia sp the other is interpreted to be hyella sp the chemistry of ooids from across the region is remarkably similar 18o and 13c values for all samples vary directly having a slope of approximately 1 the outer cortex has low 18o and 13c values of 3 4 and 0 2 respectively whereas the 18o and 13c values of the inner cortex are high with values of 1 9 and 6 8 respectively the presence of aragonite cement in microborings in the inner cortex increases the overall isotopic composition of both oxygen and carbon in the ooid causing it to appear close to equilibrium with seawater the isotopic variation in 18o and 13c within the cortex can be characterized as a mixing line between the low values in the unaltered ooid laminae and the aragonite cement in the microborings the most exterior portion of the ooid has very high mg ca values and is interpreted as an amorphous calcium carbonate acc coating there are two other phases in the cortex both being aragonite the outer cortex has a higher mg ca ratio and lower sr ca ratio than the inner cortex this difference in chemistry is a result of the presence of aragonite cement in the inner cortex stable isotopic and trace element results coupled with sem investigations indicate that microbes do not play a role in ooid formation but instead alter the texture and chemistry of ooids after they have formed a new model of ooid formation is proposed whereby a veneer of acc precipitates on an ooid while it is at the sediment water interface the active phase this veneer of acc later recrystallizes to aragonite needles and a new cortex layer is formed the stationary phase observations from this study lead to a deeper understanding of the chemical processes involved in ooid genesis which allow a better understanding of paleoenvironments hosting ooid formation

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

evidence for recent volcanism on mars from crater counts impact craters help characterize the age of a planetary surface because they accumulate with time they also provide useful constraints on the importance of surface erosion as such processes will preferentially remove the smaller craters earlier studies of martian crater populations revealed that erosion and dust deposition are important processes on mars they disagreed however on the age of the youngest volcanism these earlier studies were limited by image resolution to craters larger than a few hundred metres in diameter here we report an analysis using new images obtained by the mars global surveyor spacecraft of crater populations that extend the size distribution down to about 16 m our results indicate a wide range of surface ages with one region lava flows within the arsia mons caldera that we estimate to be no older than 40 100 million years we suggest that volcanism is a continuing process on mars

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

wind and climate on mars several apparently conflicting inferences in matthew p golombek s perspective on martian climate 1 1 can be reconciled by recognizing that surface sediment features associated with rocks at the pathfinder landing site indicate the direction of the strongest winds only in the present climate

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

upper cambrian carbonate platform evolution elvinia and taenicephalus zones pterocephaliid ptychaspid biomere boundary northwestern wyoming abstract the pterocephaliid ptychaspid biomere boundary is an upper cambrian extinction horizon that can be used to correlate mixed carbonate siliciclastic strata in northwestern wyoming the biomere boundary is defined by the change between the elvinia and taenicephalus zones of the standard north american trilobite zonation it is also signaled chemostratigraphically by a positive trend in carbon isotope ratios thus both biostratigraphic horizons and carbon isotope data are used to constrain the position of the biomere boundary in northwestern wyoming study of 21 stratigraphic sections reveals the presence of distinct facies belts for the middle and upper elvinia zone and lower taenicephalus zone these are interpreted to reflect deposition in three distinct paleoenvironments 1 oolitic shoal 2 intrashelf basin and 3 inner platform margin following a major hiatus in sedimentation in wyoming during much of pterocephaliid biomere time a healthy shallow water carbonate factory was reestablished during elvinia zone time this produced abundant deposits of oolitic and skeletal packstone in the shoal environment and an areally extensive thrombolite boundstone facies in the intrashelf basin setting approximately coincident with the biomere boundary extinction event irvingella major time ooid production was terminated and the thrombolites were drowned the correlation of this major change in the pattern of sedimentation allows for the recognition of parts of two depositional sequences in the study area this drowning event sequence boundary may also be coeval with the accumulation of transported shallow water carbonate deposits in an inner platform margin environment during early taenicephalus zone time sedimentation rates were apparently reduced and the entire study area is marked by thin concentrations of trilobite and brachiopod shells the nonskeletal carbonate factory was never reestablished in the former shoal environment where thick successions of homogeneous lime mud accumulated during the rest of taenicephalus zone time flat pebble conglomerate and shale accumulated in the storm influenced intrashelf basin during this time sea level change and changes in seawater chemistry likely both played roles in the drowning event across the elvinia taenicephalus boundary a positive trend in carbon isotope values across the pterocephaliid ptychaspid biomere boundary is observed in the six sections analyzed this is consistent with previous studies that indicated that the mass extinction occurs within a regional shift towards positive values and this may reflect the development of low oxygen conditions on the shelf the global significance of these isotopic trends changes in relative sea level and the mass extinction has not been confirmed although recent study of the extinction event at the base of the pterocephaliid biomere has revealed a similar trend towards positive carbon isotope values that is worldwide

defining the 3d geometry of thin shale units in the sleipner reservoir using seismic attributes acknowledgments the seismic interpretation and image processing was carried out in the seislab facility at the university of aberdeen sponsored by bg bp and chevron seismic imaging analysis was performed using geoteric ffa and analysis of seismic amplitudes was performed in petrel 2015 schlumberger we would like to thank the nddc rg11766 10 for funding this research and statoil for the release of the sleipner field seismic dataset utilized in this research paper and also anne kari furre and her colleagues for their assistance we also thank the editor alejandro escalona and the two anonymous reviewers for their constructive and in depth comments that improved the paper

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

carbonate and organic matter sedimentation and isotopic signatures in lake chungara chilean altiplano during the last 12 3 kyr abstract sediments in lakes in the andean volcanic setting are often made up of diatomaceous ooze together with volcaniclastics and small amounts of carbonates despite their scarcity carbonates along with organic matter provide significant paleoenvironmental information about lake systems this study focuses on the carbonates in lake chungara their morphologies distribution and origin deduced from the isotopic markers these markers reflected changes in the water and the biomass between the onset of the holocene and around 9 6 cal kyr bp these changes are marked by general increases in toc tn and tn 15nair and by fluctuating values of toc 13cvpdb in its sediments and are probably related to major shifts in the lake surface volume associated with rises in lake level an increase in salinity around 10 cal kyr bp is thought to be linked to a short dry period giving rise to the onset of carbonate production the mid holocene arid period between 7 3 and 3 5 cal ka bp with a maximum of aridity around 6 0 cal kyr bp was deduced from 18ovpdb values in the endogenic carbonates these results match the reconstructions in lake titicaca based on benthic diatoms and paleoshore levels offshore sediments mainly consist of a diatomaceous ooze laminated in the lower half unit 1 and banded massive with tephra layers in the upper half of the sequence unit 2 toc 13cvpdb and the c n ratio confirm that phytoplankton was the main source of organic matter in these sediments shallower sediments units 3 to 5 developed in platform and littoral settings providing evidence of subaqueous macrophytes and to a lesser extent land plants carbonate content ranges between 0 1 and 6 wt in offshore settings 30 to 40 m water depth and reaches the maximum values in the lower part of unit 2 carbonate minerals low magnesium calcite and minor amounts of high magnesium calcite and aragonite are scarce and are arranged in mm thick layers commonly forming cm thick levels or bioclasts carbonate layers are made up of euhedral to subhedral spindle shaped calcite crystals and to a lesser extent aragonite needles all in the m range aragonite spheroids coexist in littoral sediments with other carbonate shapes and charophyte remains where carbonate reaches locally up to 20 wt co2 photosynthetic depletions related to seasonal phytoplankton blooms were responsible for the high frequency deposition of mm thick carbonate layers the average values for 13cvpdb in lake water plankton and sediments of lake chungara as organic matter or as carbonate are around 15 higher than commonly reported values in other lakes this 13c enrichment is attributed to carbon assimilation from a dic affected by methanogenesis in which hco3 is the dominant species the 13cvpdb and 18ovpdb variations and their covariation in endogenic carbonates suggest that lake water volume and lake level increased along the holocene

changes in the pelagic fine fraction carbonate sedimentation during the cretaceous paleocene transition contribution of the separation technique to the study of bidart section abstract the bidart section southwestern france displays an almost complete late maastrichtian early danian transition as the cretaceous paleocene k p crisis mainly affects carbonate producers the aim of this study is to precisely evaluate the evolution of pelagic sediments composition across the boundary a new protocol for separation of fine carbonates into discrete size fractions was used 1 to precisely evaluate in weight of carbonate and not only number of particles per gram of rock the contribution of each carbonate component type foraminifera calcareous nannofossils and nonbiogenic carbonates calcite microparticles and calcareous macrocrystals and 2 to measure their isotopic signature and thus constrain the origin of these carbonate particles the maastrichtian pelagic carbonates are mostly biogenic 70 of the bulk carbonate but also contain nonbiogenic carbonates calcite microparticles 4 m due to their isotopic signature calcite microparticles cannot result from fragmentation and partial dissolution of biogenic remains and the most likely origin appears to be early diagenetic processes and or export from shallow water platforms the calcareous macrocrystals derive from an early diagenetic process the lowermost danian sediments are dominated by nonbiogenic carbonates between 50 and 80 of the bulk carbonate nevertheless two kinds of biogenic components are present thoracosphaera operculata and maastrichtian like calcareous nannofossils thoracosphaera operculata is an opportunistic calcareous dinoflagellate which frequently occurs at high abundances in basal danian sediments and is certainly in situ this is confirmed by it showing a danian isotopic signature the maastrichtian like calcareous nannofossils are nannofossils of taxa that indisputably occur in the maastrichtian but which do not occur consistently in later danian strata they show a maastrichtian type isotopic signature which strongly supports the inference that they are mainly reworked from the maastrichtian this study shows that nonbiogenic carbonates are important components of the pelagic carbonate even during periods of normal sedimentation in the crisis interval the biogenic carbonate production drops sharply while calcite microparticles and carbonate macrocrystals remain almost unaffected and represent the major part of the carbonate in the earliest danian sediments finally the sediments return to a composition similar to that of the maastrichtian in m pseudobulloides foraminifera zone

implications for neoarchaean ocean chemistry from primary carbonate mineralogy of the campbellrand malmani platform south africa the precipitation of calcite and aragonite as encrustations directly on the seafloor was an important platform building process during deposition of the 2560 2520 ma campbellrand malmani carbonate platform south africa aragonite fans and fibrous coatings are common in unrestricted shallow subtidal to intertidal facies they are also present in restricted facies but are absent from deep subtidal facies decimetre thick fibrous calcite encrustations are present to abundant in all depositional environments except the deepest slope and basinal facies the proportion of the rock composed of carbonate that precipitated as encrustations or in primary voids ranges from 0 to 65 depending on the facies subtidal facies commonly contain 20 35 in situ precipitated carbonate demonstrating that neoarchaean sea water was supersaturated with respect to aragonite carbonate crystal growth rates were rapid compared with sediment influx rates and the dynamics of carbonate precipitation were different from those in younger carbonate platforms the abundance of aragonite pseudomorphs suggests that sea water ph was neutral to alkaline whereas the paucity of micrite suggests the presence of inhibitors to calcite and aragonite nucleation in the mixed zone of the oceans

discriminating stromatolite formation modes using rare earth element geochemistry trapping and binding versus in situ precipitation of stromatolites from the neoproterozoic bitter springs formation northern territory australia stromatolites consist primarily of trapped and bound ambient sediment and or authigenic mineral precipitates but discrimination of the two constituents is difficult where stromatolites have a fine texture we used laser ablation inductively coupled plasma mass spectrometry to measure trace element rare earth element ree y and th concentrations in both stromatolites domical and branched and closely associated particulate carbonate sediment in interspaces spaces between columns or branches from bioherms within the neoproterozoic bitter springs formation central australia our high resolution sampling allows discrimination of shale normalised ree patterns between carbonate in stromatolites and immediately adjacent fine grained ambient particulate carbonate sediment from interspaces whereas all samples show similar negative la and ce anomalies positive gd anomalies and chondritic y ho ratios the stromatolites and non stromatolite sediment are distinguishable on the basis of consistently elevated light rees lrees in the stromatolitic laminae and relatively depleted lrees in the particulate sediment samples additionally concentrations of the lithophile element th are higher in ambient sediment samples than in stromatolites consistent with accumulation of some fine siliciclastic detrital material in the ambient sediment but a near absence in the stromatolites these findings are consistent with the stromatolites consisting dominantly of in situ carbonate precipitates rather than trapped and bound ambient sediment hence high resolution trace element ree y th geochemistry can discriminate fine grained carbonates in these stromatolites from coeval non stromatolitic carbonate sediment and demonstrates that the sampled stromatolites formed primarily from in situ precipitation presumably within microbial mats biofilms rather than by trapping and binding of ambient sediment identification of the source of fine carbonate in stromatolites is significant because if it is not too heavily contaminated by trapped ambient sediment it may contain geochemical biosignatures and or direct evidence of the local water chemistry in which the precipitates formed

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