factors controlling the burial of organic carbon in laminated and bioturbated sediments off nw mexico implications for hydrocarbon preservation factors controlling the burial of organic carbon oc in late quaternary sediments on the nw mexican continental margin are assessed using a suite of box and piston cores strategically located on the shelf slope rise with respect to the intense oxygen minimum in this region an oc concentration maximum occurs on the mid slope below the core of an intense water column o2 minimum due to current winnowing on the outer shelf the preferential accumulation of organic matter in fine grained deposits and the offshore decrease in the settling flux of organic detritus the organic matter at all water depths is overwhelmingly marine hydrogen indices hi are higher on the slope 300 mg hc g toc than on the shelf 300 mg hc g toc where current winnowing has promoted organic matter degradation but there is no difference in hi in slope sediments accumulating under well oxygenated and o2 deficient conditions the degree of winnowing appears to be the primary factor affecting the preservational quality of organic matter deposited on this margin rates of accumulation of oc and opal are all higher in the interglacial intervals when compared with the glacial deposits over the last 140 000 yr however matrix corrected hi values in the mid and lower slope cores are invariant and are similar to values in the laminated intervals from the oxygen minimum site thus cyclic changes in organic carbon accumulation on this margin have been controlled by production variations rather than differential preservation hi values in late quaternary sediments from several continental margins including nw mexico and euxinic basins correspond to type ii kerogen irrespective of bottom water o2 concentrations therefore the preservation of oil prone kerogen in productive margin settings does not appear to be restricted to sediments deposited under conditions of low bottom water o2 concentrations as envisioned in models of petroleum source rock deposition

dissolution of biogenic carbonates effects of skeletal structure abstract dissolution of biogenic shallow water carbonates exposed on deep sea moorings indicates that skeletal structure is important for the rate of disintegration of biogenic carbonates besides mineralogy and grain size of particles the aragonites and high mg calcites used represent a wide spectrum of mineralogies and types of skeletal framework the particles were deployed at different water depths on a mooring in the drake passage for 52 days weight loss curves for the various types of particles show the relative importance of the different structural factors for the disintegration of these biogenic carbonates organic coatings intraskeletal pore spaces and sizes and shapes of individual crystallites in the skeletons may be more important than carbonate mineralogy and particle size in cases the presence of internal sediments cement aggregates and natural contaminations and of diatoms incorporated during growth into carbonate skeletons strongly influence the disintegration of the skeletal materials the first step of particle disintegration is the selective removal of impurities this step is analogous to a cleaning of specimens it is succeeded by initial dissolution by strong dissolution and finally by disintegration

bryozoans as carbonate sediment producers on the cool water lacepede shelf southern australia abstract modern sediments on the lacepede shelf and adjacent slope are typical mid latitude cool water palimpsest deposits dominated by bryozoan mollusc and quartz particles bryozoan bioclasts form autochthonous and to a lesser extent allochthonous accumulations across the grain size spectrum from mud to boulder size the bryozoan skeletons are grouped on the basis of architecture and style of disarticulation fragmentation into nine morphotypes erect rigid forms comprise 1 fenestrate 2 foliose 3 flat robust branching and 4 delicate branching morphotypes large adeona sp skeletons are recognised as a separate type of fenestrate form because of their distinctive shape mineralogy and restricted environment erect flexible forms are either 5 articulated branching or 6 articulated zooidal other important morphotypes are 7 encrusting 8 nodular arborescent and 9 vagrant differences in skeletal architecture result in distinct particle types encrusting morphotypes on soft organic substrates and the wide variety of erect rigid forms are broken into fragments erect flexible forms disintegrate upon death into definitive sand and silt size grains encrusting morphotypes on hard substrates nodular arborescent and vagrant forms remain whole and when large accumulate in place morphotypes are mineralogically distinct and so bryozoan sediment composition is environmentally controlled shallow water

neritic and pelagic carbonate sedimentation in the marine environment ignorance is not bliss synthesis of available data allows us to define general patterns of late quaternary carbonate production and sedimentation in the global ocean during high stands of sea level the neritic and pelagic environments appear to sequester approximately similar amounts of carbonate whereas during low stands of sea level the decreased neritic zone produces and accumulates approximately an order of magnitude less carbonate assuming that global accumulation of deep sea carbonates remains more or less constant during glacially induced changes in sea level the ocean becomes depleted with respect to calcium carbonate during high stands and recharges during low stands before we can achieve a better understanding of the global carbonate system however we need a better understanding of key environments and processes a production and accumulation on continental shelves both as potential sinks accumulation and as sources export to the deep sea b a better measure of pelagic carbonate production and c late quaternary late pleistocene and holocene mass accumulation rates in the deep sea

influence of water column anoxia and sediment supply on the burial and preservation of organic carbon in marine shales abstract previous work has suggested that the laminated organic rich and bioturbated organic poor shales of the camp run member of the late devonian early mississippian new albany shale formed under anoxic and oxygenated bottomwater conditions respectively and that the interbedding of the two facies was due to the vertical oscillation of a water column anoxic oxic boundary where it impinged on the basin margin we have extended this analysis by examining the chemical and mineralogical differences between the two shale facies in a single borehole core by seeking evidence for deposition of the laminated shales under bottom water oxia or anoxia and by determining whether the laminated shales formed when the carbon supply to the sea floor was higher the results of this study show that the laminated and bioturbated shales are mineralogically and chemically distinct relative to al an index of the aluminosilicate content si ti fe p na ba co cr cu mo ni v zn and zr are all higher whereas mn ca mg and sr are lower in the laminated compared with the bioturbated shales the differences are due to a higher quartz feldspar titanite ilmenite and zircon content in the laminated shales probably indicating a coarser grain size and the greater abundance of manganoan calcite in the bioturbated shales dissolved oxygen was present in bottom waters during the deposition of some of the laminated shale intervals because of the presence of manganoan calcite a phase that can only form in sediments with an oxic surface in addition the organic matter preserved in the two shale types is isotopically different 13corganic values are 1 9z permil lighter on average in the laminated compared with bioturbated intervals possibly indicating a larger fraction of terrestrial organic matter in the latter 15n values are 1 9z permil lighter on average in laminated compared with bioturbated intervals possibly indicating a larger fraction of terrestrial organic matter in the latter 15n values are 1 9z permil lighter on average in laminated compared with bioturbated intervals suggesting that nutrient drawdown was less during the deposition of the organic rich laminated shales the chemical mineralogical and isotopic contrasts between the two shale facies of the camp run member indicate that the conditions of sedimentation were different during their deposition the difference was possibly related to variations in sea level which would have caused the camp run shoreline to move closer to and farther from the core site causing in turn the deposition of coarser and finer grained sediments that contained different mixtures of marine and terrestrial organic matter bottomwater conditions were anoxic during deposition of most laminated intervals bottom water anoxia or dysoxia led to decreased burial and preservation of the essential nutrient phosphorus in the laminated organic rich shales relative to the rocks deposited beneath better oxygenated bottomwaters increased availability of phosphorus in the water column on long timescales leads to increased productivity and a higher settling flux of organic matter causing bottom water oxygen levels to fall this is consistent with the nitrogen isotope evidence suggesting that production was probably higher during the deposition of the organic rich shales thus production variations coupled with enhanced sedimentary regeneration of phosphorus from sediments related to low oxygen bottom water concentrations provide a general mechanism for the formation of the alternating facies of this member of the new albany shale

first cycle quartz arenites in the orinoco river basin venezuela and colombia modern first cycle quartz arenites are forming in the orinoco drainage basin by at least two distinct mechanisms common to both mechanisms is an environment of intense chemical weathering and extended time over which weathering can occur in the llanos andean foreland basin extended time is provided by temporary storage of orogenically derived sediments on extensive alluvial plains in lowland portions of the guayana shield long soil mineral residence times are produced by very low erosion and transport rates resulting from low relief and tectonic quiescence both weathering intensity and the duration of weathering are important in the modification of sand composition first cycle quartz arenites are produced in diverse tectonic settings and cannot be used by themselves to infer paleotectonic settings of ancient sequences furthermore although tectonic setting is of great importance in determining sand composition modifications during transport and deposition can overprint the effects of the tecto

endolithic biodegradation of cool water skeletal carbonates on scott shelf northwestern vancouver island canada abstract biodegradation of shell material is widespread in the cool water skeletal carbonate deposits on scott shelf northwestern vancouver island and is especially evident in the large aragonitic bivalves glycymeris and humilaria major primary sediment contributors ten types of endolithic microborings have been identified in the shells including representatives of green algae e g ostreobium quekettii blue green algae e g plectonema terebrans scytonema sp fungi bacteria and clionid sponges as well as macroborings of phoronids polychaetes and naticid gatropods microcrystalline carbonate is not precipitated in vacated bores boring physically weakens the shells rendering them more prone to mechanical abrasion during sediment transport and bioturbation and to biological abrasion by grazing benthos tumbling experiments demonstrate that the rate of carbonate mud production is much greater for bored as compared to fresh bivalve shells and that mud production rates decrease with tumbling time because most endolithic microborings are confined to the periphery of grains boring also increases significantly the porosity and surface area of skeletal grains and destroys their organic matrix making them susceptible to maceration and dissolution on cool water shelves fostered by the generally low rates of carbonate production and accumulation many aragonitic bivalve shells on scott shelf have become thoroughly degraded through a combination of endolithic microboring maceration and dissolution within about 1000 years in ambient sea water in geologic terms such selective taphonomic loss of skeletal material may be considerable in ancient temperate shelf limestones and should be evaluated when interpreting their paleoecology and paleoenvironments

controls on marine carbonate cement mineralogy review and reassessment abstract commonly cited models for controls on variations in carbonate mineralogy are only partly successful in accounting for natural mineralogical trends this limited success may result in part from a failure to recognize or address the ramifications of assumptions inherent to such models in general changes from calcite to aragonite or vice versa and variations in mgco3 incorporation in calcite are interpreted as resulting from 1 kinetic rather than thermodynamic effects 2 the effects of fluid chemistry rather than hydrologic biological or substrate effects 3 the effects of single parameters of fluid chemistry rather than the interactive or aggregate effects of several components and or 4 inorganic rather than organic parameters of fluid chemistry recent laboratory data suggest the necessity of a new approach to modeling controls on mineralogy these data indicate that no single fluid parameter is likely to be a sole or dominant control on carbonate mineralogy in natural environments in some cases thermodynamic stability differences may be as important as kinetic factors in determining mineralogy furthermore the effects of substrate hydrologic and biological factors and of organic compounds cannot be ruled out for most natural situations although at present reliable data are lacking to quantify the influence of these factors on mineralogy future models of controls on carbonate mineralogy need to consider the aggregate and interdependent effects of all of these factors

major and trace element geochemistry in zeekoevlei south africa a lacustrine record of present and past processes this study reports a multi parameter geochemcial investigation in water and sediments of a shallow hyper eutrophic urban freshwater coastal lake zeekoevlei in south africa zeekoevlei receives it

taphonomic processes affecting late quaternary molluscs along the coastal area of buenos aires province argentina southwestern atlantic molluscan concentrations were abundantly deposited along the coastal area of buenos aires province argentina southwestern atlantic by different sea level episodes during the late quaternary most of them represent holocene ridges highly energetic depositional environments subparallel to the modern shore where the main associations preserved are parautochthonous and others mainly autochthonous associations occurring in tidal flats seaward of the ridges or coastal lagoon deposits accumulated behind them low energetic analysis of three physical taphonomic signatures disarticulation fragmentation and abrasion of the molluscan skeletons of four bivalve species tagelus plebeius mactra isabelleana pitar rostratus brachidontes rodriguezi which exhibit different shell shape and thickness conditions habitats and life habits and represent the most widely distributed and palaeoecologically significant taxa along the area of study support the distinction of both palaeoenvironments characterized by low and high taphonomic grades respectively the taphonomic alteration breakage and abrasion of most shells seems to have been a direct consequence of the amount of reworking of the material especially during periods of exposure along an extensive beach zone more than a consequence of transport from a distant original habitat abrasion signatures are the only probably correlated with after death time span shells showing a near modern aspect and original luster being most probably younger than those with chalky or polished appearance the infaunal species analysed typically living on clays and silts or fine sandy bottoms of the infralittoral or intertidal t plebeius m isabelleana are better preserved less fragmented and abraded than shells of the epibyssate b rodriguezi which lives on hard bottoms of the intertidal and supralittoral areas more exposed to highly unstable conditions with longer episodes of exposure although these nearshore concentrations may represent a time averaging of 1000 yr b p taking into account the ecological requirements of the most constant and dominant taxa together with the taphonomic analysis of shells from parautochthonous and autochthonous associations they still represent valid indicators of past marginal marine conditions in the area but are only reliable to establish long term palaeoenvironmental variations

carbonate mud sedimentation on a temperate shelf bass basin southeastern australia bass basin 66 000 km2 is unusual as a carbonate dominated basin because calcitic carbonate muds are accumulating in relatively shallow 70 85 m water depths in the central part of the basin 20 000 km2 the carbonate muds are produced by the primary accumulation and disintegration of nannoplankton as well as through the biodegradation of skeletal carbonate grains accumulating on the sea floor the muds are transported to the south of the basin to the end of tidal current transport paths where they accumulate in the lowest energy environment available in enclosed bass basin although they are still subject to periodic reworking by storm energy the basin margins consist of coarser partially palimpsest carbonate sands whereas the central muds overlie a late pleistocene disequilibrium surface in cores these muds are up to 1 m thick and thin away from the centre towards the margins bioturbation is important in modifying the textural character of the muds by increasing the grain size as faecal pellets and therefore creating a sea floor not in equilibrium with modern hydrodynamic conditions radiocarbon dates for an early post glacial marine transgression embayment facies in the basin gave ages of 10 290 250 to 11 660 300 years b p an age of 8700 710 years b p was obtained for the base of the modern strait facies these ages and facies thicknesses were used to establish holocene sedimentation rates of 12 cm 1000 y for the basin centre falling to 6 cm 1000 y towards the margins

surficial cool water carbonate sediments on the otway continental margin southeastern australia abstract the otway continental margin of southern australia is a swell dominated open cool water carbonate platform it is partitioned into a series of depth related zones of biological sediment production and accumulation by three hydrodynamic interfaces 1 abrasion depth 70 m 2 swell wavebase 130 m and 3 storm wavebase 250 m the otway shelf profile drops steeply from the strandline the shallow shelf lies largely between 30 and 70 m and is a high energy zone of carbonate sediment production intensive destructive diagenesis and erosion the seafloor is characterized by widespread exhumed limestone substrates which host dense encrusting assemblages of molluscs sponges bryozoans and red algae loose sediment accumulation is patchy and consists of a thin quartz rich palimpsest gravel veneer the middle shelf 70 to 130 m is a zone of swell wave shoaling and production and accumulation of coarse mega rippled 2 d dunes bryozoan sands deep shelf areas 130 to 180 m accumulate finer bioclastic sands foraminiferal echinoid bryozoan that are intensely bioturbated and episodically reworked by storms grains are variably fragmented and marginally bored at the shelf edge and top of slope 180 to 350 m nutrient rich upwelling currents support laterally extensive aphotic biostromal hermal bryozoan sponge coral communities which generate and accumulate thick successions of muddy skeletal sand sediments of the upper slope 350 to 500 m are a bioturbated mixture of periplatform bioclastic debris and pelleted foraminiferal nannofossil mud turbidites and resedimentation features are common bioturbation and shelf derived skeletal content decrease progressively downslope and pelagic muds dominate below 500 m lower slope areas are cross cut by gullies and have low accumulation rates the base of slope is characterized by sediment apron deposits consisting of interbedded shelf derived coarse grained turbidites and pelagic ooze

when is an old shell really old discrimination and comparison between relatively young recently dead and relatively old less recently dead shells has been an important component of studies on modern death assemblages we employed amino acid dating to determine whether or not various shell and taphonomic attributes could be used to categorize donax into age time since death categories e g old vs young as expected biological interactions and whether the specimen was a right or left valve were not time dependent abraded shells shells lacking sheen those showing evidence of dissolution fragments and chipped shells tended to be relatively old but many old shells retained the species original living characteristics with regard to all of these attributes except color and sheen shell color was the only attribute yielding a satisfactory division of shells into two age categories old vs young the formation of a typical beach assemblage requires the gradual addition and taphonomic alteration of shells over many hundreds of

geochemical appraisal of palaeovegetation and climate oscillation in the late miocene of western bulgaria abstract palaeobotanical methods and geochemical techniques were used to assess plant contribution and palaeoenvironment for the staniantsi basin bulgaria the aim was to connect palaeovegetation change and climate oscillation based on pollen and statistical analysis with organic geochemical proxies for a late miocene lacustrine to paludial sedimentary succession three samples from lignite marl cycles were studied the biomarker assemblage and bulk chemical data indicated that gymnosperms were not important in the palaeomire the presence of des a triterpenoids 17 21 seco triterpenoids hopanes a high content of a d ring monoaromatic hopane and aromatized triterpenoids suggested that photochemical and microbial processes significantly contributed to the alteration of the organic matter om a prolonged period of high water table and severe mechanical destruction promoted microbial activity prior to burial and enhanced decay a geochemical appraisal of short term climate oscillation ca 21 7 kyr was attempted within the limitations of the small number of samples studied the cycles are expressed as lignite marl clay layers combined with cyclic changes in swamp vegetation related to cyclic changes in groundwater level and inundation of the basin in periods of low water level swamp phase lignite accumulation took place preliminary results for selected samples suggest that the oscillation may be reflected in the content of friedelin vs possible degradation products the ratio of a chromatographic peak tentatively assigned as a norfriedel 8 en 10 one to friedelin is proposed as a means of detecting short term environmental cycles where values 1 reflect periods of inundation however time series analysis using densely sampled lignite clay layer oscillations are needed to confirm the value of this biomarker ratio for environmental reconstruction

rare earth elements in river waters we measured rare earth element ree concentrations in river waters to characterize the suspended and dissolved river flux of the ree to the oceans the ree pattern of river suspended materials is sensitive to drainage basin geology a positive correlation is observed between la yb ratios and nd model ages for the rivers studied major rivers have light ree enriched patterns relative to the north american shale composite nasc with la yb n 1 6 2 7 river water dissolved material 0 2 m is heavy ree enriched relative to suspended material and the most pronounced negative ce anomalies occur in rivers of high ph light ree concentrations vary by approximately 3 orders of magnitude and are inversely related to ph and major cation concentrations from these data we estimate that typical major river runoff has heavy ree depleted suspended material with la yb n 1 9 we conclude that the terrigenous input to the oceans from major rivers is heavy ree depleted relative to shales from the available data average river water dissolved material appears to be heavy ree enriched with la yb n 0 4 estuarine removal processes lower the dissolved ree river flux by approximately 60 and result in a flux that is more heavy ree enriched with la yb n 0 2 calculated oceanic residence times with respect to river input range from 2300 to 21 000 years are shortest for ce and greatest for the heavy ree and la such long residence times may suggest the presence of additional sources of ree in seawater

major and trace elements of river borne material the congo basin the congo river basin is the second largest drainage basin in the world after the amazon the materials carried by its main rivers provide the opportunity to study the products of denudation of a large fraction of the upper continental crust of the african continent this paper presents the chemical composition of the different phases carried in the congo rivers and is followed by a companion paper devoted to the modelling of major and trace elements the congo river between bangui and brazzaville as well as its main tributaries including a few organic rich rivers also called black rivers were sampled during the 1989 high water stage the three main phases suspended load dissolved load and bedload were analysed for twenty five major and trace elements concentrations normalized to the upper continental crust show that in each river suspended sediments and dissolved load are chemical complements for the most soluble elements ca na sr k ba rb and u while these elements are enriched in the dissolved loads they are considerably depleted in the corresponding suspended sediments this is consistent with their high mobility during weathering another type of complementarity is observed for zr and hf between suspended sediments and bedload related to the differential velocity of suspended sediments and zircons which are concentrated in bedloads compared to other rivers absolute dissolved concentrations of ca na sr k ba rb and u are remarkably low surprisingly high dissolved concentrations are found in the congo waters for other trace elements e g rees especially in the black rivers on a world scale these concentrations are among the highest measured in rivers and are shown to be ph dependent for a number of dissolved trace elements the dissolved loads are systematically normalized to the suspended loads for each river in order to remove the variations of the element abundances owing to source rock variations normalized diagrams for rees are presented and extended to the other elements they strongly support the argument that the apparent higher solubility of trace elements in the congo waters is due to the presence in the dissolved load of a colloidal phase as a result of 0 2 m filtration an important result is that these colloids are strongly depleted in fe and al with respect to the other elements finally the comparison of the dissolved suspended and sandy transport fluxes of each element in the congo basin rivers shows that although the proportions of for example the rees in the dissolved loads of the majority of the congo basin rivers is close to 10 of the total transport flux up to 80 of the rees are transported by the so called dissolved load in the black rivers

measurements of beach pebble attrition in palliser bay southern north island new zealand the beaches of palliser bay exposed to oceanic waves from the south are composed of mixed sand and gravel derived from mountain ranges of greywacke and argillite that rise above both sides of the bay beach gravel tracer experiments each using up to 75 tonnes of limestone pebbles were conducted at three sites increase in the roundness of these tracer pebbles at each site during the year of observation was compared with the increase in roundness and rate of weight loss of limestone and greywacke pebbles in a laboratory tumbler the comparison enabled estimation of the rate of attrition of the natural beach pebbles a weight loss of 41 per year was found for pebbles at the most exposed site and 15 and 7 at the other two sites

temperate skeletal carbonate sediments on scott shelf northwestren vancouver island canada abstract scott shelf 2000 km 2 centred at scott islands northwestern vancouver island is mainly shallower than 150 m topographically diversified and floored by bedrock outcrops and terrigenous lithic gravels and sands left stranded following the post glacial rise in sea level 13 000 yrs b p it lies in a zone of vigorous wind wave currents and strong tidal flows but is largely starved of modern terrigenous sediment as a consequence it is slowly accumulating a thin discontinuous blanket of clean skeletal carbonate sands and gravels admixed to varying degrees with the underlying terrigenous deposits principal skeletal contributors are infaunal bivalves on coarse sandy and gravelly substrates barnacles on low amplitude gravelly ridges bryozoans on bedrock outcrops and boulders and benthic foraminifera on fine sands in deep 100 m waters south of the scott islands living carbonate benthos are scattered and generally sparse occupying specific ecologic niches and the shelf wide rate of carbonate production is low skeletons are fragmented transported and mixed during storms and are concentrated within bedrock hollows and crevices and shallow depressions between gravel ridges where infaunal bivalves are abundant the carbonates are dominated by aragonite but otherwise the skeletal hashes are predominantly calcitic many grains and especially aragonitic ones are corroded and weakened by epilithic and endolithic bioerosion and probably also by marine and dissolution the most corroded shells have ages of only about 1000 yrs so that their preservation potential is low the character of scott shelf skeletal carbonate deposits reflects their temperate latitude cold water heritage

carbonate dissolution in nearshore terrigenous muds the role of physical and biological reworking because shallow marine sediments generally underlie waters which are saturated with respect to arago nite and calcite physical chemical dissolution of carbonate in these environments is sometimes discounted the present investigation as well as others before it show such an assumption to be unwarranted in this case study of terrigenous mud deposits from long island sound u s a demonstrates extensive dissolution of molluscan shell debris near the sediment water interface net buildup of carbonate varies significantly between different regions of the sound despite fairly uniform production of shell debris and similar total sedimentation rates aside from this apparent discrepancy between rate of supply and net carbonate sedimentation evidence for carbonate dissolution comes from the pitting of shells elevated ca levels in pore water and calculated undersaturation of pore waters with respect to carbonate minerals models of pore water distributions and carbonate supply imply dissolution ra

reefal coralline algal build ups within the arctic circle morphology and sedimentary dynamics under extreme environmental seasonality abstract carbonate frameworks secreted by phototrophic organisms within the arctic circle are not well documented underwater surveys of the inner shelf off troms northern norway 70 n reveal extended fixed algal build ups which are fringed by rhodolith belts affected by storms reefd growth by coralline algae under temperature and light regimes of extreme seasonality is made possible because of a decoupling of carbon fixation during summer and utilization of stored carbon during the period of winter darkness although the annual growth of the framework constructing algae is comparatively low the annual carbonate production rate is similar to subtropical tropical counterparts because of a remarkably high standing stock early diagenetic alteration is restricted to intraparticle cementation processes which start in vivo bioerosional destruction is the dominant control on the preservation of high latitude build ups preservation of holocene autochthonous coralline algal biostromes is enhanced by rapid burial during storm events redeposition during storms is the most important process in forming a distinct sedimentary facies zonation

carbonate cement fabrics displayed a traverse across the margin of the bahamas platform near lee stocking island in the exuma cays abstract consolidated to friable carbonate rocks found in the lee stocking island area in the exuma cays include 1 reef rock 2 channel stromatolites 3 shallow water hardgrounds 4 beachrock rimming the islands and 5 pleistocene bedrock the most common cement fabrics observed are aragonitic fibers which include acicular fan druse and square tipped coarse fibers cementing beachrock and stromatolites and an isopachous needle fiber rim cementing hardgrounds and stromatolites less common are high mg calcite bladed textures of the reef rock and stromatolites two types of blades are present the more common stubby variety of either high mg or low mg calcite and an elongated variety of high mg calcite which was found in only three beachrock samples aragonitic micrite envelopes usually surround grains in beachrock hardgrounds and stromatolites but only in association with fibrous cement an aragonitic crust cements the surfaces of lime mud beds of the tidal channel while a high mg calcite cryptocrystalline cement occurs in all the rock types calcified algal filaments of high mg calcite from the abundant green and blue green algae in the area are a primary cement in stromatolites and a secondary cement in hardgrounds and beachrock a low mg calcite equant spar cements the pleistocene samples and is associated with meteoric diagenesis and cementation of the pleistocene surface cement precipitation coincides with the path of the cool oceanic water from exuma sound as it warms and loses co 2 and moves up onto the bank near lee stocking with the incoming tide cryptocrystalline cement is the first and commonest cement forming to the seaward while platformward fibrous cements become predominant as suggested by their crystal size and location on the shelf margin we think that the reef rock cryptocrystalline material are the fastest forming of the cements where the incoming oceanic water is more saturated with respect to calcium carbonate and undergoes the most significant warming the rate of the warming and degassing process is thought to increase in the tidal channel though the cementation rate is thought to fall slightly in response to a reduced availability of calcium carbonate on the platform interior further warming and degassing are believed to cause cement precipitation and the development of hardgrounds but these may form at a slower rate than that of the margin though this rate is still quite high cementation gradients occur from the tidal channel to the intertidal zones of 1 west norman s pond cay where cement fabric suggests a reduced calcium carbonate availability and 2 west lee stocking island where a change in mineralogy suggests a change in water chemistry thus a sequence of cement fabrics and mineralogies can be traced micritic textures occur in a more seaward position fine fibrous aragonite fibers in a more lagoonal and levee position and coarser aragonite fibers and mg calcite cements in the intertidal and supratidal position this sequence is thought to track the evolution of the water mass

microbial degradation of the marine prymnesiophyte emiliania huxleyi under oxic and anoxic conditions as a model for early diagenesis long chain alkadienes alkenones and alkyl alkenoates to investigate the effect of bacterial diagenesis on the distributions of algal lipids the marine haptophyte emiliania huxleyi was incubated with estuarine sediment slurries under defined conditions oxic sulfate reducing and methanogenic rapid initial degradation of lipids corresponded to increased bacterial populations both of which subsequently declined under oxic conditions the algal c31 dienes were rapidly and completely degraded within 178 days extensive degradation of the characteristic c37 methyl alkenones occurred under all conditions up to 85 under oxic conditions however the u37k index remained essentially constant except for a slight increase at the longest oxic incubation time under anoxic conditions the alkyl alkenoates were preferentially degraded relative to the alkenones and changes in the aa36 index occurred these results demonstrate that lipids are part of the labile organic matter however they were generally degraded at multiple rates which slowed during incubation resulting in their partial preservation hence laboratory experiments conducted over only short periods cannot predict the preservation potential of lipid components furthermore preservation differed between sulfate reducing and methanogenic conditions so the roles of anaerobic processes need to be considered individually with regard to oxic vs anoxic preservation

the rate of taphonomic loss in modern benthic habitats how much of the potentially preservable community is preserved abstract the post mortem fate of shells added by natural mortality to modern death assemblages was studied to determine the importance of taphonomic loss during the initial stages of the formation of a fossil assemblage individuals are added to the living community in pulses usually by larval settlement these input pulses usually are followed by discrete input pulses into the death assemblage the decay of these discrete input pulses in the death assemblage can be used to measure the rate of taphonomic loss as expressed by the half life of a pulse for instance if a species has a half life of 125 days and the input pulse begins with 500 individuals then the number of shells remaining after one half life 125 days is 250 shells at two sites on the texas coast the longest half lives did not exceed one year for any of the 13 species which produced input pulses during the study period little net addition to the death assemblage could be detected in spite of the transitory addition of hundreds of shells as input pulses these rapid decay rates show that death assemblages do not accumulate at the rate at which organisms die some species such as the infaunal bivalve tagelus plebeius did not decay measurably we predict that decay rates drop substantially with depth in the sediment so that individuals which die at depth or are rapidly reworked below the sediment surface have a greater chance of preservation the final composition of death assemblages is probably little related to the actual numerical input from the potentially preservable component of the living community

synsedimentary cemented calcarenite layers in oligo miocene cool water shelf limestones eucla platform southern australia abstract the abrakurrie limestone is a bryozoan rich cool water open shelf unit of oligocene middle miocene age beneath the nullarbor plain the sediments have been in the meteoric diagenetic environment never buried more than a few hundred meters since middle miocene time the unit is composed of meter scale subtidal cycles each of which is capped by an erosion surface the upper several centimeters of most cycles are well lithified forming distinctive hard layers in otherwise friable carbonates intergranular cements in these layers are now isopachous rinds of low magnesium calcite crystals are inclusion rich generally fibrous and have a speckled cathodoluminescence signature cements in intergranular pores at the tops of cycles are overlain by interstitial marine sediment the cem nts are interpreted to have been marine precipitates in the form of magnesium calcite originally containing from 8 12 mole mgco3 and having a 18o value of 1 0 0 5 versus pdb stable carbon and oxygen isotope values in these interpreted marine hardgrounds are distinctly more positive than values from underlying soft sediments reflecting alteration under lower water rock ratio conditions due to reduced permeabilities covariance in stable isotope values from different localities suggests diagenesis in groundwaters of mixed composition fluid inclusions indicate neomorphism in fluids with salinities equal to or slightly greater than seawater alteration is envisaged to have taken place in a paleogroundwater system of variably mixed saline continental waters and seawater occurrence of similar to identical cemented layers and cements in coeval cool water new zealand shelf carbonates suggests that such cementation was a sporadic but widespread phenomenon on mid cenozoic platforms in this area by inference cool water carbonates of all ages may contain units lithified on the seafloor by cements containing low to intermediate amounts of magnesium cool water is not an inhibitor to synsedimentary cementation

an introductory perspective on non tropical shelf carbonates abstract this brief account introduces the topic of non tropical shelf carbonate deposits both modern and ancient specific aspects of which are described and discussed in the 22 accompanying papers in this special issue of sedimentary geology non tropical shelves are loosely defined as lying poleward of the present limit of development of hermatypic coral reefs or beyond the mean annual surface water isotherm of about 20 c typically near 30 latitude most geoscientists have traditionally regarded shallow water carbonate formation as a low latitude phenomenon however wherever the rate and extent of terrigenous sediment dilution is small the potential exists for extensive deposits of skeletal carbonate to accumulate on shelves beyond the sub tropics the facies attributes of the modern non tropical carbonates are distinctive and contrast significantly with the structural lithological faunal floral mineralogical and diagenetic character of their tropical counterparts in the rock record non tropical limestones probably occur throughout the phanerozoic but generally remain to be identified and or interpreted as such they are presently described mainly from the ordovician permian and tertiary quaternary periods of major continental glaciations and accentuated latitudinal thermal gradients a uniformitarian approach to the recognition and interpretation of ancient shallow marine limestones requires acceptance of both tropical and non tropical carbonate facies models

the role of nutrient availability in bioerosion consequences to carbonate buildups abstract zooxanthellate organisms which are among the major carbonate producers on coral reefs are highly adapted to nutrient deficient conditions and tend to be outcompeted by filamentous or fleshy algae if nutrients are abundant reef dwelling bioeroding organisms on the other hand seem to increase in abundance with increasing availability of nutrient and food resources maximum rates of calcium carbonate production in a reef system are comparable in magnitude to maximum rates of bioerosion the dynamic interplay between accretion destruction of coral reefs is therefore likely to be strongly influenced by nutrient availability nutrient availability may also influence development of carbonate facies in reef systems that develop in oligotrophic waters skeletal sands and framework should dominate carbonate buildups in systems that develop in mesotrophic waters enhanced bioerosion should reduce much of the skeletal material to carbonate muds increasing the prevalence of mud rich facies eutrophic conditions should result in bioeroded hardgrounds and carbonate depositional hiatuses when carbonate accumulation rates fail to equal or exceed rates of local relative sea level rise reefs drown bored and lithified pavements with crusts of ferromanganese oxides phosphates or glauconite often separate shallow reef from overlying deeper water deposits nondeposition bioerosion and evidence for low redox potentials at the sediment water interface all implicate excess nutrient availability as a factor in reef drowning some carbonate depositional hiatuses and bioerosional surfaces are local phenomena evidence of responses to local increases in nutrient availability resulting from submergence of soils runoff or topographic upwelling others appear to reflect regional or worldwide events because many carbonate producing organisms have life history strategies specialized to nutrient deficient conditions they are unable to compete when nutrients become plentiful events that triggered ocean turnover or sharp increases in the rate of deep ocean circulation would have increased sea surface nutrient availability worldwide such eutrophication could have caused widespread extinctions in reef communities because eutrophication promotes bioerosion details of the fossil records of those events are probably lost in bioerosional hiatuses

non tropical carbonate deposits on the modern new zealand shelf abstract extensive ca 50 000 km 2 shallow marine platforms 70 caco 3 despite proximity to a tectonically active plate margin in these regions the terrigenous sediment supply is presently low and growth of epibenthos is fostered by firm substrates rock gravels shells seaweeds and the generally energetic nature and high nutrient levels of open shelf waters rapid transition into adjacent terrigenous dominated facies is characteristic irrespective of water depth the carbonates are coarse grained and fragmental carbonate mud is rare calcite dominates over aragonite high mg calcite widespread off northern new zealand is rare in the south skeletal material is dominated by bryozoans and bivalve molluscs with significant local contributions from foraminifers barnacles calcareous red algae and echinoderms the name bryomol is suggested for this distinctive temperate region skeletal carbonate facies which can be usefully subdivided based on dominant zoarial growth forms of the bryozoan component known to be habitat related bioerosion is an important mechanism of skeletal fragmentation and degradation many grains especially aragontic bivalves are infested by endolithic borers and have low preservation potential ages of skeletal material in the surficial deposits range from more than 20 000 years b p to modern which is consistent with both low rates of carbonate production and sediment accumulation and the wide range in preservation state of grains some data suggest that the skeletal carbonates are dispersed and mixed mainly during infrequent movement of sand ribbons sand waves and sand sheets driven by storm assisted tidal flows tracts of modern palimpsest and relict carbonates can occur in juxtaposition the facies characteristics of the new zealand shelf carbonate deposits contrast significantly with those of the classical bahaman type carbonate model however they are similar to those reported from many other mid to high latitude carbonate shelves and afford good analogues for most onland occurrences of new zealand cenozoic limestones

the geochemistry of aliphatic and polar organic tracers in sediments from lake bera malaysia abstract the molecular distributions of homologous aliphatic lipid compounds n alkanes n alkanoic acids n alkanols n alkanals and n alkan 2 ones were investigated in sediments of lake bera malaysia as well as in plant waxes from the vicinity using gas chromatography mass spectrometry in addition triterpenoids of the oleanane ursane lupane and friedelane series and their diagenetic products were also identified using the aquatic proxy p aq it was found that the n alkanes present in surface sediments s1 s5 and s6 correspond to an input from emergent macrophytes whereas in sediments collected from open water s2 s3 and s4 they correspond to input from submerged floating macrophytes the presence of n alkan 2 ols a commonly found homologous series in the sediments as well as in the plant wax is novel here in addition the presence of 24 methylcycloart 22 enol and 24 ethylcycloart 22 enol in sediments as well as the plant wax of pandanus helicopus a dominant reed around the lake also supports the input of plant biomarkers to the sediments the occurrence of 5 membered ring triterpenoid derivatives as major diagenetic products of the triterpenoids identified e g friedelan 2 3 dione shows that the degradation of the organic matter in the sediments occurs mainly under oxic conditions also the presence of ring a degraded triterpenoids dihydroputranjivic acid 3 nor 2 4 seco friedelane 4 and 4 3 norfriedelanes a norfriedel 10 one and a norfriedel 8 en 10 one and the aromatized triterpenoid hydrocarbons mono and triaromatic derivatives as minor diagenetic products indicates photochemical and microorganism mediated alteration reactions of natural products in the sediments additional novel polyfunctional friedelane derivatives namely friedelane 2 3 dione friedelane 1 3 diol 2 one and friedel 1 10 ene 1 3 diol 2 one were identified for the first time in this study these novel polyfunctional triterpenoids were inferred to derive from the oxidation of the triterpenoids from higher plants in this oxic tropical environment for future reference purposes the mass spectra of the identified polar triterpenoids are also presented here this study provides important data on the biogeochemistry of organic matter in a tropical lake of se asia

the fluvial geochemistry and denudation rate of the guayana shield in venezuela colombia and brazil abstract the guayana shield is composed of early to mid precambrian igneous and metamorphic basement rocks with a quartzitic platform cover the complete absence of limestones and evaporites allows a clear chemical expression in the stream data of the primary weathering of the basement in a humid tropical environment total erosion rates are extremely slow 10 m m y with equal contributions from the dissolved and suspended loads however the former is largely silica with ratios of si to total cation equivalents si tz ranging to in excess of three weathering is complete to kaolinite and gibbsite i e the environment is one of active laterisation with a penetration rate of the weathering front into the fresh substrate about twice the denudation rate in basins of relatively homogeneous lithology rb sr isochrons constructed from the river data agree with the whole rock ages from the drainages thus all the common refractory rb containing minerals k feldspar mica are completely dissolved the thick lateritic regolith that is accumulating as a result of this intense weathering is a common relict feature on other southern hemisphere shields in the absence of active tectonics or greatly accelerated mechanical erosion the weathering rates of these basement rocks must be quite insensitive to environmental change

geochemistry of a tropical lake lake leopoldo on pseudo karst topography within the roraima group guayana shield venezuela abstract a geochemical assessment was performed on lake leopoldo a tropical lake located on the guayana shield venezuela the lake water was characterized ca2 mg2 na k cl no 3 no 2 so 4 2 nh 4 fe and p lacustrine setting the origin transport and fate of sedimentary material in this highly unusual lacustrine setting lake sediments were analyzed for textural mineralogical and chemical composition including the rare earth element ree contents in addition to toc 13c 15n and elemental c n ratios the molecular distributions of several biomarker compounds were determined and used to assess organic matter source and transport to the lake lake leopoldo developed on a sequence of quartz arenites of the precambrian roraima group the area conforms to the definition of pseudo karst morphology the results show that the lake is oligotrophic where primary production seems to be limited by nutrients in the sediments organic matter shows an overwhelming higher plant derived origin certain elements k v cu th ba hg and lree displayed enrichments with organic matter while others zr hf si sn pb and ta displayed an association with the mineral fraction the results obtained in this study suggest that the lake water characteristics and the affinities of trace elements for sediment organic fractions are controlled by biogeochemical processes within the surrounding forest soil environment prior to the transport of the terrestrially derived organic matter to the lake the most significant geochemical finding is that organic matter can play an important role in the distribution of trace metals in such highly oligotrophic terrains of humid tropical environments

origin of a cool water oligo miocene deep shelf limestone eucla platform southern australia the abrakurrie limestone is an areally extensive bryozoan rich unit within the eucla platform a tertiary carbonate shelf which caps the central part of the southern australian continental margin the onshore portion the topic of this study has been exposed since middle miocene time and lies beneath the nullarbor plain the rocks are fine sand to granule sized calcarenites composed of bryozoans bivalves benthic foraminifera and echinoids with lesser numbers of brachiopods solitary corals and serpulids they conspicuously lack significant numbers of planktonic foraminifera and coralline algae most bryozoan remains are from delicate branching cyclostomes although delicate branching robust branching foliose fenestrate multilaminar encrusting and free living cheilostomes are variably abundant in specific units the poorly lithified sequence is punctuated by well cemented layers with erosional tops which are interpreted as hardgrounds the limestone is interpreted as a cool water deep shelf deposit which accumulated in water depths generally greater than 50 m on the inner part of the eucla platform a model which involves deposition and cementation on a carbonate shelf swept by open ocean swells is proposed to explain the style of sedimentation the shelf is envisaged as partitioned by the depth of the zone of wave abrasion sediments were produced throughout but accumulated only below this depth when the seafloor was above this depth it was an environment of cementation and erosion the vertical sequence correlates in a general way with the global sea level model for the mid cenozoic while accumulation rates for southern australian carbonates are similar to rates of cool water carbonate deposition elsewhere c 2 5 cm kyr 1 the rate of abrakurrie accumulation is much less suggesting that significant time periods are represented by the hardgrounds

biodegradation of temperate water skeletal carbonates by boring sponges on the scott shelf british columbia canada abstract boring sponges are a major agent of shell destruction and in the process an important contributor of fine grained carbonate detritus in temperate skeletal carbonate deposits on scott shelf northwestern vancouver island intensive boring is encouraged by the generally low rates of carbonate production and sedimentation skeletal grains become considerably weakened by sponge boring and break preferentially along the directions of sponge excavated tunnels and galleries within the shell boring also facilitates shell dissolution by significantly increasing the surface area weight ratio of shells and by breaking down the organic matrix of shells the large aragonitic bivalves glycymeris and humilaria appear especially susceptible to sponge biodegradation and have become thoroughly bored and corroded within about 1000 years in ambient shelf waters thus their preservation potential is low selective loss of aragonitic biota from ancient cool water shelf skeletal carbonate deposits is anticipated and is evident in many temperate latitude occurrences of new zealand cenozoic limestones

methane derived high mg calcite submarine cement in holocene nodules from the fraser delta british columbia canada carbonate nodules and slabs in late holocene shelly terrigenous deposits of the modern fraser river delta 49 n are formed close to the seafloor by precipitation from saline pore waters of mainly fibrous to bladed crystals of high mg 10 20 mol mgco3 calcite cement as coalescing isopachous crusts on grains previous reports that the cement is low mg calcite are not supported by this study highly negative 13c values of 7 to 59 for the cements indicate that the bulk of their carbonate carbon was derived from the microbiological degradation of organic matter in the deltaic deposits during shallow burial in particular the production of biogenic methane ch4 by anaerobic bacterial fermentation its upward migration chemical or biological oxidation to co2 and neutralization in the near surface sediment and diffusion to microenvironments relatively enriched in organic components are a possible set of conditions influencing the process and sites of carbonate cementation methane derived mg calcite appears also to be the major submarine cement in several other modern occurrences of lithified shallow water terrigenous sands and muds at non tropical latitudes

climatic oceanic forcing on the organic accumulation across the shelf during the early cambrian age 2 through 3 in the mid upper yangtze block ne guizhou south china abstract the organic rich sediments were widely deposited over the entire yangtze block during the early cambrian late nemakit daldynian to botomian in the mid upper yangtze region northeastern guizhou south china they comprise in ascending order the niutitang jiumenchong and lower bianmachong formations which are dominated by black shales except the middle one characterized by interbedded shales limestones three third order depositional sequences are identified in the two studied sections located on the upper slope to basin of the open shelf the organic rich sediments were mostly deposited notably during transgressions on the shallower upper slope margin toc up to 25 wt where they are characterized by co increases in c p fe and ba concentrations indicating the highest organic productivity and coupled c p and fe cycling there in contrast in the shelf basin the concomitant organic rich sediments yield lower organic toc

solid speciation and fractionation of rare earth elements in a spodosol profile from northern sweden as revealed by sequential extraction a sequential extraction has been carried out on seven soil samples from a spodosol profile in till with granitic composition the soil profile was sampled in northern sweden five fractions were selected for extraction a ch 3coona extractable exchangeable adsorbed carbonate b na 4p 2o 7 extractable labile organics c 0 25 m nh 2oh hcl extractable amorphous fe oxyhydroxides mn oxides d 1 m nh 2oh hcl extractable crystalline fe oxides and e kclo 3 hcl extractable organics and sulphides extracted rare earth elements ree were determined with high resolution icp ms in addition to the soil stream water suspended particulate matter was also analysed for ree total concentrations in the soil samples show that the ree have been fractionated during weathering in the acidic ph 4 28 e horizon all ree are depleted relative to the unweathered till the depletion decreases with increasing atomic number also in the b horizon ph 5 86 the ree are depleted although to a lesser extent compared to the e horizon secondary phases in the b horizon fractionate the ree in different ways rare earth elements extracted in extractions a and c were enriched in the intermediate ree relative to heavy and light ree if normalized to local till in extractions b and d the heavy ree were enriched relative to the light ree and in extraction e a large enrichment of light ree relative to heavy ree was found stream water suspended particulate matter is enriched in all ree relative to local till the light ree are more enriched than the heavy ree normalized la lu ratios in the suspended matter ranged from 1 5 to 2 4 and were negatively correlated with stream water discharge as well as with content of detrital inorganic matter in the suspended load the only extracted phase that potentially can explain the high normalized la lu ratio in the suspended matter is the phase extracted in extraction e which has a ratio of 2 0 4 2 the normalized la lu ratio for the rest of the extractions ranged from 0 5 to 1 2 it is concluded that organic matter seems to be important for the particulate transport of ree in northern coniferous areas

maceration of calcareous skeletons in the december issue of sedimentology 1979 26 two papers on the disintegration of calcareous skeletons appeared in both articles the authors alexandersson 1979 fitzgerald parmenter milliman 1979 describe skeletal degradation producing fragments and even single calcium carbonate crystallites in the present marine environment at least part of this maceration alexandersson 1978 is attributed to the breakdown of the skeletal organic matrix on the sea bottom both studies were carried out on samples from the sediment surface of marine cold water regions there another destructive process is active etching and dissolution of the calcareous particles by the ambient carbonate under saturated water e g alexandersson 1975 1978 the possible coexistence of these biological and physicochemical destructive processes and the lack of information from cores have left open questions in the above mentioned two studies some of these questions were discussed by me lewy 1975 my observations and conclusions may now have a wider significance

chemical weathering in the drainage basin of a tropical watershed nsimi zoetele site cameroon comparison between organic poor and organic rich waters abstract this study deals with the weathering processes operating at the scale of a small catchment nsimi zoetele cameroon and is focused on the role of organic colloids on mineral weathering and transport of elements in natural waters samples of river spring and groundwaters from nsimi zoetele were filtered through membranes of decreasing pore size 0 22 m 0 025 m or 300 000 da 5000 da to separate colloidal fractions from the truly dissolved one major and trace elements and dissolved organic carbon doc were analysed in each fraction two kinds of waters can be distinguished in the catchment clear and coloured waters clear waters exhibit low concentrations of major and trace elements and doc elements are carried in these solutions in a true dissolved form except al and rare earth elements rees by contrast the higher abundances of al fe and trace elements in coloured waters are controlled by the colloidal fraction thermodynamic equilibrium calculations show that clear waters are in equilibrium with kaolinite and iron oxi hydroxide which are major minerals in the weathered soil for coloured waters the aqueous speciation of ca mg cu fe al la and th was calculated taking into account the complexes with humic acids speciation calculations for cu fe al la th show a strong complexation with humic acids in good agreement with the results of the filtration experiments by contrast although filtration experiments show a strong control of major cations by organic matter for example 75 for ca speciation calculations reveal that their complexes with humic ligands do not exceed a few percent of total dissolved elements this discrepancy is explained as an artefact induced by the organic colloids and occurring during the filtration procedure finally both filtration experiments and speciation calculations show that organic matter plays an important role in natural doc rich waters organic acids increase significantly the dissolution rates of silicates and oxi hydroxides and thus the amounts of solutes and of complexed elements leaving the catchment

a skeletal assemblage classification system for non tropical carbonate deposits based on new zealand cenozoic limestones abstract cenozoic limestones in new zealand are mainly skeletal grainstones and packstones formed under non tropical climatic conditions in open marine shelf or ramp environments following petrographic analysis of the nature and abundance of the skeletal components in nearly 500 samples of these limestones a complete linkage cluster analysis identified seven major skeletal assemblages that may be regarded as subdivisions of the single foramol skeletal association defined by lees and buller 1972 for temperate region carbonate deposits the seven assemblages are given contracted names as follows a barnamol barnacle bivalve dominated b bimol bivalve dominated c bryomol bryozoan bivalve dominated d echinofor echinoderm benthic foraminiferal dominated e nannofor nannofossil planktonic foraminiferal dominated f rhodalgal calcareous red algal dominated and g rhodechfor calcareous red algal echinoderm benthic foraminiferal dominated a composite triangular classification diagram has been devised for naming the skeletal assemblage of an unknown sample on the basis of its three main skeletal components the diagram successfully characterises more than 85 of the new zealand cenozoic limestone samples and also appears to be applicable for the skeletal assemblage designation of many overseas examples of non tropical carbonate deposits limitations relate mainly to locally common skeletal types e g serpulids brachiopods that are presently not incorporated into the new zealand based scheme the general ecological preferences of the main skeletal contributors in each of the seven skeletal assemblages form a basis for relating the assemblages to broad shelf habitats consequently as well as the benefits of providing a more consistent skeletal assemblage terminology for comparative studies between different workers the scheme can assist with the paleoenvironmental interpretation of non tropical skeletal carbonate facies

geomorphology of the gran sabana guayana shield southeastern venezuela abstract the forms of the gran sabana have been developed mainly on rocks of the gently folded precambrian roraima group a sequence of quartzites and siliceous conglomerates with intercalated jasper chert arkose siltstone and shale locally intruded by diabase dikes and sills two planation surfaces are preserved on the gran sabana the auyan tepui 2000 2900 m elevation and the wonken 900 1200 m surfaces with a speculative mesozoic initial age they are still extending table mountains dominate the landscape they comprise pediments foothills precipitous scarps hundreds of meters high a rim and a flattish summit the table mountain topography is controlled by joint systems water percolates along joints and bedding planes dissolves the siliceous cement the quartzite disaggregates and large blocks are isolated and collapse accumulating at the foot of the scarps and on top of the table mountain summits the percolating water forms large and intricate cave systems which frequently emerge in the scarp zone as high waterfalls these mark the lowest level of erosion of the table mountains karstic structures are prominent in the summit zone the evolution of the table mountain topography has been controlled by a combination of tectonic structural stratigraphic climatic and temporal factors

geochemistry of the amazon 1 precipitation chemistry and the marine contribution to the dissolved load at the time of peak discharge analyses of precipitation and surface water are used to estimate the fluxes of marine cyclic salts through that part of the amazon river system draining past obidos 80 of the basin at the time of peak discharge in june amazon precipitation chemistry can be devided into two principal components marine and terrestrial the marine component determined from analyses of marine rain consists of na k mg ca and cl in approximately sea salt proportions with s doubly enriched the excess sulfur is probably derived from gas phase inputs the terrestrial component makes an important contribution of k ca s and n much of which can be related to biological emissions the emission of reduced sulfur in the marine and terrestrial environment and nitrogen in the terrestrial environment is responsible for a natural acid rain in the amazon region with a ph from 4 7 to 5 7 this is about one tenth the acidity of polluted urban rain the chloride content of lowland rivers which drain regions lacking significant geologic sources of chloride shows a systematic decrease in chloride with increasing distance from the ocean this trend is used to define the cyclic salt background for amazonian surface waters cyclic salts in general make only a minor contribution relative to terrestrial inputs to the chemistry of amazon basin rivers even those draining intensely weathered terrains an estimated 17 6 cl 6 9 na 1 3 mg 3 6 s 0 4 k and 0 1 ca of the dissolved load at obidos during peak discharge is cyclic

lack of evidence for enhanced preservation of sedimentary organic matter in the oxygen minimum of the gulf of california comment and reply the role of bottom water oxygenation in the degradation and preservation of marine sedimentary organic matter is the subject of controversy two opposing theories have been proposed to explain the typically higher organic carbon concentrations found in laminated fully anoxic sediments and sedimentary rocks when compared to their bioturbated more oxygenated counterparts one theory holds that preservation of organic matter is enhanced in laminated sediments because of slower net rates of organic carbon decomposition under fully anoxic conditions e g cranfield 1989 1992 alternatively it has been suggested that organic rich laminated sediments reflect high levels of primary productivity in the surface waters and consequently increased proportions of organic matter in the particulate flux deposited at the water sediment interface rather than enhanced preservation in the sediments e g pedersen and calvert 1990 calvert et al 1992 compared data on modern surface sediments of the guaymas basin accumulating insides as well as outside the zone where the oxygen minimum impinges on the sea floor the choice of sediments deposited in close proximity minimizes the effects related to variations in the nature and mass flux or organic matter reaching the water sediment interface the data were collected on the first 2 5 cm of gravity cores with themore high sediment accumulation rates in the guaymas basin see calvert et al 1992 organic matter in the sediments analyzed has been subjected to early diagenetic processing for only short periods of time at most 10 20 years calvert et al reported the absence of systematic variations of the total organic carbon concentrations and rock eval hydrogen and oxygen indices with bottom water oxygen concentrations this led them to conclude that bottom water oxygen levels exert no major influence on the concentration and composition in terms of the h c and o c ratios of organic matter in the sediments 10 refs less

effects of early sea floor processes on the taphonomy of temperate shelf skeletal carbonate deposits abstract cool water shelf carbonates differ from tropical carbonates in their sources modes and rates of deposition geochemistry and diagenesis inorganic precipitation marine cementation and sediment accumulation rates are absent or slow in cool waters so that temperate carbonates remain longer at or near the sea bed early sea floor processes occurring between biogenic calcification and ultimate deposition thus take on an important role and there is the potential for considerable taphonomic loss of skeletal information into the fossilised record of cool water carbonate deposits the physical breakdown processes of dissociation breakage and abrasion are mediated mainly by hydraulic regime and are always destructive impact damage reduces the size of grains removes structure and therefore information and ultimately may transform skeletal material into anonymous particles abrasion is highly selective amongst and within taxa their skeletal form and structure strongly influencing resistance to mechanical breakdown dissolution and precipitation are the end members of a two way chemical equilibrium operating in sea water in cool waters inorganic precipitation is rare there is conflicting opinion about the importance of diagenetic dissolution of carbonate skeletons on the temperate sea floor but test maceration and early loss of aragonite in particular are reported dissolution may relate to undersaturated acidic pore waters generated locally by a combination of microbial metabolisation of organic matter strong bioturbation and oxidation of solid phase sulphides immediately beneath the sea floor in otherwise very slowly accumulating skeletal deposits laboratory experiments demonstrate that surface to volume ratio and skeletal mineralogy are both important in determining skeletal resistance to dissolution biological processes on the sea floor include encrustation and bioerosion encrustation a constructive process may be periodic or seasonal and can be reversed it produces both information and material bioerosion in contrast is destructive and permanent in temperate areas bioerosion may destroy even very large shells during their long residence at the sea floor on the order of hundreds to thousands of years overall processes on the temperate sea floor may combine to destroy more carbonate than they produce and the preservation potential of temperate shelf carbonate into the rock record may be significantly affected where preservation does occur in such a destructive regime the effects of early sea floor processes will be key determinants of the deposit resulting in a taphofacies characteristic of temperate shelf carbonate sediments

epipsammic diatoms as borers an observation on calcareous sand grains abstract sem examination of intertidal calcareous sand grains from the inlet of the tidal lagoon anse de kernic located on the northern coast of brittany indicates that some species of epipsammic diatoms are carbonate borers epipsammic diatoms that remain attached to the sand grains after washing are found in cavities and depressions some of them notably of the genus cocconeis are found in characteristically shaped cavities which are interpreted as made by the diatoms themselves the occurrence of the borings indicates that epipsammic diatoms can play a role in the weathering process of calcareous substrates

rapidity of marine carbonate cementation implications for carbonate diagenesis and sequence stratigraphy perspective abstract lithification is a rapid process but how rapid is rapid a year after one of my visits to joulter s cay bahamas i found a sardine can from my previous visit the cementation of carbonate particles in and around the can was surprising 382 g of ooids and skeletal material had filled the can and lithified therein or had become cemented to the outside of the can the rapidity of marine carbonate cementation implies that during highstand of sea level shallow water platform carbonate sediments are subject to marine lithification which retards erosion this conclusion is at variance with that of other workers who rely on freshwater cementation during lowstands to generate cement which retards reworking clearly the historical debate over highstand versus lowstand of shallow water marine carbonates is not yet over