early diagenetic silica deposition in algal cysts and spores a source of sand in black shales abstract devonian black shales deposited on the north american craton contain abundant tasmanites cysts although these are typically flattened because of compaction a small proportion are filled with diagenetic silica the latter are spherical to ellipsoidal 0 1 0 5 mm and filled with chalcedony microquartz and megaquartz and single quartz crystals chalcedonic cyst fillings are preserved best in cheat and phosphate nodules whereas megaquartz and single quartz crystals are most common in shale matrix together with colloform textures this suggests that the various silica types originated from recrystallization of early diagenetic silica deposits thin sandstone beds in the chattanooga shale e g bransford sandstone contain abundant quartz sand that is much coarser than the detrital quartz component of the underlying black shales because of this their quartz component is thought to have been transported over considerable distances from the basin margin however because certain shale horizons contain as much as 10 silicified cysts that upon reworking could have yielded quartz grains of fine to coarse sand size the quartz component of these sandstone beds may actually have formed in situ indeed petrographic examination of the sandstone beds shows them to contain quartz grains with morphological and textural features of cyst quartz e g rounding sphericity chalcedony pyrite inclusions lobate grain margins thus silica deposition in algal cysts may provide a significant component of intrabasinal quartz sand in shale sequences distinction of this type of quartz from extrabasinal detrital quartz is important to the reconstruction of the depositional history of shale sequences

the record of the messinian salinity crisis in the tertiary piedmont basin nw italy the alba section revisited abstract the alba succession tertiary piedmont basin nw italy preserves the northernmost record of the messinian salinity crisis msc and was deposited on the southern margin of a wide wedge top basin related to the involvement of the piedmont basin in the apennine compressional tectonics pre msc sediments consist of a cyclic succession of marine euxinic shales and calcareous marls deposited under the influence of precession modulated climate changes and document the progressive restriction of the basin prior to the onset of the msc they are followed by the primary lower gypsum unit plg deposited during the first msc stage from 5 96 to 5 60 ma these sediments show a clear precession related cyclic stacking pattern and record the lateral transition from a shallow water marginal setting in the sw to a deeper one in the ne in marginal settings six plg cycles are recognised truncated by an erosional unconformity placed at the base of the post evaporitic sediments the lowermost five cycles are composed of massive and banded selenite beds separated by thin shale intervals a sharp change evidenced by the appearance of the branching selenite facies is recorded by the 6th gypsum bed that represents a distinctive marker bed here called sturani key bed that can be mapped throughout the study area basinward the lower plg cycles are transitional to decimetre thick carbonate rich layers interbedded to euxinic shales that are overlain by the sturani key bed above the marker bed other seven plg cycles are present the gypsum beds form thinner bodies compared to the sturani key bed and are characterised by a greater amount of fine grained terrigenous fraction suggesting an increase of continental runoff related in turn to humid climate conditions at the end of the first msc stage plg cycles are followed by slumped mudstones and clastic gypsum beds that correspond to the resedimented and chaotic facies resedimented lower gypsum deposited in the mediterranean basins during the second msc stage from 5 60 to 5 55 ma they are in turn overlain by continental and brackish water facies with lago mare fossil assemblages recording the final stage of the msc the messinian succession of alba provides the opportunity to reconstruct the lateral facies transition between marginal and distal settings and to shed new light on the deep water msc sedimentary record moreover the appearance of the branching selenite facies from the 6th plg cycle upward provides a tool for properly placing the piedmont record in the msc chronostratigraphic framework allowing us to investigate the influence of climate gradients on the sedimentary response to the mediterranean salinity crisis

evaporitic and biosiliceous cyclic sedimentation in the miocene of the gulf of suez depositional and diagenetic aspects abstract the middle to late miocene evaporite bearing formations which outcrop along the gebel zeit and gems highs in the southwestern gulf of suez egypt consist of thick calcium sulphate beds rhythmically interbedded with marlstones siltstones and claystones containing various amounts of biogenic silica as well as pure diatomites the calcium sulphate beds are composed mainly of gypsum and anhydrite subaqueous crustallisation in subpermanent brine ponds of marine origin predominated during deposition of the selenite and laminated gypsum early diagenetic interstitial growth of nodular anhydrite occurred episodically when the water level dropped in response to increasing salinity associated with the precipitation of halite in the deeper troughs nevertheless most of the nodular facies resulted from the burial conversion of gypsum to anhydrite which has been sometimes rehydrated into gypsum a last phase of gypsum dehydration has occurred at the surface related to the present hot and dry climatic conditions the biosiliceous deposits contain either well preserved opal a or diagenetically altered opal ct clinoptilolite quartz and even clays diatom frustules such diagenetic changes are commonly described in the oceanic biosiliceous deposits the fossil content of these diatomitic layers implies shallow marine conditions on the shelves of highs induced by block faulting the cyclic succession of evaporited and biosiliceous deposits recorded the alternation of high sea level stands associated with high organic productivity probably enhanced by seasonal input of nutrients and low sea level stands associated with hypersaline conditions and evaporite deposition locally the sharp contact between the diatomites and gypsum argues for a rapid evolution of salinity towards high concentration up to gypsum saturation and more the formation of finely laminated diatomitic sediments is known to require either high organic productivity or anoxic conditions in bottom intermediate waters and eventually both processes in the gebel zeit and gemsa areas a local association of the biosiliceous sediments with diagenetic carbonates and native sulphur resulting from processes of bacterial sulphate reduction confirms that anoxic conditions occurred in the deep water body or in the sediments large amounts of organic compounds were involved in these diagenetic processes immature organic matter was consumed during an early diagenetic stage of transformation it is assumed that this organic matter was supplied either by the biosilica rich interbeds or the organic rich laminated carbonates the initial organic content of the biogenic deposits was impoverished as early as the first stages of sedimentation and diagenesis although a large part of the organic matter could have been destroyed by these early diagenetic processes and further oxidation in outcrops these sediments can be considered as potential source rocks in nearby less exposed sequences

origins and paleoceanographic significance of laminated muds from the ulleung basin east sea sea of japan abstract a detailed facies analysis of long piston core sediments from the ulleung basin reveals four mud facies laminated mud crudely laminated mud bioturbated mud and homogenous mud the laminated and homogeneous muds were most likely deposited from turbidity currents whereas the bioturbated and crudely laminated muds were formed by hemipelagic pelagic sedimentation under well oxygenated and poorly oxygenated bottom water conditions respectively a detailed analysis of vertical distribution of the mud facies together with a chronostratigraphic framework derived from correlative tephra layers with known eruption ages reveals paleoenvironmental changes during the late quaternary during the period between 49 and 23 ka eustatic lowering of sea level facilitated frequent slope failure resulting in repeated deposition of fine grained turbidites on the basin floor normal hemipelagic sediments were intensively bioturbated under well oxygenated bottom water conditions during the period between 23 and 13 to 11 ka deposition from turbidity currents continued but hemipelagic sediments were non bioturbated under poorly oxygenated bottom water conditions the bottom water deoxygenation is attributed to further lowering of sea level during the last glacial period and consequent intensified stratification in the water column by freshwater supply to the nearly isolated sea primary laminae preserved in the crudely laminated mud generally exhibit low compositional bimodality in backscattered electron images which suggests low surface primary productivity and presence of superficial micro bioturbation after rapid rise of sea level at 13 to 11 ka the slopes were stabilized and pelagic sedimentation has prevailed under well oxygenated bottom water conditions

biological forcing of hemipelagic sedimentary laminae evidence from odp site 893 santa barbara basin california core observation x radiography and scanning electron microanalysis of distinctly laminated indistinctly laminated and nonlaminated sediments from odp site 893 show that laminated as well as nonlaminated i e massive hemipelagic sediments can form in the absence of bioturbation this view contrasts with conventional sedimentological interpretations equating laminated hemipelagic sediments with anoxic water masses i e absence of bioturbation and nonlaminated sedimentary intervals exclusively with oxygenation episodes that permitted infaunal colonization we conclude that heterogeneities in the texture and or composition of sediment supply are necessary for the production of laminated sediments the absence of hydraulic and biological reworking permits their preservation laminae at site 893 have two prominent components biosilica and terrigenous detritus compositional contrasts between adjacent laminae in x radiographs reflect short term seasonal and subseasonal heterogeneities in sediment flux core intervals with large contrasts in density and or composition between adjacent laminae are termed as having high bimodality hb hb couplets produce distinctly laminated sediments that intergrade vertically with indistinctly laminated and nonlaminated sediments adjacent laminae in indistinctly laminated sediments possess minimal contrasts in bulk density and composition nonlaminated sediments are compositionally and texturally homogeneous many biosiliceous laminae responsible for distinctly laminated hb couplets record mass flocculation and sedimentation of ungrazed diatom frustules formed during discrete bloom events ecological interpretation of diatomite laminae suggests that many were effectively self sedimenting i e the production of gelatinous exudates by phytoplankton facilitated their aggregation and rapid sedimentation the results indicate that lamination style preserves meaningful ocean climate data concerning episodicity heterogeneity and export efficiency of biologically mediated sedimentary flux in upwelling dominated coastal ecosystems

cyclicities in triassic platform carbonates synchronizing radio isotopic and orbital clocks sedimentary cycles recorded in young sediments are often attributed to fluctuations of the earth s climate on a 10 4 10 6 year scale which in turn is governed by periodic variations in solar insolation linked to orbital milankovitch parameters a spectacular example of cyclic stratal patterns in ancient deposits is the middle triassic latemar carbonate platform w dolomites n italy based on spectral analyses from previous studies a superimposition of precession 20 ka and eccentricity 100 ka controlled sea level fluctuations has been suggested to account for the stacking hierarchy at latemar with 20 ka being assigned to each highest order depositional cycle zircon u pb isotopic ages from volcanic ash layers within the cyclic succession corroborated by biostratigraphic constraints suggest that the average time interval for every individual cycle is significantly smaller than the shortest milankovitch period and therefore challenge previously published interpretations relating distinct spectral peaks to the above mentioned hierarchy however our new spectral data indicate that cyclicities resembling milankovitch characteristics might exist but on an entirely different scale our findings show that frequency spectra should only be interpreted in combination with robust age control they also encourage the search for complementary mechanisms controlling carbonate deposition

varieties of a single cosmopolitan diatom species associated with surface water masses in the north pacific abstract a time series of 65 sediment trap samples representing 48 month changes in diatom flora and 62 sediment core tops from the western north pacific ocean were analyzed varieties of a single diatom species were found to be characteristic of two surface water masses bounded by the kuroshio current thalassionema nitzschioides var incurvata var inflata and var parva dominate in warm saline waters in the subtropical gyre while t nitzschioides sensu stricto is abundant in less saline waters northwest of this current time series data on percent abundances for the three t nitzschioides varieties in the t nitzschioides complex are compared with path variations of the current south of japan inter annual fluctuations in percent abundance are linked to changes in surface water masses over the sediment traps in association with migration of the kuroshio paths this finding makes the percent abundance a useful tool for deciphering path migrations of the kuroshio current in the geologic past down core analyses of two sediment cores collected beneath the modern kuroshio current were performed a sharp increase in percent abundances from 12 to 9 kyr bp with an abrupt change around 11 kyr bp records the previously reported transition of the kuroshio current from a predominantly offshore path to its modern path path migrations of the kuroshio current may be inferred from percent abundances of these diatoms which comprise a new proxy for delineating the paleoclimate of the northwestern pacific in relation to changes in northward heat transport from the western equatorial pacific ocean by the western boundary current

occurrence of thick ethmodiscus oozes associated with a terminal mid pleistocene transition event in the oligotrophic subtropical south atlantic abstract within generally calcareous sediment sequences layers of variable thickness of the giant diatom ethmodiscus were found in five cores recovered in the subtropical south atlantic between 23 and 33 s from both sides of the mid atlantic ridge two types of oozes occur almost monospecific layers of ethmodiscus and layers dominated by ethmodiscus with several accompanying tropical subtropical oligotrophic water diatoms the two thickest ethmodiscus layers occur in geob3801 6 around 29 s and accumulated during late mis 14 and mis 12 respectively downcore concentrations of ethmodiscus valves range between 3 4 104 and 2 3 107 valves g 1 we discuss the ooze formation in the context of migration of frontal systems and changes in the thermohaline circulation the occurrence of ethmodiscus oozes in sediments underlying the present day pelagic low nutrient waters is associated with a terminal event of the mid pleistocene transition at around 530 ka when the ocean circulation rearranged after a period of reduced nadw production

sedimentary stable isotope and micropaleontological records of paleoceanographic change in the messinian tripoli formation sicily italy abstract the tripoli formation 6 96 5 98 ma of the central sicilian basin provides a good record of the paleoceanographical changes that affected the mediterranean during the transition from slightly restricted conditions to the onset of the messinian salinity crisis the falconara gibliscemi section has been selected for an integrated approach at a high resolution scale using sedimentology stable isotopes of the carbonates and microfossils the sedimentary succession includes 46 precession controlled cycles resulting from the periodical increase in biosiliceous productivity diatomites that followed the deposition of marls and pinkish laminites which appear as sapropel type deposits induced by the oceanic fertilization by terrestrial nutrients during wet periods higher scale environmental changes are superimposed to this precession forced rhythmicity there is a general trend of increasing basin restriction from near marine conditions at the base of the tripoli to semi closed settings in its uppermost part which are the prelude of the salinity crisis this pattern reflects the hydrological response of the mediterranean to the progressive decrease of the atlantic inputs and an enhanced influence of the climate on depositional conditions however this evolution is not linear and shows successive phases of different duration during the first period until 6 71 ma open atlantic mediterranean exchanges maintained relatively stable marine conditions the second period 6 71 6 29 ma marks an important step in the basin restriction with a wider range of salinity fluctuations and an increased bottom stagnation the 6 71 ma event which is correlated at a mediterranean scale may have resulted from shallowing of the mediterranean gateway under a tectonic control this shallowing reduced the oceanic inputs resulting in an increased climatic constraint of the mediterranean hydrology during the third period 6 29 6 03 ma an increase of the surface water salinity resulted in stressful conditions for the marine microfauna the 6 29 ma change is a major step in the restriction that may be correlated with the intensification of the glaciation recorded in the atlantic which could have enhanced the effects of the tectonic closure the last two cycles 48 and 49 that underlie the calcare di base witnessed the rapid transition to a semi closed mediterranean setting characterized by large variations of salinity from diluted to hypersaline conditions under a dominant climatic control and by the nearly complete disappearance of the marine organisms long trend environmental changes recognized within the tripoli formation resulted from a complex set of interfering factors controlling the water fluxes exchanged between the mediterranean and the world ocean most of the rapid changes identified in falconara gibliscemi at 7 16 6 71 and 6 29 ma that occurred simultaneously in the western and eastern mediterranean were mainly controlled by the stepwise tectonic closure of the atlantic connections although a glacio eustatic overprint cannot be completely excluded

a model for carbonate to terrigenous clastic sequences the development of carbonate sequences that grade toward a source region into terrigenous clastic sediment is poorly understood in both carbonate and terrigenous clastic regimes environments of rapid sedimentation have comparable rates of deposition 102 to 102 5 cm 1 000 yr slowest continuous rates are also comparable 100 5 to 101 cm 1 000 yr because of the inhibiting effect of terrigenous deposition on carbonate skeletal production the slowest rates in a transect tend to occur where mixed lithologies are deposited midway between coarse clastic and pure carbonate sediments this leads to a bimodal distribution of rapid depositional rates and invariably to the development of a basin in the intervening area to develop a model we combine concepts of depositional rate with knowledge of the middle ordovician carbonate to terrigenous clastic sequence of the southern appalachians deposition began on a shallow water ramp sloping gently eastward but everywhere within the photic zone an episode of tectonic subsidence differentiated this ramp into a shallow shelf on the northwest and a deep starved basin on the southeast separated by a linear shelf margin with organic build ups subsequent development of the sequence consisted of the filling of the deep basin first with pelagites then with turbidites during which the carbonate shelf was isolated from terrigenous deposition once the basin filled terrigenous deposition began to affect the shelf inhibition of carbonate deposition near the shelf edge led to formation of successively smaller basins within the old shelf area after the initial pulse of tectonic subsidence deposition of the sequence can be explained by sediment loading subsidence thus the ordovician sequence coupled with the concept of bimodal distribution of sedimentation rates in carbonate to terrigenous clastic facies patterns allows development of a model for basins adjacent to active tectonic lands five model stages are derived by application of calculated deposition rates and sediment loading along a transect away from the terrigenous source these stages are 1 an initial carbonate ramp stage followed by 2 a carbonate shelf starved deep basin stage 3 a carbonate shelf turbidite basin terrigenous shelf stage 4 a carbonate shelf narrowed and shallowed basin broad terrigenous shelf stage and finally 5 a stage in which the initial basin is filled a subsequent starved basin forms on the old carbonate shelf and a new carbonate shelf margin forms a few to tens of kilometres farther from the terrigenous source the model should be useful in predicting depositional facies in analogous basins

late cretaceous seasonal ocean variability from the arctic palaeoclimate records for the arctic ocean and its environs in the late cretaceous greenhouse era 65 99 million years ago are sparse using electron microscope techniques applied to cores of shallow buried cretaceous laminated sediments from the alpha ridge of the arctic ocean davies et al have obtained a seasonally resolved sediment record from the cretaceous that permits a reconstruction of the annual cycle of marine productivity and flux to the sediment they find that the period was characterized by seasonal diatom productivity in a stratified ocean with possible winter sea ice cover although the modern arctic ocean is regarded as a barometer of global change and an amplifier of global warming little is known about its state in the greenhouse period of the late cretaceous epoch 65 99 million years ago here a seasonally resolved cretaceous sedimentary record from the alpha ridge of the arctic ocean is presented evidence suggests seasonal diatom productivity in a stratified ocean with possible winter sea ice cover the modern arctic ocean is regarded as a barometer of global change and amplifier of global warming1 and therefore records of past arctic change are critical for palaeoclimate reconstruction little is known of the state of the arctic ocean in the greenhouse period of the late cretaceous epoch 65 99 million years ago yet records from such times may yield important clues to arctic ocean behaviour in near future warmer climates here we present a seasonally resolved cretaceous sedimentary record from the alpha ridge of the arctic ocean this palaeo sediment trap provides new insight into the workings of the cretaceous marine biological carbon pump seasonal primary production was dominated by diatom algae but was not related to upwelling as was previously hypothesized2 rather production occurred within a stratified water column involving specially adapted species in blooms resembling those of the modern north pacific subtropical gyre3 or those indicated for the mediterranean sapropels4 with increased co2 levels and warming currently driving increased stratification in the global ocean5 this style of production that is adapted to stratification may become more widespread our evidence for seasonal diatom production and flux testify to an ice free summer but thin accumulations of terrigenous sediment within the diatom ooze are consistent with the presence of intermittent sea ice in the winter supporting a wide body of evidence for low temperatures in the late cretaceous arctic ocean6 7 8 rather than recent suggestions of a 15 c mean annual temperature at this time9

the abad composite se spain a messinian reference section for the mediterranean and the apts a high resolution integrated stratigraphy is presented for the abad marls of the sorbas and nijar basins in se spain preevaporitic messinian of the western mediterranean detailed cyclostratigraphic and biostratigraphic analyses of partially overlapping subsections were needed to overcome stratigraphic problems in particular encountered at the complex transition from the lower to the upper abad the resulting abad composite section contains a continuous stratigraphic record from the tortonian messinian boundary up to the transition to the messinian evaporites of the yesares member all together 18 calcareous plankton events were recognized which were shown to be synchronous throughout the mediterranean by means of detailed bed to bed cyclostratigraphic correlations the magnetostratigraphy allowed the identification of the four magnetic reversals of chron c3an in the upper abad details in the sedimentary cycle patterns allowed the abad composite to be astronomically calibrated this calibration to the 658n summer insolation curve of solution la90 1 1 yielded astronomical ages for all sedimentary cycles calcareous plankton bioevents ash layers and paleomagnetic reversals up to now the abad composite is the only astronomically well calibrated section that provided a reliable cyclostratigraphy magnetostratigraphy and calcareous plankton biostratigraphy as such it will serve as a reference section both for the pre evaporite messinian in the mediterranean as well as for the messinian interval in the astronomical polarity time scale q 2001 elsevier science b v all rights reserved

sequence stratigraphy of carbonate evaporite basins models and application to the upper permian zechstein of northeast england and adjoining north sea the successions of carbonate evaporite basins which are periodically cut off from the world ocean are considered in terms of their sequence stratigraphy two basin types are distinguished those where incomplete drawdown takes place and those showing complete drawdown and wholesale basin desiccation it is argued that following relative sea level falls sequences begin with the evaporites precipitated as marginal lowstand gypsum wedges in the first basin type and as lowstand basin centre halite fills in the second the evaporites pass up into carbonate sediments which are mainly deposited on shallow water platforms around the basin during transgressive and highstand systems tracts the concepts and models developed are applied to the zechstein upper permian strata of northeast england and adjoining north sea seven sequences are proposed with some containing thin parasequences the sequence approach to carbonate evaporite basins which considers both basin margin and basin centre facies gives a more encompassing perspective of sedimentation than the traditional cycle concept with its emphasis on the basin centre

miocene intensification of upwelling along the california margin as recorded in siliceous facies of the monterey formation and offshore dsdp sites

the fall dump a new perspective on the role of a shade flora in the annual cycle of diatom production and export flux investigations of diatom fluxes recorded in laminated sediments using scanning electron microscope techniques together with evidence from sediment trap studies have contributed to a reappraisal of the annual cycle of diatom production and export flux we propose that where there is a strong seasonal thermocline and nutricline a number of diatoms hitherto regarded as typical sparse summer flora characteristic of oligotrophic waters are able to generate substantial production at depth these species including rhizosolenia spp stephanopyxis palmeriana thalassiothrix spp and some coscinodiscus spp may represent a shade flora that have adapted to grow in low light conditions and or to regulate their buoyancy to move between a deep nutrient source and the euphotic zone although rates of growth and primary production are substantially lower than species characteristic of spring bloom or upwelling conditions the total primary production integrated over the several month period of summer stratification may be as significant as the spring bloom or greater the term fall or autumn bloom as a counterpart of the spring bloom is therefore a misnomer whereas the spring bloom involves a rapid burst of reproduction and sedimentation the fall dump is the sedimentation of a long lived episode of production lasting the duration of the seasonal thermocline and triggered by the fall winter mixing that breaks down stratification the fall dump may produce as much or in some cases more export production than the spring bloom the results of this study suggest that a reorientation of thinking on diatom ecology and palaeoecology may be required

interactive aggregation and sedimentation of diatoms and clay sized lithogenic material monospecific diatom cultures thalassiosira punctigera and skeletonema costatum were incubated in rotating cylinders together with clay suspensions present in a range of concentrations 5 100 mg kaolinite l 21 and as different minerals 50 mg l 21 kaolinite smectite illite and clay sized quartz powder the addition of lithogenic suspensions to diatom cultures accelerated the formation of visible aggregates in the roller tanks by a factor of 3 aggregate size decreased and density increased proportionally to the amount of kaolinite added to the diatom cultures in the presence of kaolinite and illite aggregate sizes were smaller and sinking rates lower than in the presence of smectite and quartz the influence of lithogenic matter on the sinking velocities of aggregates was ambiguous compound aggregates sank faster with increasing amounts of lithogenic matter present in cultures of t punctigera until a certain ratio between lithogenic and biogenic material was reached further increasing the amount of lithogenic matter did not increase sinking rates significantly in contrast increasing the concentration of kaolinite added to cultures of s costatum could decrease sinking velocities of the evolving compound aggregates this nonlinear behavior is argued to be primarily a function of aggregate composition on aggregate sizes and excess densities although the possibility of a mutual acceleration of vertical flux of algae and clay is confirmed the results show that the presence of lithogenic material could also decrease the downward flux of phytoplankton biomass rapid sinking of phytoplankton derived organic carbon via sedimentation of aggregates contributes significantly to the vertical transport of marine particulate organic carbon poc a process known as the biological pump volk and hoffert 1985 rapid sinking of clay sized lithogenic material can be inferred from sediment trap data wefer 1989 laboratory studies on marine aggregate formation and properties have focused on pure phytoplankton aggregates kiorboe et al 1990 riebesell et al 1991 kiorboe and hansen 1993 or on monomineralic clay aggregates kranck 1973 however coaggregation of phytoplankton and clay is a familiar phenomenon in aquatic environments first drawings of marine snow show sundry marine organisms and mineral grains embedded in a mucous matrix which dominates the aggregates by volume and appears to hold the solid compounds together tsujita 1953 avnimelech et al 1982 described enhanced floc formation and sedimentation of freshwater algae in the presence of clay and vice versa the fact that lithogenic and biogenic particles are often found in deep sediment traps during the same time interval in spite of independent occurrence in the surface water also suggests coaggregation and cosedimentation of these particles deuser et al 1983 wefer 1989 ittekkot 1993 while small amounts of both biogenic and lithogenic particles are present in all marine environments areas with elevated concentrations of lithogenic particles often show a high phytoplankton abundance at the same time and are thus particularly interesting in terms of interaction between phytoplankton and lithogenic material examples for marine en

silicate weathering in anoxic marine sediments two sediment cores retrieved at the northern slope of sakhalin island sea of okhotsk were analyzed for biogenic opal organic carbon carbonate sulfur major element concentrations mineral contents and dissolved substances including nutrients sulfate methane major cations humic substances and total alkalinity down core trends in mineral abundance suggest that plagioclase feldspars and other reactive silicate phases olivine pyroxene volcanic ash are transformed into smectite in the methanogenic sediment sections the element ratios na al mg al and ca al in the solid phase decrease with sediment depth indicating a loss of mobile cations with depth and producing a significant down core increase in the chemical index of alteration pore waters separated from the sediment cores are highly enriched in dissolved magnesium total alkalinity humic substances and boron the high contents of dissolved organic carbon in the deeper methanogenic sediment sections 50 150 mg dm 3 may promote the dissolution of silicate phases through complexation of al3 and other structure building cations a non steady state transport reaction model was developed and applied to evaluate the down core trends observed in the solid and dissolved phases dissolved mg and total alkalinity were used to track the in situ rates of marine silicate weathering since thermodynamic equilibrium calculations showed that these tracers are not affected by ion exchange processes with sediment surfaces the modeling showed that silicate weathering is limited to the deeper methanogenic sediment section whereas reverse weathering was the dominant process in the overlying surface sediments depth integrated rates of marine silicate weathering in methanogenic sediments derived from the model 81 4 99 2 mmol co2 m 2 year 1 are lower than the marine weathering rates calculated from the solid phase data 198 245 mmol co2 m 2 year 1 suggesting a decrease in marine weathering over time the production of co2 through reverse weathering in surface sediments 4 22 15 0 mmol co2 m 2 year 1 is about one order of magnitude smaller than the weathering induced co2 consumption in the underlying sediments the evaluation of pore water data from other continental margin sites shows that silicate weathering is a common process in methanogenic sediments the global rate of co2 consumption through marine silicate weathering estimated here as 5 20 tmol co2 year 1 is as high as the global rate of continental silicate weathering

unconventional reservoir potential of the upper permian zechstein group a slope to basin sequence stratigraphic and sedimentological evaluation of carbonates and organic rich mudrocks northern germany the late permian zechstein group in northeastern germany is characterized by shelf and slope carbonates that rimmed a basin extending from eastern england through the netherlands and germany to poland conventional reservoirs are found in grainstones rimming islands created by pre existing paleohighs and platform rimming shoals that compose steep margins in the north and ramp deposits in the southern part the slope and basin deposits are characterized by debris flows and organic rich mudstones lagoonal and basinal evaporites formed the seal for these carbonate and underlying sandstone reservoirs the objective of this investigation is to evaluate potential unconventional reservoirs in organic rich fine grained and or tight mudrocks in slope and basin as well as platform carbonates occurring in this stratigraphic interval therefore a comprehensive study was conducted that included sedimentology sequence stratigraphy petrography and geochemistry sequence stratigraphic correlations from shelf to basin are crucial in establishing a framework that allows correlation of potential productive facies in fine grained organic rich basinal siliceous and calcareous mudstones or interfingering tight carbonates and siltstones ranging from the lagoon to slope to basin which might be candidates for forming an unconventional reservoir most organic rich shales worldwide are associated with eustatic transgressions the basal zechstein cycles z1 and z2 contain organic rich siliceous and calcareous mudstones and carbonates that form major transgressive deposits in the basin maturities range from over mature gas in the basin to oil generation on the slope with variable toc contents this sequence stratigraphic and sedimentologic evaluation of the transgressive facies in the z1 and z2 assesses the potential for shale gas oil and hybrid unconventional plays potential unconventional reservoirs might be explored in laminated organic rich mudstones within the oil window along the northern and southern slopes of the basin although the zechstein z1 and z2 cycles might have limited shale gas potential because of low thickness and deep burial depth to be economic at this point unconventional reservoir opportunities that include hybrid and shale oil potential are possible in the study area

opal sedimentation shifts in the world ocean over the last 15 myr biogenic silica opal accumulation records were used to trace mechanisms consequence and geographic pattern of shifts in the main locus of opal deposition of the world ocean over the last 15 myr over this time interval the main opal sink seems to have moved from the north atlantic to the pacific equatorial pacific eastern equatorial pacific eastern boundary current upwelling systems california namibia peru and finally to the southern ocean the interplay between opal deposition and a series of climatic tectonic oceanographic and biologic events has been analyzed and discussed these events include the cenozoic global cooling trend intensified glaciation in antarctica late miocene early pliocene biogenic bloom development of northern hemisphere glaciation nhg closing of the panama seaway transition of the climate system from a monopolarto a bipolar glaciated world mid pleistocene revolution mpr nutrient availability evolution of diatoms and c4 plants and changes in continental weathering rates while the observed shifts are mostly traceable to oceanic reorganizations and global climatic evolution conditions favorable to opal deposition involve the above mentioned complex mix of processes for this reason the interpretation of opal deposition records might not always be straightforward we however believe that it can still provide clear indications of large scale oceanographic reorganizations in the geological past d 2004 elsevier b v all rights reserved

western versus eastern mediterranean paleoceanographic response to astronomical forcing a high resolution microplankton study of precession controlled sedimentary cycles during the messinian abstract planktic foraminifera and stable isotopes were analyzed in samples from four precession controlled sedimentary cycles from southeastern spain and the island of gavdos greece to reconstruct the paleobiologic and paleoceanographic response of the western and eastern mediterranean to astronomically driven climate variability during the messinian although the lithological succession in the western mediterranean cycles sapropel homogeneous marl diatomite homogeneous marl is different from that in the eastern mediterranean sapropel diatomite homogeneous marl the pelagic biological succession is similar four stages were recognized in both environments characterized by the dominance of warm oligotrophic foraminifera stage 1 neogloboquadrinids and globorotaliids stage 2 globigerina bulloides stage 3 and globigerinita glutinata stage 4 the first stage occurs during deposition of the upper part of the sapropels in spain and the sapropels and part of the diatomites on gavdos this stage is inferred to relate to high summer insolation and strong surface water stratification with winter temperatures not high enough to allow eutrophic foraminifera growth stage 2 occurs during the deposition of the lower homogeneous marls in spain and the middle upper part of the diatomites in greece most likely indicating stratification with colder winter temperatures and a nutricline above the euphotic layer the end of surface water stratification is indicated during stage 3 by the replacement of the neogloboquadrinids by g bulloides which occurs at the base of the diatomites in spain and near the top of the diatomites in greece we link this event to the appearance of dryer climates in the mediterranean at times of relatively low summer insolation in the northern hemisphere which would result in higher surface salinities deep water convection and deep water ventilation stage 4 defined by the dominance of g glutinata and low concentrations of planktic foraminifera appears linked to the base of the sapropels both in the western and eastern mediterranean the onset of sapropel deposition occurred at a time of relatively high surface water salinities as suggested by the heavy 18o in orbulina universa and prior to the development of surface water stratification evident in the subsequent increase in warm oligotrophic foraminifera gavdos diatomites were deposited under anoxic conditions whereas sorbas diatomites were formed in an oxygenated environment the difference between western and eastern mediterranean diatomites would be due to differences in hydrographic conditions between the two studied settings

microbioirrigation of marine sediments in dysoxic environments implications for early sediment fabric formation and diagenetic processes it is manifest in the study of dysoxic sediments from the geological record that infaunal burrowing is considered so severely limited by the lack of dissolved oxygen as to be nonexistent although the effects of megafauna and macrofauna on sedimentary and geochemical processes are well known the effects of meiofauna are largely ignored here we document abundant meiofauna in the recent severely dysoxic laminated sediments from the santa barbara basin california margin and also microcavities and microtunnels in laminated deglacial sediments from palmer deep west antarctic peninsula that we interpret to be open relict nematode burrows santa barbara basin box core subcores were sieved to quantify metazoan abundance and others were embedded with resin for examination of meiofaunal life positions using confocal microscopy metazoan densities in the surface centimeters of sediment range from 80 7 to 117 9 cm 3 and nematode populations together with their abundant burrows remain quite high to at least 3 cm scanning electron microscope analysis of fractured surfaces in palmer deep sediments revealed that the rigid diatom ooze framework aids the preservation of 50 m diameter open nematode burrows these structures were observed to at least 40 m below the seafloor surface this is the first description of a nematode produced open burrow network preserved in the geological record optical microscopy of resin embedded thin sections revealed widespread sediment redistribution without significant lamina disruption the implications of abundant nematode burrows in surface sediments and their preservation in the geological record are wide ranging for both modern and ancient dysoxic marine environments including for determining early sediment fabric production geochemical processes and diagenetic reactions in the oxic and suboxic zones

structural setting of western monferrato alps apennines junction zone nw italy new data on the structural setting of the torino hill and western monferrato domains provide a clearer picture of the kinematic evolution of the alps apennines junction zone in the alps apennines junction left lateral underthru sting of ligurian units below alpine crust has often been in voked to explain the underthrusting of the insubric domain and the nw ward movement of the adriatic indenter a transpres sive fault system rio freddo fault zone developed in the monferrato epi ligurian succession is here understood as the surficial expression of a deep seated thrust along which the alpine metamorphic basement of the torino hill has overrid den the apenninic ligurian nappes since the paleogene up to burdigalian in the western monferrato the regional sinistral transpression differentiated separated crustal blocks during the oligocene early miocene and kilometer thick successions we re steepened by nnw sse faults that acted as reverse and or left lateral faults double vergent se nw low angle shear zones then cut the rio freddo fault zone in post langhian ti mes they have been here related with the eastward tectonic transport of the torino hill and relative alpine crust toward the monferrato domain which occurred before the seismic line scale middle miocene unconformity was established this po ses the problem of the exact age of this regional unconformi ty which in our case should be at least serravallian finally the monferrato and torino hill domains were coupled and both were nw ward translated by the padan thrust front in the pliocene the youngest strike slip and reverse faults of the western monferrato may be tear or compartmental faults that partitioned the padan thrust motion whose propagation could be controlled by pre middle miocene tectonic structures the structural complexity of the study area is here understood as the inheritance of deep crosscutting regional structures that forced the convergent structural units of the alpine and insubric do mains to follow different tectonic transport directions since the oligocene

are the large filamentous microfossils preserved in messinian gypsum colorless sulfide oxidizing bacteria the thick gypsum deposits formed in the mediterranean basin during the messinian salinity crisis incorporate dense mazes of filamentous fossils which were interpreted as algae or cyanobacteria thus pointing to a shallow marine subtidal or intertidal environment the data presented here reveal that these filaments represent remains of colorless vacuolated sulfide oxidizing bacteria this interpretation is supported by the presence of small crystal aggregates of iron sulfide pyrite and associated polysulfide within the filamentous fossils pyrite and polysulfide are considered to result from early diagenetic transformation of original zero valent sulfur globules stored within the cells which is a clade diagnostic feature of living and degraded sulfur bacteria in addition to filamentous fossils the studied gypsum crystals contain remains of euryhaline and stenohaline diatoms and clay rich aggregates interpreted as alteration products of marine snow floccules this peculiar fossil assemblage reflects conditions of increased productivity in the water column triggered by high fluxes of nutrients into the basin during phases of enhanced riverine runoff and freshwater discharge this study confirms that gypsum evaporites have great potential to preserve the early stages of the taphonomic alteration of bacterial cells shedding light on the paleoecology of ancient hypersaline environments

sequence stratigraphy and cyclic development of basal zechstein carbonate evaporite deposits with emphasis on zechstein 2 off platform carbonates upper permian northeast germany abstract although the basal zechstein is classically subdivided into two cycles z1 and z2 lower part it can alternatively be subdivided into three complete third order depositional sequences zs1 zs3 and the associated lowstand systems tract transgressive systems tract of a fourth sequence zs4 third and higher order sea level fluctuations influenced the depositional and diagenetic history of the carbonate evaporite alternations within the basal zechstein the main subject of the present study the zechstein 2 carbonate stassfurt carbonate or ca2 is interpreted to encompass both transgressive predominantly in slope and basinal settings and highstand systems tracts of zechstein sequence zs3 as well as part of the lowstand systems tract of the fourth zechstein sequence zs4 in northeastern germany hydrocarbon reservoirs of the ca2 developed along both northern and southern margins of the southern permian basin on top of the topography of the zechstein 1 cycle z1 this z1 topography resulted from deposition of the werra anhydrite a1 the highstand systems tract of the second zechstein sequence zs2 the northern ca2 carbonate platform is consistently narrow and fringed by coated grain and composite grain bars and shoals it is characterized by a steep fault controlled slope which precluded the formation of in situ shallow water grain dominated carbonates during relative sea level lowstands displaced shallow water coated grain and composite grain packstones wackestones may however occur as slope fan deposits turbidites in contrast the southern carbonate platform is wide tectonically dissected and fringed by grain dominated shallow water carbonates coated grain and composite grain bars and shoals zechstein 2 shallow water carbonates detectable by basal zechstein seismic anomalies also developed on pre existing topographical highs in off platform positions three depositional cycles ca2 cycles i ii and iii most probably reflecting high amplitude fourth order sea level fluctuations can be identified within the stassfurt carbonate our observations suggest that sea level variations possibly reflecting cyclicity in the milankovitch frequency band 0 1 0 4 ma and the pre zechstein topography pronounced during formation of the werra anhydrite were the most important factors in controlling the depositional as well as the diagenetic history of the zechstein 2 carbonate in off platform positions

the anatomy of a sulphate platform and adjacent basin system in the leba sub basin of the lower werra anhydrite zechstein upper permian northern poland the lower werra anhydrite zechstein upper permian deposits of the teba area originated in a deep basin setting in shallow to deep water conditions facies changes occur within small distances and suggest fluctuating boundaries between well defined basins and platforms this pattern of local platforms and adjacent basins developed during deposition in basinal areas the sequence is clearly transgressive whereas on platforms accumulation kept pace with subsidence after an initial transgression nodular anhydrite represents a polygenetic deposit which formed at different times with respect to deposition massive anhydrite with pseudomorphs after upright growth gypsum crystals suggest rapid precipitation in a subaqueous environment and or fluctuating but generally high salinity conditions massive clastic sulphate originated due to periodic high energy events and resedimentation or due to brecciation possibly connected with salinity fluctuations and the dissolution of halite massive textureless anhydrite is locally porous and passes upward into breccia indicating a strongly saline environment bedded anhydrite is considered to form in shallow water environments and laminated anhydrite in deep water bedded anhydrites contain portions which are graded intercalations of sulphate turbidites and upright growth gypsum suggest fluctuating water depths with comparatively deep water during turbidite deposition but shallower conditions during upright growth gypsum deposition the sequence observed in slope zones at platform basin margins detrital parautochthonous sulphate sand to graded beds to basinal laminites indicates that redistribution processes were important at the onset of the lower werra anhydrite deposition bathymetric relief existed between the central part of the basin and its margins where carbonate platforms remained subaerially exposed formation of local platforms and adjacent basins required a relatively high subsidence rate as pre existing relief cannot account for the total accumulated thickness of the lower werra anhydrite deposits one implication of this is that the main argument against the shallow water shallow basin evaporite basin model i e a very fast rate of subsidence may not be valid for the eba lower werra anhydrite basin

depositional processes of the zhushadong and mantou formations early to middle cambrian shandong province china roles of archipelago and mixed carbonate siliciclastic sedimentation on cycle genesis during initial flooding of the north china platform to understand the depositional processes and environmental changes during the initial flooding of the north china platform this study focuses on the lower to middle cambrian zhushadong and mantou formations in shandong province china the succession in the jinan and laiwu areas comprises mixed carbonate and siliciclastic deposits composed of limestone dolostone stromatolite thrombolite purple and grey mudstone and sandstone a detailed sedimentary facies analysis of seven well exposed sections suggests that five facies associations are the result of an intercalation of carbonate and siliciclastic depositional environments including local alluvial fans shallowing upward carbonate siliciclastic peritidal cycles oolite dominant shoals shoreface and lagoonal environments these facies associations successively show a transition from an initially inundated tide dominated carbonate platform to a wave dominated shallow marine environment in particular the peritidal sediments were deposited during a large number of depositional cycles these sediments consist of lime mudstone dolomite stromatolite and purple and grey mudstones these shallowing upward cycles generally resulted from carbonate production in response to an increase of accommodation during rising sea level the carbonate production was however interrupted by frequent siliciclastic input from the adjacent emergent archipelago the depositional cycles thus formed under the influence of both autogenetic changes including sediment supply from the archipelago and allogenic control of relative sea level rise in the carbonate factory a low relief archipelago with an active tidal regime allowed the development of tide dominated siliciclastic and carbonate environments on the vast platform siliciclastic input to these tidal environments terminated when most of the archipelago became submerged due to a rapid rise in sea level this study provides insights on how a vast cambrian carbonate platform maintained synchronous sedimentation under a tidal regime forming distinct cycles of mixed carbonates and siliciclastics as the system kept up with rising relative sea level during the early stage of basin development in the north china platform

productivity generated annual laminae in mid pliocene sapropels deposited during precessionally forced periods of warmer mediterranean climate abstract paleoproductivity estimates for a sequence of five mid pliocene mediterranean sapropels illustrate the importance of insolation maxima in enhancing organic carbon accumulation well laminated sapropelitic intervals in the northern apennines have been studied by a combination of sedimentological and micropaleontological analysis detailed electron microscope description bulk chemical composition carbon isotopic composition and elemental ratios each sapropel formed during precessional minima lasted 7 5 to 10 kyr which is the same duration calculated for coeval counterparts in odp sites in the eastern mediterranean the organic carbon mass accumulation rates of the studied sapropels show the same values of those calculated in the coeval mediterranean sapropels suggesting that the same productivity conditions were controlling sapropel formation in the whole mediterranean despite the differences in depositional setting and the strong variations in sedimentation rate mat forming diatoms play an important role in increasing the settling velocity allowing rapid sinking of organic matter and preventing bacterial remineralization in the water column the consequent partial oxygen depletion at the seafloor increases preservation which is therefore considered an effect rather than a cause of the organic carbon accumulation

paleoecological significance of laminated diatomaceous oozes during the middle to late pleistocene north atlantic ocean iodp site u1304 abstract we examined diatom assemblages in a series of remarkable laminated diatomaceous ooze ldo horizons in the marine sediments from integrated ocean drilling program iodp site u1304 to reconstruct the middle to late pleistocene paleoceanographic evolution of the northern north atlantic ocean four confirmed diatom biohorizons combined with calcareous nannofossil and paleomagnetic stratigraphies established the chronological framework for the material the planktonic araphid needle like species thalassiothrix longissima was the greatest contributor to the ldo facies from the results of a principal component analysis using the percent abundances of 65 significant p 5 diatom taxa except for tx longissima which was extremely dominant in almost all horizons observed we identified two principal component pc axes taxa probably associated with the stratigraphic distribution of the major zonal marker neodenticula seminae ranging from 1 26 to 0 84 ma in this ocean loaded on pc1 with a high value pc2 was related to the ocean surface temperature the stratigraphic variability of the pc2 score indicated that switching between warm and cold water assemblages occurred concurrently with ldo deposition or extreme tx longissima dominance episodes in several horizons particularly after 0 84 ma suggesting that the subarctic convergence sac oceanic front passed over site u1304 during pleistocene glacial interglacial cycles our floral evidence supports the model of nearly monospecific ldo formation caused by the enhanced physical accumulation of particular diatoms such as tx longissima on the other hand nd seminae which probably contributes to spring phytoplankton blooms in the modern ocean was present only between 1 26 and 0 84 ma in this area thus we infer that the main contributor of export flux in the regional annual primary production cycle would have shifted drastically from one of a spring phytoplankton bloom leader nd seminae to minor but mass dump assemblages tx longissima etc in the mid pleistocene

diatoms in northeast pacific surface sediments as paleoceanographic proxies abstract fossil diatom total abundances valves g in 54 surface sediment samples from the northeast ne pacific ocean reflect the position of high primary production associated with coastal upwelling and that possible biases associated with dilution or dissolution are small diatom species assemblages defined by q mode factor analysis in 30 samples with abundant diatoms are related to modern oceanographic properties five statistical assemblages given by five specific diatom species and or groups are related to upwelling chaetoceros spores subtropical thalassionema nitzschioides subarctic rhizosolenia hebetata transitional neodenticula seminae and freshwater freshwater diatoms ecological environments these factors are significantly correlated with primary productivity temperature nutrient concentrations and salinity although the strongest relationship is that between diatom assemblages and productivity however it is not possible to distinguish between coastal and open ocean curl driven upwelling based on chaetoceros spores relative percentages by themselves or on the floral factors

production of giant marine diatoms and their export at oceanic frontal zones implications for si and c flux from stratified oceans from a synthesis of recent oceanic observations and paleo data it is evident that certain species of giant diatoms including rhizosolenia spp thalassiothrix spp and ethmodiscus rex may become concentrated at oceanic frontal zones and subsequently form episodes of mass flux to the sediment within the nutrient bearing waters advecting towards frontal boundaries these species are generally not dominant but they appear selectively segregated at fronts and thus may dominate the export flux ancient thalassiothrix diatom mat deposits in the eastern equatorial pacific and beneath the polar front in the southern ocean record the highest open ocean sedimentation rates ever documented and represent vast sinks of silica and carbon several of the species involved are adapted to a stratified water column and may thrive in deep chlorophyll maxima thus in oceanic regions and or at times prone to enhanced surface water stratification e g during meltwater pulses they provide a mechanism for generating substantial biomass at depth and its subsequent export with concomitant implications for si export and c drawdown this ecology has important implications for ocean biogeochemical models suggesting that more than one diatom functional type should be used in spite of the importance of these giant diatoms for biogeochemical cycling their large size coupled with the constraints of conventional oceanographic survey schemes and techniques means that they are undersampled an improved insight into these key species will be an important prerequisite for enhancing our understanding of marine biogeochemical cycling and for assessing the impacts of climate change on ocean export production

salt as a 3d element in structural modeling example from the central european basin system abstract based on a well constrained 3d structural model of the central european basin system 3d modeling has been carried out to understand the regional scale regularities between movements of the upper permian zechstein salt and the major post permian tectonic events the 3d reconstruction of the paleo thickness of upper permian salt at the end of the permian period demonstrates that areas of thickened upper permian salt correlate with the present day distribution of the largest salt structures within the central european basin system this spatial correlation implies that the distribution of salt thickness at the end of deposition partially controlled the structural style of the basin during post permian phases of salt tectonics moreover the results of 3d salt redistribution for post permian times indicate that the initiation of salt movements took place in the triassic the triassic extensional events triggered several phases of salt movements within the coeval depocenters of the central european basin system such as the glueckstadt graben the horn graben the norwegian danish basin the fjerritslev trough and the himmerland graben and the polish basin during the late jurassic early cretaceous extensional transtensional tectonic event the strongest salt movement occurred within the central graben the lower saxony basin and the pompeckj block the intensity of the late jurassic early cretaceous phase of salt tectonics is essentially declining from the marginal areas of the central european basin system towards the horn graben the glueckstadt graben and adjacent areas the late early cretaceous early late cretaceous is characterized by tectonic quiescence without strong salt movements over the entire study area the next regional phase of salt movements was triggered by late cretaceous early cenozoic inversion involving almost the entire central european basin system in terms of renewed salt movements inversion related thick skinned salt tectonics is observed along the nw se trending elbe fault system and the teisseyre tornquist zone where strongest compressional deformations were localized on the contrary a thin skinned character of salt movements was prevailing within the largest part of the north german basin post inversion cenozoic subsidence was also accompanied by salt activity related either to diapiric rise due to regional shortening and or to local roughly e w directed extension the cenozoic phase of salt movements was especially prominent within the marginal troughs of the glueckstadt graben

records of seasonal flux in holocene laminated sediments gulf of california holocene laminated sediments from guaymas basin gulf of california mexico have in the past been described as annual couplets of sedimentation couplets comprise an alternation of diatom and terrigenous rich sediment reflecting winter and summer flux to the sediment respectively new data from backscattered electron imagery bsei show that a three component pattern of sedimentation is preserved comprising 1 a lithogenic lamina consisting of clays silt and minor diatomaceous material deposited during the summer and autumn wet season 2 a mixed flora diatomaceous lamina deposited during the early winter collapse of the thermocline and onset of water column mixing and 3 a near monospecific flora lamina of chaetoceros resting spores or more rarely skeletonema costatum representing deposition from or at the end of the spring coastal upwelling bloom recent sediment trap biogenic and terrigenous flux data also show three to four major flux events per year direct comparisons are possible between bsei records and recent sediment trap flux data to facilitate high resolution palaeoceanaographic and palaeoclimatic reconstructions throughout the holocene

chemostratigraphy and palaeo environmental characterisation of the cambrian stratigraphy in the amadeus basin australia abstract the sedimentary succession of the central australian amadeus basin consists of neoproterozoic to carboniferous sedimentary rocks and contains shallow marine subtidal carbonates of middle to late series 2 to furongian cambrian age a combination of sequence stratigraphy geochemistry and mineralogy shows a transgressive 2nd order cyclicity deposited between 511 490 ma and a change from arid low energy to humid high energy depositional environments this is reflected in an initially evaporitic sequence with upward decreasing halite and anhydrite abundance and transition from oxygenated to anoxic conditions reflected by the fe mineral species change from hematite to pyrite during transgression sequence boundaries of several 3rd order cycles consisting of hst carbonate rocks and lst siltstones correlate with globally recognised sequence boundaries linked to the inferred eustatic sea level record for the upper two series of the cambrian system the carbon isotope record for this 1400 m thick succession in combination with biostratigraphic age correlation allowed the identification of the globally recognised steptoean positive carbon isotope excursion spice drumian carbon isotope excursion dice and redlichiid olenellid extinction carbon isotope excursion roece

southern ocean laminated diatom ooze mat deposits and potential for palaeo flux studies odp leg 177 site 1093 laminated diatom ooze samples collected during odp leg 177 were analysed using scanning electron microscope sem and optical microscopy to test their potential as high resolution records of polar front hydrography surface production and export sem analysis from two intervals marine isotope stage mis 29 and 12 11 respectively recovered from 50 s in the atlantic ocean odp site 1093 hole a sections 13h 4 0 18 cm and 23h 4 0 22 cm show abundant and well preserved thalassiothrix antarctica mats thought to be indicative of rapid export from the surface and deposition in the sediment a preliminary analysis of laminae succession points to a possible annual couplet triplet succession of laminae and suggests exceptionally high local sedimentation rates of 57 and 80 cm kyr 1 for mis 12 11 and 29 respectively such high accumulation rates imply that local export from the surface layer and sequestration of biogenic silica and organic matter to the sediments may have been much higher than previously suggested resume des depot sedimentaires lamines de diatomites recueillis pendant le leg 177 de l ocean drilling program odp ont ete analyses en microscopie electronique a balayage sem et en microscopie optique ces enregistrements sedimentaires presentent un fort potentiel pour reconstituer avec une haute resolution la production de matiere exportee depuis la couche de surface de l ocean au niveau du front polaire l analyse en sem des stades isotopiques 29 et 12 11 sur du materiel collecte a 50 s dans le secteur atlantique de l ocean austral site odp 1093 forage a sections 13h 4 0 18 cm et 23h 0 22 cm revele des depots abondants et bien preserves de tapis de thalassiothrix antarctica suggerant une exportation rapide jusqu au sediment de materiel biogene genere a la surface de l ocean une analyse preliminaire des successions laminees par couple ou triplet indique des vitesses d accumulation sedimentaire exceptionnellement elevees 57 et 80 cm kan 1 respectivement pour les stades 12 11 et 29 de telles vitesses d accumulation suggerent que l exportation de matiere depuis la couche de surface de l ocean et ou la preservation de silice biogene dans le sediment peuvent etre bien plus elevees qu on ne le pensait jusqu a present

the upper miocene diatomaceous sediments of the northernmost mediterranean region a lamina scale investigation of an overlooked palaeoceanographic archive

variation in biogenic and detrital fluxes and formation of laminae in late quaternary sediments from the peruvian coastal upwelling zone abstract studies of laminated sediments from the peru margin using back scattered electron imagery bsei provide a high resolution record of variation in detrital and biogenic fluxes and highlight the uncertainty involved in preserving a record of surface processes in bottom sediments intermittently laminated sediments which predominate in the peru forearc basins record episodic major diatom bloom events from which there was adequate flux to counteract water column processes of dissolution and comminution a distinctive sub millimetre lamination with a limited occurrence during isotope stage 5 comprises a regular alternation between silt rich and clay rich couplets which may represent annual deposition with irregularly interlaminated diatom ooze containing near monospecific bloom assemblages the absence of diatom ooze layers between silt clay couplets may be controlled by variation of either the intensity of upwelling and or the nutrient content of upwelled waters macroscopically obvious 0 5 3 cm thick pelleted or homogeneous zones which occur throughout the sub millimetre laminated sequence represent periods of increased oxygenation with a probable 8 16 year duration occurring at irregular intervals of a few years to a few hundred years a slight but pervasive microscopic bioturbation is common within laminated intervals which would macroscopically be described as anaerobic sediments

pelletal structures in peruvian upwelling sediments late quaternary sediments from the peru upwelling zone have been investigated using backscattered electron imagery the dysaerobic to anaerobic conditions which persist within the strong oxygen minimum zone intersecting the shelf and upper slope sediments have resulted in the preservation of a wide range of microscopic fine structure relating to biological activity two forms of pelletal structures are present pellets and aggregate structures 1 pellets are oval in cross section with a mean diameter of 610 m and sharp boundaries three common pellet types have been identified a crushed diatom and clay pellets b intact diatom pellets and c silt pellets 2 aggregate structures are smaller than pellets with a mean diameter of 360 m they have indistinct boundaries and may be composed either of pure diatom ooze or a mix of clay silt and diatoms the size shape and mode of occurrence of aggregate structures suggest that they are the product of a meiofauna redistributing the topmost sediment during periods of very slight increases in oxygen most of the crushed diatom clay pellets and intact diatom pellets are probably also the product of a surficial deposit feeding fauna tolerant of low dissolved oxygen levels such as polychaete worms silt pellets may record the chamber building activities of agglutinating foraminifera

marine planktic diatom correlation of lower messinian deposits in the western mediterranean abstract samples from late neogene mediterranean outcrops were analysed for diatom content the presence of certain species in the tripoli formation caltanissetta basin sicily and in the beida formation cheliff basin nw algeria permits correlation between these sections and the paleomagnetic reversal record the occurrence ofthalassiosira praeconvexa thalassiosira miocenica thalassiosira convexa var aspinosa andasterolampra acutiloba indicates that the tripoli and beida formations were deposited over an interval ranging from the upper half of paleomagnetic epoch 6 into the lower half of epoch 5 5 6 to 5 9 ma previous diatom correlations were too young the occurrence ofnitzschia reinholdii andasterolampra acutiloba in pre evaporitic diatomites from the upper turre formation se spanish sorbas basin is used to place deposits at this site tentatively in paleomagnetic epoch 5 or 6

an intra basinal mechanism for marine evaporite cyclicity marine evaporites such as the zechstein permian nw europe consist of thinning upward sulphate halite potash cycles whose origin is poorly understood an intra basinal mechanism presented here explains well their mineral composition and cycle development it involves the progressive obstruction of ocean connections by sulphate platform progradation causing a chain reaction of outflow reduction and subsequent accelerated sulphate precipitation numerical modelling shows this to be a self accelerating process that ultimately triggers halite and potash precipitation isostatic compensation of the salt load explains the formation of accommodation space for subsequent cycles each about half the thickness of the previous cycle

a laminated sediment sequence from the northern north atlantic ocean and its climatic record a remarkable 3 m thick sequence of laminated diatom ooze ldo has been recovered in the north atlantic ocean the ldo is monospecific and consists of the pennate diatom thalassiothrix longissima biostratigraphy and benthic oxygen isotope stratigraphy indicate that the ldo was deposited during substage 5e eem in analogy to recent observations of massive surface concentrations of diatoms in the equatorial pacific and neogene examples we suggest that the ldo represents numerous depositional events of thalassiothrix mats that were physically accumulated at a subarctic convergence front the occurrence of the ldo combined with other climate proxies indicates substantial geographic stability of ocean fronts in the north atlantic during substage 5e

sequence stratigraphy in the middle ordovician carbonate successions yeongheung formation mid east korea paleogeographic implications for a land detached ramp type platform abstract this study reveals the origin of the stratigraphic discrepancy between the yeongweol and taebaek groups and reconstructs paleogeographic model for the taebaeksan basin on the sino korean craton skc the global sea level fall in the earliest middle ordovician would have subaerially extensively exposed the lower ordovician peritidal successions forming the solution collapsed breccia zones on the sauk tippecanoe bounding surface the successive rise in sea level generated the fine grained siliciclastic successions in the taebaek group during early transgression since the carbonate production of the group was hampered by the strong input of siliciclastic sediments from terrestrial sources however such successions are absent in the yeongweol group because the yeongweol platform was distant from the siliciclastic sources and sustained carbonate production due to isolation from terrestrial influences the yeongweol group is interpreted as a land detached isolated ramp type platform as it responded to sea level fluctuations with restricted terrestrial influence whereas the taebaek group can be described as a land attached ramp type platform affected by a large input of siliciclastic sediments during lowstand and early transgression based on the comparison of stratigraphic patterns and configurations of land attached and detached ramp type successions this study concludes that the yeongweol sequence would have been formed on a land detached isolated carbonate ramp type platform located at the skc along the northeastern gondwana margin

self sedimentation of phytoplankton blooms in the geologic record abstract understanding the formation of laminated organic rich sediments is an essential topic for researchers interested in fossil fuels biogeochemical cycles earth s environmental history and global change biologists have very recently demonstrated that some marine phytoplankton blooms actively govern their own sedimentation by the formation of sticky transparent gels that facilitate rapid aggregation accelerated sinking and efficient export flux here we present fossil evidence of unfragmented low diversity phytoplankton assemblages preserved as sedimentary laminae and irregular flocs that are attributable to a similar phytoplankton driven sedimentary mechanism we term self sedimentation the geological evidence suggests that self sedimentation precludes significant heterotrophic grazing propels the formation of some conspicuous hemipelagic sedimentary laminae and results in efficient carbon and opal flux to the sediments we suggest that the self sedimentation phenomenon may have broad implications for the geological history of biogeochemical cycling oceanic ecological dynamics and abrupt atmospheric environmental change broader recognition of the self sedimentation phenomenon as explicitly defined here is a prerequisite to testing these unconventional hypotheses

upper permian zechstein supergroup carbonate evaporite platform palaeomorphology in the uk southern north sea abstract seismic interpretation geological mapping and depth conversion of the zechstein supergroup z2 cycle using high quality well calibrated three dimensional 3 d seismic has revealed the complex palaeomorphology of a deeply buried ancient carbonate shelf margin on the southwestern margin of the southern permian basin the new mapping shows that the margin comprises a series of corrugated embayments and promontories which extend up to 5 km 3 mi into the basin and locally have a palaeobathymetric relief of 200 m 650 ft well calibration across the margin demonstrates a strongly bipartite lateral thickness distribution between a high velocity anhydrite dominated shelf and a lower velocity halite dominated basin strong lateral and vertical velocity variations in the upper permian zechstein supergroup are known to have major impacts upon seismic imaging and depth conversion in the spb the resulting uncertainty remains one of the major challenges when interpreting and assessing the prospectivity of the underlying upper permian rotliegend group leman sandstone formation reservoirs in the uk southern north sea an understanding of the zechstein shelf edge s 3 d physiography and its velocity variation has implications for the delineation of traps containing the prospective reservoirs that lie below recognition of the complexity and effects of this shelf margin contributed to the recent juliet discovery whose position outside of the presently defined rotliegend group play fairway suggests that prospectivity is more extensive along the basin margin than previously thought as such the work provides a means by which to identify and delineate new structures and extend the life of this mature yet prolific gas province

middle triassic orbital signature recorded in the shallow marine latemar carbonate buildup dolomites italy a new time frequency analysis of sea level controlled carbonate platform cycles in the middle triassic latemar massif dolomites italy reveals a strong depositional signature with characteristics of dominant forcing by climatic precession modes corresponding to long and short precession components at 1 21 7 k y and 1 17 6 k y underwent amplitude modulations matching earth9s orbital eccentricity with major frequency components at 1 400 k y 1 125 k y and 1 98 k y obliquity appears as a minor component at 1 35 4 k y the latemar signature thus constitutes the oldest pristine milankovitch signature yet observed in the geologic record its fidelity rivals that of the pliocene pleistocene record originally used to confirm the theory of orbitally forced climates this evidence deepens a widely noted disagreement between radiometric and cyclostratigraphic time scales for the latemar buildup the latemar cycles indicate that orbitally forced sea level oscillations were operative in the ice free middle triassic hothouse world

orbitally controlled oscillations in planktic communities and cyclic changes in western mediterranean hydrography during the messinian we use quantitative analyses of the planktic foraminiferal assemblages and stable isotope analyses of the sorbas section in the western mediterranean to reconstruct the cyclical changes in surface and deep water hydrographic conditions during the late miocene prior to the messinian salinity crisis oscillations in winter and summer temperatures linked to cyclical fluctuations in the hydrographic conditions and nutrient availability were the main mechanisms driving the cyclical changes in the planktic foraminiferal assemblages during the messinian the winter intensification and southward expansion of the northern cool and dry winds during precession maxima lowered sea surface temperatures below 14 15 c favoring water convection and the upward mixing of nutrients with the consequent increase in the proportion of cold eutrophic water foraminifera the low summer temperatures below 24 c inhibited the growth of warm oligotrophic water foraminifera as occurs today in the mediterranean by contrast at times of precession minima the low influence of the high latitude air masses caused winter temperatures to remain relatively high which combined with the lower surface salinities led to the formation of a permanent pycnocline winter conditions prevented the growth of cold eutrophic water foraminifera while high summer temperatures and the formation of a permanent pycnocline stimulated the growth of warm oligotrophic water foraminifera the prevalence of stable water stratification during winter due to the large density gradient between the surface and intermediate waters prevented deep water formation and slowed down the rates of oxygen supply to the bottom resulting in the formation of the sapropels a sharp decrease in 13c of benthic and planktic foraminifera that occurred between 6 8 and 6 7 myr is related to an increase in the residence time of mediterranean waters because similar changes have been observed in other regions of the mediterranean we conclude that this change records a significant reduction in the atlantic mediterranean water exchange at that time mediterranean climatic amplification increased throughout the messinian as the atlantic mediterranean water exchange was progressively more limited and oscillations in the ratio of planktic benthic foraminifera and of warm oligotrophic cold eutrophic species are good records of this increasing amplification which culminated at 5 95 myr with the deposition of gypsum pelitic layers that mark the onset of the messinian salinity crisis

transition from marine to hypersaline conditions in the messinian tripoli formation from the marginal areas of the central sicilian basin abstract three sections of the early messinian tripoli formation from the northern and southern margins of the central sicilian basin serra pirciata torrente vaccarizzo and marianopoli have been studied with the aim to reconstruct the sedimentary and environmental changes which occurred during the transition between marine conditions and the evaporitic events of the salinity crisis recorded in the overlying calcare di base formation a detailed biostratigraphic and cyclostratigraphic study provided the opportunity of cycle by cycle correlations between the marginal sections and the reference section of falconara the main paleoenvironmental changes are recorded by 1 the evolution of calcareous microfossils towards low diversity and their complete disappearance 2 the composition of the carbonate fraction which commonly changes from calcite mostly related to calcareous microfossils to authigenic carbonate phases consisting of either calcite dolomite and or aragonite 3 the appearance of shallow water deposits and evaporite pseudomorphs 4 the variation of the stable isotope composition of the carbonate fraction indicative of large fluctuations of the freshwater dilution evaporation balance with a general trend towards hypersaline conditions both mineralogical and isotope data indicate that the dolomite precipitated generally from concentrated pore waters while other carbonates formed in the water column submitted to large fluctuations of salinity except locally the increase in salinity did not reach concentrations high enough to precipitate significant volumes of evaporites as during the deposition of the overlying calcare di base the transition from normal marine to hypersaline conditions is recorded diachroneously in the three sections respectively in cycle 34 6 32 ma at serra pirciata cycle 42 6 15 ma at torrente vaccarrizzo and cycle 44 6 12 ma at marianopoli these changes represent the hydrological and sedimentary response to the fragmentation of a large marine basin into different subbasins which ended in the more or less complete closure recorded by the deposition of the overlying evaporite rich calcare di base fm true onset of the evaporitic event two hypotheses not exclusive may explain this change either the regional consequence of the compressive tectonics or the incipient drop of the global sea level which led to the salinity crisis our results confirm that the salinity crisis was preceded by evaporative episodes in the evolution towards hypersaline conditions which occurred diachroneously as a response to the diversity of paleogeographical settings even if the paroxysmal phase seems to have affected the basin at the same time

sequence stratigraphy of a carbonate evaporite succession zechstein 1 hessian basin germany the evaporitic hessian zechstein basin is a sub basin of the southern zechstein basin situated at its southern margin twelve facies groups were identified in the zechstein limestone and lower werra anhydrite in order to better understand the sequence stratigraphic evolution of this sub basin which contains economically important potassium salts four different paleogeographic depositional areas were recognized based on the regional distribution of facies siliciclastic carbonate carbonate carbonate evaporite and evaporite shallowing upward successions are developed these allow the establishment of parasequences and sequences as well as correlation throughout the hessian basin and into the southern zechstein basin two depositional sequences are distinguished zechstein sequence 1 and zechstein sequence 2 the former comprises the succession from the variscan basement up to the lowermost part of the werra anhydrite including the kupferschiefer as part of the transgressive systems tract the highstand systems tract is defined by the zechstein limestone in which two parasequences are developed in large parts of the hessian basin zechstein sequence 1 is capped by a karstic subaerial exposure surface interpreted as recording a type 1 sequence boundary that formed during a distinct brine level fall low lying central areas central hessian sub basin werra sub basin however were not exposed and show a correlative conformity topography was minimal at the end of sequence 1 widely developed perilittoral sabkha and salina shallowing upward successions indicate a renewed rise of brine level interpreted as a transgressive systems tract because of inflow of preconcentrated brines from the southern zechstein basin to the north this marks the initiation of zechstein sequence 2 which comprises most of the lower werra anhydrite in the central hessian sub basin situated proximal to the brine inflow and on the ridges within the hessian basin physico chemical conditions were well suited for sulphate precipitation to form a thick cyclic succession it consists of four parasequences that completely filled the increased accommodation space in contrast only minor sulphate accumulation occurred in the werra sub basin situated further southwards and distal to the inflow as a result of substantially different sulphate precipitation rates during increased accommodation water depth in the region became more variable the werra sub basin characterized by very low sedimentation rates became increasingly deeper through time trapping dense halite brines and precipitating rock salt deposits werra halite this self organization model for an evaporitic basin in which depositional relief evolves with sedimentation and relief is filled by evaporite thereafter contradicts earlier interpretations that call upon the existence of a tectonic depression in the werra area which controlled sedimentation from the beginning of the zechstein

quantitative seismic stratigraphic interpretation of the evaporite sequence in the santos basin abstract on the account of the absence of modern analogues the interpretation of evaporite bearing basins relies extensively on seismic and well data recent studies describe the intrasalt kinematic geometries and compositions relating the change of reflection patterns in seismic data to variations in evaporite sequences however these interpretations are only qualitative this study describes a step further in proceeding from qualitative to quantitative interpretation of an evaporite dominated interval it integrates geophysical and geological foundations by combining rock physics seismic inversion and statistical techniques to deliver seismic driven facies volume of the evaporite sequence in the santos basin offshore brazil synthetic seismogram demonstrate the ambiguities and uncertainties in relating the seismic amplitude to salt types rock physics analysis indicates that acoustic impedance is an effective differentiator of salt types in comparison to seismic amplitude therefore we perform seismic inversion to transform interface property into layer property the acoustic impedance volume mitigated significantly the risk of seismic interpretation in the salt sequence the bayesian classification integrates acoustic impedance and rock physics analysis to deliver the seismic driven facies volume which categorizes bittern salts halite and anhydrite this enables the refinement of the seismic stratigraphic interpretation of intrasalt sequences of the ariri formation based on the sedimentary cycles of evaporites additionally we calculate proportion maps to evaluate the spatial variability of salt deposition we observe an increase in bittern salts and anhydrite proportions from the bottom to the top of the sequence the comparison between well based and seismic based proportions testifies to the quantitative potential of seismic data the approach provides quantitative analysis of the evaporite sequence and can be applied to other salt bearing basins

cyclostratigraphy and astrochronology of the tripoli diatomite formation pre evaporite messinian sicily italy the ongoing debate about the messinian salinity crisis in the mediterranean is fuelled in part by the lach of an adequate time control the most accurate and at the same time detailed constrains are nowadays provided by the astronomical dating technique here we present an astronomical age model for the cyclically bedded tripoli diatomite formation on scily pre evaporite messinian italy based on an intergrated stratigraphic study of three key sections falconara gibliscemi and capodarso characteristic sedimentary cycle pattenrs allow i the sections to be cyclostratigraphically correlated the bed to bed correlations being confirmed by high resolution planktonic foraminiferal biostratigraphy and ii the tripoli cycles to thecalibrated to the astronomical record despite minor misfits the correctness of thetuning is evident from the match between precession obliquity interference in the astronomical target and its reflection in the sedimentary cycle record the tuning provides absolute astronomically dated at 7 005 ma indicating that the onset of diatomite formation is diachronous in the mediterranean since itstarted 300 000 years earlier on sicilt than on gavdos south of crete the top of the tripoli and thus the onset of the salinity crisis proper on sicily arrives at 5 98 ma

sequence stratigraphic context of extensive basin margin evaporites middle jurassic gypsum spring formation wyoming u s a abstract ancient marine evaporites are commonly far more extensive than any known modern evaporite system this is true not only for basin central saline giants but also for evaporites that occur in basin margin settings and the origin of these basin margin systems is unclear extensive basin margin evaporites of the middle jurassic gypsum spring formation are well exposed along the eastern flank of the bighorn basin in wyoming usa and a facies and sequence stratigraphic analysis was undertaken to understand the origin of these evaporites and associated deposits the gypsum spring formation was deposited on a northwestward dipping mixed evaporate carbonate siliciclastic ramp with facies and depositional environments that include distal ooid shoal grainstone open marine shallow subtidal peloidal skeletal wackestone to packstone restricted shallow subtidal laminated lime mudstone peritidal dolostone and microbialite as well as red mudstone and gypsum deposited on mud flats sabkhas and salinas three depositional sequences are present with the j 1 j 1a and j 1b sequence boundaries characterized locally by chert pebble lags silicified horizons and iron rich paleosols evaporites accumulated in a basin margin position with 1 6 m beds of gypsum forming in salinas and sabkhas and nodular anhydrite and gypsum forming in red mudstones deposited on sabkhas and mud flats the limited lateral extent of salina type gypsum beds and the variable number of these beds among the exposures indicates substantial autocyclic control on the formation of salinas salinas are well developed in the tst of the j 1 sequence and sabkhas and mud flats are well developed in the j 1 hst the j 1a hst and the j 1b tst the great lateral extent of the gypsum spring evaporites reflects the original expanse of the depositional environment in which they formed and is not the result of diachroneity of a narrow evaporite precipitating belt such as the modern day coastal sabkhas of the trucial coast the great lateral extent of gypsum spring evaporites suggests that they were precipitated mainly from continental brines on an expansive desert mudflat

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

diatom mats in gulf of california sediments implications for the paleoenvironmental interpretation of laminated sediments and silica burial preservation of laminations in hemipelagic and pelagic sediments is routinely attributed to reduced oxygen conditions that inhibited benthic activity as such the degree of preservation of laminae is used commonly as a paleo oxygenation indicator recent evidence from deep sea pelagic sediments in the eastern equatorial pacific and north atlantic however demonstrates that preservation of laminae may occur by rapid deposition of ungrazed thalassiothrix longissima diatom mats that overwhelm the benthos in otherwise oxic bottom waters here we report that similar diatom mat laminae are common in hemipelagic sediments of the guaymas basin in the gulf of california have aided preservation of the laminated sediment sequence and have also enhanced opal burial examples of diatom mats influencing sediment preservation are also described from the northern california margin and from the miocene monterey formation southern california thus even in classical settings of laminated sediment occurrence caution needs to be taken in correlating degree of lamina preservation with paleo oxygen levels such studies must be underpinned by detailed microfabric and micropaleontological analyses

depositional sequences in shallow carbonate dominated sedimentary systems concepts for a high resolution analysis abstract oxfordian and berriasian sections representing shallow water carbonate dominated sedimentary systems have been studied in the swiss and french jura in spain and in normandy they all display a hierarchical stacking of depositional sequences facies evolution and stacking pattern allow to define elementary small scale medium scale and large scale sequences some depositional sequences display well marked sequence boundaries others are limited by transgressive or maximum flooding surfaces the hierarchical organisation of such sequence stratigraphic elements implies that sea level fluctuations were an important factor in their formation and that these fluctuations had different frequencies the superposition of high frequency sea level changes on a long term sea level trend leads to repetition of diagnostic surfaces defining sequence boundary and maximum flooding zones wherein the corresponding high frequency surfaces are well developed chronostratigraphic tie points permit us to estimate the duration of large scale sequences this time control and the observed hierarchical stacking suggest that the high frequency sea level changes were controlled by climatic cycles in the milankovitch frequency band the variability of stacking pattern and facies evolution between sections illustrates the complexity of the studied environments furthermore because of the minimal accommodation space available in these shallow water settings much of the geologic time is not recorded nevertheless detailed analysis of the depositional sequences allows the interpretation of the evolution of the sedimentary system with a high time resolution thus there is a potential to monitor sedimentological ecological and diagenetic processes on a time scale of 20 to 100 ka

effect of zechstein supergroup z1 cycle werrahalit pods on prospectivity in the southern north sea mapping of the werraanhydrit formation a component part of the upper permian zechstein supergroup in the southern north sea highlights the profound effect that large isolated and hitherto poorly documented halite pods have on the assessment of gas prospects and existing fields in the basin seismic interpretation of well calibrated three dimensional volumes demonstrates that the pods ascribed to the werrahalit member occur throughout an approximately 20 km 12 mi wide west northwest east southeast striking belt that transects the basin they commonly have a pronounced lensoid cross sectional profile and a northwest southeast striking elongate lozenge shape in plan view integration of the geometrical evidence with facies and thickness data suggests that the pods resulted from original deposition in an evaporating salina setting their occurrence appears to have been influenced in part by topography generated by the effect of compaction above fault systems that had affected the rotliegend group deposition direct calibration of the halite pods demonstrates that they have a significantly lower velocity than the anhydrite dominated host in which they are encased the velocity effect has led to major discrepancies between prognosed and actual depth for deeper highly prospective rotliegend group leman sandstone formation gas reservoir targets significantly the identification of and allowance for differential velocities has a potential to improve the seismic imaging and the definition of sub zechstein traps in the area the results provide a new found basis on which to reassess the gas volumes in producing fields e g leman and tristan the economic viability of undeveloped discoveries e g tristan north west and the validity of both tested and undrilled prospects

paleoenvironments stratigraphic evolution and reservoir characteristics of the upper cretaceous yingjisha group southwest tarim basin abstract the upper cretaceous yingjisha group in the southwestern tarim basin is an important host of hydrocarbons with the highest quality reservoirs are a product of depositional environment diagenesis and tectonics the yingjisha group is composed in ascending order of the kukebai wuyitak yigziya and tuyiluok formations strata are thickest in the kashgar sag in the northwest and the yecheng sag up to 635 m in the southeast and thin over the central qimugen uplift less than 70 m six facies associations 1 6 are interpreted respectively as braided fluvial lagoon nearshore shelf playa lake restricted carbonate platform and open carbonate platform to platform margin stratigraphic evolution of the yingjisha group recorded in vertical changes in inferred depositional environments is related to changes in water depth and climate at least two transgressive kukebai and yigziya formations regressive wuyitak and tuyiluok formations cycles are recognized that can be correlated with the global eustatic curve of haq et al 1988 detailed paleoenvironmental analysis forms the basis for understanding and predicting the occurrence of reservoir rocks in this mixed siliciclastic carbonate succession high energy braided fluvial sandstones and carbonate platform and platform margin grainstones locally dolomitized comprise the best reservoirs the most important diagenetic processes in sandstones that resulted in porosity and permeability changes are 1 mechanical compaction 2 cementation and 3 replacement and dissolution of unstable clastic grains and cements whereas in carbonate rocks the dominant diagenetic processes are cementation and dissolution fracturing played an important role in enhancing the quality of sandstone and especially carbonate reservoirs notably in terms of their permeability by connecting previously isolated pores stratigraphic evolution of the yingjisha group resulted in stacking of seal rocks above reservoirs during the late transgressive and regressive phases of sedimentation

composition and origins of laminae in late quaternary and holocene sediments from the santa barbara basin late quaternary and holocene sediments from the santa barbara basin have been analyzed using back scattered electron imaging bsei techniques to identify the origins of laminae and document intra and interannual ocean climate variability over the last glacial cycle laminae observed in x ray radiography and bsei are entirely defined by variations in the relative abundance and grain size of biogenic and terrigenous components there is no evidence to suggest that bacterial mat mediation may have played a role in lamina formation the simplest lamination type comprises silt rich silt poor couplets that originate from seasonal variation in the supply of silt to the basin the second lamination type contains an additional lamina of diatom ooze to form terrigenous sediment diatomaceous triplets the thicker diatom ooze laminae commonly exhibit a vertical succession of diatom floras which represents intra annual variation in productivity the variation in thickness and assemblage of diatom ooze laminae is probably a function of the amount of diatom flux which is controlled by upwelling dri ven primary productivity in the basin laminated sediments deposited during cooler periods generally contain thicker diatom ooze laminae consistent with more vigorous regional circulation and increased upwelling

a rapidly deposited pennate diatom ooze in upper miocene lower pliocene sediment beneath the north pacific polar front abstract rapidly deposited thalassionema thalassiothrix pennate diatom oozes previously have been described in upper miocene lower pliocene sediment beneath the frontal boundary of the eastern equatorial pacific here we document a new occurrence of thalassionema thalassiothrix ooze in upper miocene lower pliocene sediment beneath the frontal boundary of the subarctic north pacific the ooze is a 6 m interval of siliceous sediment at ocean drilling program odp sites 885 886 that was rapidly deposited between approximately 5 0 and 5 9 ma bulk sediment in this interval may contain greater than 85 pennate diatom tests there are also abundant laminae and pockets that are composed entirely of thalassionema and thalassiothrix diatoms the presence of a rapidly deposited ooze dominated by pennate diatoms indicates unusual past conditions in the overlying surface waters time coincident deposition of such oozes at two distinct frontal boundary locations of the pacific suggests that the unusual surface water conditions were causally linked to large scale oceanographic change this same oceanographic change most likely involved 1 addition of nutrients to the ocean or 2 redistribution of nutrients within the ocean the occurrence and origin of pennate diatom oozes may be a key component to an integrative understanding of late neogene paleoceanography and biogeochemical cycling

deep water changes the near synchronous disappearance of a group of benthic foraminifera from the late miocene mediterranean abstract benthic foraminiferal and geochemical data from the monte del casino section in northern italy are employed to reconstruct the sequence of events preceding the late miocene mediterranean salinity crisis we evaluate the effects of eustatic sea level changes and tectonic events affecting the atlantic mediterranean connections changing benthic foraminiferal assemblages record changes in the deep water environment that can be explained by progressive isolation of the mediterranean basin the results of the analyses of stable oxygen and carbon isotopes and redox sensitive elements are in line with the benthic faunal trends at 7 16 ma a first major step in isolation of the basin is indicated by the nearly simultaneous disappearance of a group of deeper water benthic species usually found in middle to lower slope environments at the same time stable isotopes and redox sensitive elements indicate cooling and decreasing bottom water oxygenation after 6 8 ma gradual development of water mass stratification probably accompanied by increasing bottom water salinity is indicated by all proxies the cyclic pattern of homogeneous and sapropelitic sediments in the section is related to astronomical parameters and allows a detailed correlation with mediterranean sections at considerable geographic distances we compared the benthic foraminiferal faunas with those from the metochia section gavdos greece deposited at a similar water depth 1000 m to find that the benthic foraminifera at both sites indicate a similar synchronous paleoenvironmental development all proxies indicate that the onset of the late miocene mediterranean salinity crisis dates back to the earliest messinian the development of the mediterranean basin toward an evaporite trap is a result of interactions between a 400 kyr eccentricity related climatic effect superimposed upon gateway dynamics at the atlantic mediterranean connections benthic foraminifera accurately record the steps in increasing isolation of the mediterranean basin

meso cenozoic basin and carbonate platform development in the sw dolomites unraveled by basin modelling and apatite ft analysis rosengarten and latemar northern italy abstract the rosengarten and latemar middle triassic carbonate platforms indicate similar subsidence patterns during equivalent times of platform evolution as calculated by numerical basin reverse modelling in the case of rosengarten and latemar thermo tectonic subsidence is the most significant among all three subsidence components thermo tectonic s flexural s compaction induced s hence the influence of differential tectonics as recorded in the formation of structural highs and lows in the dolomites during the lower to middle anisian almost ceases in late anisian times basin modelling of rosengarten and latemar requires the correlation and integration of two sets of chronostratigraphic data from buchenstein fm basin and schlern dolomite fm 1 platform subsidence modelling argues for a microcycle i e a single shallowing upward cycle duration of less than 1 97 ka in the cyclic succession at latemar however previous time series analyses identify milankovitch cyclicities in larger order cycle stacking patterns and thus indicate a microcycle duration of 4 2 ka according to each model total subsidence reaches values of 650 700 m ma 4 2 ka model or 780 850 m ma 1 97 ka model during the first stage of platform evolution as both platforms successfully keep up with this subsidence peak it is evident that the carbonate factory despite its low diversity must have recovered from the p t faunal crisis after this regionally observed peak subsidence drops and triggers fast progradation at rosengarten subsidence in the study area is controlled by active strike slip tectonics and the subsequent formation of a magmatic chamber predazzo monzoni above the deep reaching truden stava line the area close to this tectonic line and to the volcanic centre of predazzo monzoni undergoes a higher heat flow pulse during the late ladinian and is possibly buried at shallower levels than the rosengarten area as shown by fission tracks in apatites the heat flow pulse almost disappears laterally underlining the localised thermal influence of the late ladinian volcano thermal event of predazzo monzoni

new data for the kinematic interpretation of the alps apennines junction northwestern italy alps and apennines are juxtaposed within an approximately 100 km wide area covered by the upper eocene to miocene successions of the tertiary piedmont basin the upper eocene oligocene evolution of this area was characterized to the north and west by the propagation of the se verging southalpine thrust fold belt that can be traced from the po plain subsurface until the torino hill saluzzese area and to the south by a high angle broadly e w oriented megashear zone that led to the juxtaposition of different crustal levels and controlled the development of a mosaic of partly independent sub basins since the latest oligocene the n verging apenninic tectonics prevailed in the collisional system and the tertiary piedmont basin evolved as a wide thrust top basin bounded to the north by the n verging monferrato arc and characterized by a tectono sedimentary evolution recording changes of subsidence and shift of depocentres in relation to crustal structures

bioturbated oxygenation event beds in siliceous facies monterey formation miocene california abstract predominatly laminated marine diatomaceous sequences deposited in oxygen deficient neogene basins typically contain bioturbated intervals that reflect episodes of improved benthic oxygenation excellent exposures of the siliceous facies of the miocene monterey formation near lompoc central california provided an opportunity to assess the ichnology and sedimentology of 200 such event beds in their general paleogeographic paleoclimatic and sea level context and hence to evaluate the character and causes of associated oxygenation episodes four recurring event bed types type i iv are distinguished on the basis of ichnologic characteristics these types fall into two general groups delineated on the basis of sedimentologic attributes primarily texture group 1 beds type i only have 1 homogeneous bioturbate primary strata that are more clastic rich and much coarser than surrounding laminated diatomites and 2 vertically extensive piped zones characterized by firmground thalassinoides paradoxicus glossifungites ichnofacies these characteristics indicate that group 1 events involve coincident increases in bottom current intensity clastic input and benthic oxygenation some group 1 beds may be the product of turbidity current transport of sediments trauma resistant crustaceans and allochtonous waters however most group 1 beds more likely record climate and sea level mediated migratio of incipient bank top conditions down the flank of a submergent topographic high group 2 beds types ii iii and iv are characterized by primary strata that are only slightly coarser and more clastic rich than surrounding laminites ichnofabrics and trace fossil assemblages which are dominated by burrows of deposit feeders thalassinoides suevicus meniscate backfilled structures and chondrites reflect normal softground substrates and quiet water pelagic deposition zoophycos ichnofacies differences in ichnologic parameters bed thickness adn presence diversity size and vertical extent of discrete burrows in primary strata and or piped zones among the bed types within this group reflect differences in magnitude duration and to a lesser extent frequency of oxygenationepisodes coincident increases in oxygenation and clastic content recorded by group 2 beds are attributed to e1 nino like climatic events gross temporal patterns in the intensity and frequency of these climatic events appear to be linked to the phases of a third order sea level cycle whereas shorter term variations 100 120 ka may have been mediated by milankovitch cyclicity relationships established between bioturbation events and relative water depth are of potential use in reconstructing basin paleogeography and or recognizing the signal of sea level change in the monterey formation and other neogene siliceous sequences

sub milankovitch and milankovitch forcing on a model mesozoic carbonate platform the latemar middle triassic italy basic shallowing upward cycles shu cycles and five bundled megacycles in the latemar platform have been widely regarded as a model example for precessional and eccentricity forcing in the mesozoic based on bio chronostratigraphic data recent studies questioned this particular type of milankovitch forcing on the latemar cyclic series we present an integrated model that incorporates i new cyclostratigraphic data ii new and existing bio chronostratigraphic data and iii new spectral analyses the basic shu cycles in the latemar reflect sub milankovitch control cycle bundles of 1 4 5 megacycles shu cycles indicate precessional forcing they do not reflect eccentricity superimposed on precessional forcing spectral analyses reveal highly significant ratios in the large scale cycle bundlings stacking patterns of 1 9 9 and 1 24 0 shu cycles represent obliquity and short eccentricity forcing both sub milankovitch and milankovitch forcing potentially controlled shallow subtidal carbonate deposition in mesozoic greenhouse times cyclostratigraphic models require an integrated approach including bio and chronostratigraphic data

north german zechstein facies patterns in relation to their substrate an attempt was made to relate the early zechstein facies distribution to the zechstein substrate comparable developments in the hessian trough and along the lower rhine embayment allow for the assumption that the creation of depocenters in the weser ems area are governed by interrelated late saalic tectonic pulses due to differences in the architecture and mobility of the substrate variations in the depositional model must be assumed

ocean drilling program leg 112 peru continental margin part 2 sedimentary history and diagenesis in a coastal upwelling environment on the shelf and upper slope off peru the signal of coastal upwelling productivity and bottom water oxygen is well preserved in alternately laminated and bioturbated diatomaceous quaternary sediments global sea level fluctuations are the ultimate cause for these cyclic facies changes during late miocene time coastal upwelling was about 100 km west of the present centers along the edge of an emergent structure that subsequently subsided to form the modern slope the sediments are rich in organic carbon and intense microbially mediated decomposition of organic matter is evident in sulfate reduction and methanogenesis these processes are accompanied by the formation of diagenetic carbonates mostly ca rich dolomites and mg calcites the downhole isotopic signatures of these carbonate cements display distinct successions that reflect the vertical evolution of the pore fluid environment from the association of methane gas hydrates burial depth and low chloride interstitial fluids we suggest an additional process that could contribute to the characteristic chloride depletion in pore fluids of active margins release of interlayer water from clays without a mineral phase change the shelf sediments also contain a subsurface brine that stretches for more than 500 km from north to south over the area drilled the source of the brine remains uncertain although the composition of the oxygen isotopes suggests dissolution of evaporites by seawater

red bed and basement sourced fluids recorded in hydrothermal mn fe as veins sailauf germany a la icpms fluid inclusion study the hydrothermal mn fe as vein mineralization at sailauf germany hosts a complex sequence of oxide and carbonate minerals that record a protracted fluid history the mineralization is related to a major unconformity that separates permian zechstein sedimentary rocks from underlying variscan crystalline basement the hydrothermal veins contain two principal mineralization stages which are mn oxides associated with calcite and hematite associated with mn calcite the fluid evolution of the hydrothermal system has been reconstructed from fluid inclusion petrography microthermometry and la icpms microanalysis coupled with stable isotope geochemistry of carbonate and oxide minerals the fluid inclusions are high salinity sodic calcic brines and the bulk fluid properties show no major differences between the mn oxide and the hematite stage la icpms analysis of major and trace elements demonstrates that the mineralization formed from chemically distinct fluid pulses characterized by variations in their k li b pb and zn concentrations the fluid that precipitated the mn oxide stage has anomalous pb zn and li b ratios which closely resemble fluids found in fracture fillings in red beds of the permian rotliegend basin by contrast the fluids associated with the hematite stage have pb zn and li b ratios typical of crustal fluids that were derived from interaction with crystalline basement both fluids possess characteristic element ratios including cl br but variable absolute concentrations of most metals this suggests that both fluids were modified by mixing with a common metal depleted brine that had a similar cl br ratio most likely a formation water from the overlying zechstein sedimentary rocks the sailauf mineralization provides insight into the protracted post variscan fluid evolution at the basement cover interface the compositionally anomalous fluid that precipitated the mn oxides is comparable to brines derived from interaction with red beds and likely represents the ore fluid of kupferschiefer type sediment hosted cu deposits conversely the fluid that deposited the hematite mineralization resembles fluids that typically form basement and sediment hosted pb zn deposits

comparison of sequences formed in marine sabkha subaerial and salina subaqueous settings modern and ancient marine evaporites occurring in modern subaqueous salina settings and subaerial sabkha settings are different subaqueous holocene evaporites occur as shoaling upward lacustrine sequences up to 10 m thick they are evaporite dominated and are composed primarily of bottom nucleated crystals that may be deposited as massive laminated or rippled units each coastal lake is dominated by laminated evaporites with subordinate carbonate sediments in plan view they show a well developed bull s eye pattern with a sulfate center and a carbonate rim in contrast subaerial sabkha evaporites occur as part of a laterally prograding shoaling upward peritidal sequence in which the supratidal unit is usually no more than 1 m thick sabkha sequences are matrix dominated not ev porite dominated with the bulk of the sulfate phase occurring as diagenetic nodules enteroliths or diapirlike structures these sulfates were formed during syndepositional diagenesis by replacement and displacement processes the various facies of the sequence tend to accumulate in belts parallel with the shoreline relative to the sea level or the brine level sabkhas tend to form over paleotopographic highs whereas salinas tend to occur in paleotopographic lows some of the characteristics that distinguish holocene subaerial and subaqueous evaporite sequences can be used to do the same for similar ancient facies even when gypsum has been converted to nodular anhydrite the distinction is important for it can be used by explorationists in the oil industry to define the paleotopography of the associated underlying porous and nonporous carbonates

formation of holocene sedimentary laminae in the black sea and the role of the benthic flocculent layer holocene black sea sediments recovered in 1988 and 1993 from box cores and gravity cores were analyzed geochemically microscopically and with backscattered electron imagery bsei in order to determine the temporal geochemical and sedimentological relationships between the benthic flocculent layer often referred to as the fluff layer and the formation of underlying laminated unit i sediments existence of a permanent benthic fluff layer in the black sea basin is suggested acting as a geochemical transition layer within which all sedimentary particles are hydraulically sorted and particles subject to dissolution or organic remineralization are altered prior to accumulation we propose that particle residence time within the benthic fluff layer is a key factor in determining sedimentary microfabric and geochemical composition of laminated unit i sediments we present a schematic model depicting the above relationships and use it to propose a paleoflux scenario for laminae formation in the unit ii sapropel

flocculent layers and bacterial mats in the mudstone interbeds of the primary lower gypsum unit tertiary piedmont basin nw italy archives of palaeoenvironmental changes during the messinian salinity crisis abstract the recognition of peculiar laminated layers atypically rich in a biogenic intrabasinal component in the mudstone intervals from the messinian late miocene primary lower gypsum unit 5 97 5 60 ma of the piedmont basin nw italy provides information on the palaeoenvironmental evolution at precessional insolation maxima these cyclic layers consist of irregular alternation of cm thick grey terrigenous laminae and whitish composite packets the latter is in turn composed of sub mm thick wrinkled dolomite rich laminae that alternate with terrigenous ones two types of layers can be distinguished i peloidal layers composed of faecal pellets and irregular diatom rich aggregates interpreted as marine snow floccules and ii filament bearing layers composed of interwoven filaments up to 150 m across corresponding to remains of beggiatoa like giant sulphide oxidising bacteria by comparison with present day settings the peloidal layers are interpreted as flocculent layers deposited on anoxic sea bottoms following episodes of phytoplankton bloom in the upper water column the filament bearing layers are considered as chemotrophic microbial mats growing on dysaerobic sea bottoms the uncommon preservation of both kinds of layers is the result of extensive dolomite precipitation in the shallow subsurface induced by bacterial sulphate reduction the laminated layers are the first reported example of a high frequency climate driven cyclicity in the non evaporitic portion of the primary lower gypsum unit their stacking pattern within the mudstone intervals reflects the gradual increase of the oxygen content of bottom waters during the humid precessional hemicycle culminating with gypsum deposition at the onset of the arid phase

new insights into productivity and redox controlled trace element ag cd re and mo accumulation in a 55 kyr long sediment record from guaymas basin gulf of california a high resolution 55 kyr long record of chalcophile and redox sensitive trace element accumulation ag cd re and mo from md02 2515 western guaymas basin is investigated in conjunction with patterns in stratigraphy and productivity high opal concentrations 59 wt representing increased diatom production coincide with laminated sediments and dilute the concentrations of organic carbon corg and metals a similarity between opal and normalized corg ag and cd concentrations suggests delivery to the sediments by diatom export production while patterns in normalized re and mo accumulation suggest a different emplacement mechanism although mo enrichment in organic rich laminated sediments typically represents anoxic conditions at other locations mo and re in guaymas basin is enriched in nonlaminated and bioturbated sediments that are representative of oxygenated conditions adsorption onto fe and or mn oxyhydroxide surfaces during oxygenation inadequately explains both the re and mo enrichments thus recently published mechanisms invoking direct re and mo removal from the water column and bioturbation assisted irrigation of re into the sediments are used to explain the counterintuitive observations in guaymas basin the md02 2515 stratigraphic and proxy records are also different from other records in the northeast pacific in that there is little correspondence with greenland dansgaard oeschger interstadials there is some correlation with heinrich events suggesting that ventilation of intermediate waters and or reduced productivity may be important in controlling stratigraphy and trace element accumulation the results question whether md02 2515 records can be compared to northeast pacific open margin records especially before 17 kyr b p

comparison of phytoplankton in sediment trap time series and surface sediments along a productivity gradient three years of sediment trap data collected by the multitracers project are compared with surface sediments for sites off the coast of southern oregon using phytoplankton microfossils sites were located to monitor a region frequently affected by plumes from coastal upwelling nearshore one occasionally affected by the largest plumes midway and an area outside the zone of coastal influence gyre diatoms overwhelmingly dominate the microfossils with silicoflagellates an order of magnitude fewer and calcareous nanofossils absent this absence suggests that the rarity of the group in the sediments may be a function of low production rather than of deepwater dissolution the midway and nearshore sites were generally similar in taxonomic composition and timing of flux maxima while the gyre site differed in both regards interannual variability in taxon composition appears to correlate with productivity the most productive site nearshore has the most consistent interannual trend and the least productive gyre exhibits the greatest variability the sediment assemblage at the nearshore site is most similar to trap samples from the late fall and winter seasons rather than the more productive spring and summer this is partly a function of differential dissolution but the relationship persists even when only resistant taxa are used two possible explanations are discussed addition of material from the continental shelf and inadequate representation of long term trends by the trapping experiment at the midway site the data are few and scattered but once the dissolution effect is removed it appears that the sediments contain material from several seasons of production at the gyre site preservation bias results in a sediment assemblage most similar to late spring and summer only partly corresponding to maximum production comparison between sites indicates that the nearshore and gyre sediments have characteristic assemblages which are similar to the corresponding trap collections the midway sediments are similar to trap samples from all 3 sites implying that the assemblage lies on a gradient between the other two and lacks a unique character of its own

dissolution and preservation of antarctic diatoms and the effect on sediment thanatocoenoses abstract comparison of southern ocean diatom populations from i surface ocean production ii underlying antarctic sediments and iii laboratory dissolution experiments demonstrates that dissolution can account for the temporal and spatial variations in sedimentary diatom assemblages observed in southern ocean sediments increasing dissolution causes relative depletions in n kerguelensis k enrichments in t lentiginosa l and slight enrichments in e antarctica a this reflects the relative susceptibility to dissolution of the three species that dominate antarctic sediments we have devised a preservation index for the southern ocean based on the ratio k k l to estimate relative extents of dissolution and applied it to natural assemblages holocene southern ocean sediments display increasing opal preservation toward higher latitudes but south of the antarctic polar front preservation decreases in the order well preserved se indian s atlantic sw indian se pacific poorly preserved dissolution also accounts for the pattern of diatom assemblages in the last glacial maximum lgm sediments of the indian and pacific sectors but in the atlantic increased e antarctica abundances at lgm must have resulted from an increase in surface ocean production of this species holocene and lgm diatoms in atlantic and pacific sector sediments are equally well preserved but in the indian sectors holocene sediments are better preserved than those of lgm age paleoceanographic and paleoclimatic transfer functions derived from factor analyses of variations in the sedimentary abundances of these three diatoms have ignored the effects of differential dissolution on thanatocoenosis and thus should be interpreted with caution

speckled beds distinctive gravity flow deposits in finely laminated diatomaceous sediments miocene monterey formation california distinctive nonlaminated intervals we term speckled beds sbs are common in finely laminated diatomites of the monterey formation at celite quarry lompoc california sbs have sharp and bedding parallel upper and lower contacts tabular geometry and are less than or equal to 10 cm thick sbs are characterized by a bedding parallel arrangement of sand to granule size diatomaceous and detrital aggregates that are uniformly distributed in a fine grained matrix of macerated biosilica and detrital mud the unique physical properties of diatomaceous sediments provided cohesive substrates and high fluid content that were prerequisite for the formation of sbs abundant hybrid fault vein structures slump folds and complexly deformed laminae in the quarry attest to syndepositional deformation of laminated diatomaceous sediments on a subaqueous slope within the oxygen minimum zone the unique features of sbs permit an interpretation of stepwise rheological transformation during mass movement via passage through critical point transformations from bingham plastic slump and or slide block to turbulent fluid turbidity current to viscous fluid exhibiting laminar flow debris flow the recognition of sbs clarifies the origin of these distinctive yet problematic deposits and clearly differentiates them from other nonlaminated intervals sb composition indicates provenance from an oxygen minimum zone and further illustrates that a hiatus is not required to form cohesive firmgrounds in diatomaceous sediments the recognition of sbs as recorders of slope failure events complements paleoseismic studies of holocene diatomaceous laminites from saanich inlet british columbia ocean drilling program sites 1033 and 1034

microfabric study of diatomaceous and lithogenic deposition in laminated sediments from the gotland deep baltic sea the deep basins of the baltic sea are commonly anoxic and finely laminated diatomaceous sediments have been deposited at intervals throughout the last 8000 yr the origin and composition of individual laminae in gotland deep sediments have often proved difficult to characterise using traditional micropalaeontological and sedimentological studies here we present a scanning electron microscope study in which lamina down to 30 m in thickness with distinct mineralogical or micropalaeontological composition have been identified and described depositional laminae sequences in the form of couplets triplets and quadruplets of diatomaceous and lithogenic laminae are observed with an average thickness of approximately 700 m diagenetic ca rhodochrosite laminae also occur within these depositional sequences examination of the diatom assemblages suggests that these bundles of laminae represent annual deposits or varves varves are relatively uncommon and typically occur in small intervals of two to five varves which are interrupted by more diffusely laminated and homogenous sediments the origin of these more massive sediments probably relates to periodic re oxygenation of the basin on inter annual time scales and destruction of varves by bioturbation