depositional facies of a mud shoreface in suriname south america a mud analogue to sandy shallow marine deposits abstract immense volumes of argillaceous muds are being deposited along the world s longest continuous mud coastline which faces the open equatorial atlantic not only are these muds deposited in an environment normally associated with sand deposits but just as in sandstones sedimentary structures within these muds record the dynamic processes that deposited them this study investigates a 100 km long portion of the suriname coast utilizing cores bathymetric surveys and field observations from nearshore and innershelf environments using terminology borrowed from sand coastlines the innershelf nearshore of suriname is divided into transition zone shoreface foreshore and backshore the transition zone consists of a mix of offshore sands and shoreface muds shoreface and foreshore are subdivided into bank and interbank zones these zones are transitory because mud banks which make up the bank zones migrate westwardly mud banks are up to 5 m high 50 to 60 km long 10 to 20 km wide and oriented at oblique angles to the coastline these banks generally consist of fluid muds which are so fluid that they interact with surface waves causing them to be altered and damped banks are attached to the shoreline and commonly produce progradation of the coastline substrate within the interbank zone is more consolidat d than within the mud banks because of lack of wave dampening wave energies are relatively high within the interbank zone producing erosion of the shoreface and shoreline sand beaches are generally formed within the erosional interbank shoreline the vertical sedimentary sequence within the shoreface muds is a record of bank migration and other sedimentary processes present wave and current energies are recorded by scour and fill features discontinuity features micro cross laminations lenticular laminations and parallel to subparallel laminations laminations appear to be created by boundary layer shearing within fluid muds which are above wave base and made mobile by wave generated shore parallel currents large scale evidence for migration of the mud banks is westward dipping acoustical reflectors within the banks these reflectors which are over a kilometer apart mark the boundaries of annual migration events biogenic structures are generally absent within the fluid bank muds but are present within the more consolid ted interbank zone and associated with discontinuity features which are buried interbank surfaces migration and vertical stacking of mud banks are recorded by vertical sequences of massive and laminated muds which were deposited by mud banks and discontinuity features which were formed by interbank zones the coastal deposits of suriname are a mud analogue to nearshore and innershelf sand deposits transition zones of both mud and sand systems reflect a transition from offshore to shoreface processes and sediments both mud and sand shoreface foreshore systems reflect sediment input and wave current and tidal processes that act on them mud banks of suriname resemble linear sand ridges on the continental shelf of the eastern u s a in shape oblique orientation to the coastline and orientation with respect to dominant direction of transport processes the sequence of massive and laminated muds with discontinuity features resembles laminated to burrowed sequences found in ancient and recent nearshore and shallow marine sands

north pacific halocline and cold climate induced by panamanian gateway closure in a coupled ocean atmosphere gcm 1 the influence of the closure of the panamanian gateway during the late cenozoic on climate in and around the north pacific is investigated by using a coupled ocean atmosphere general circulation model with an open and closed gateway in the case of an open closed gateway deep convection is present absent in the north pacific the deep convection is associated with surface saline water transported from the subtropical atlantic through the open gateway to the north pacific on the other hand with the closed gateway the lack of saline water transport from the atlantic induces halocline formation over the subarctic pacific with cold climate the deep convection in the north pacific leads to a vigorous thermohaline circulation with larger meridional heat transport and causes warmer climate in and around the north pacific these results are generally consistent with paleoceanographic and paleoclimatic estimates related to the closure of the panamanian gateway

deposits from wave influenced turbidity currents pennsylvanian minturn formation colorado u s a turbidity currents generated nearshore have been suggested to be the source of some sandy marine event beds but in most cases the evidence is circumstantial such flows must commonly travel through a field of oscillatory flow caused by wind generated waves little is known however about the interactions between waves and turbidity currents we explore these interactions through detailed process oriented sedimentological analysis of sandstone event beds from the pennsylvanian minturn formation in north central colorado u s a the minturn formation exhibits a complex stratigraphic architecture of fan delta deposits that developed in association with high topographic relief in a tectonically active setting an 20 35 m thick unconformity bounded unit of prodelta deposits consists of dark green shale and turbidite like sandstone beds with tool marks produced by abundant plant debris some of the sandstone event beds most abundant at distal localities contain reverse to normal grading and sequences of sedimentary structures that indicate deposition from waxing to waning flows in contrast proximal deposits in some cases less than a kilometer away contain abundant beds with evidence for deposition by wave dominated combined flows including large scale hummocky cross stratification we interpret the majority of these event beds as a record of deposition from hyperpycnal flows i e turbidity currents generated directly from highly concentrated river plumes which accelerated and decelerated in response to a rising and falling flood discharge additional support for this interpretation includes the following 1 a variety of sole marks including flute and gutter casts as well as tool marks made by relatively large up to tens of centimeters across woody debris i e groove prod and chevron marks 2 consistent unimodal orientation of sole marks and abundant ripple cross stratification which indicate strong downslope directed flow 3 a well documented sedimentological framework for the formation of fan delta deposits adjacent to nearby highlands and 4 plant fossils typical of middle to high elevation habitats that are abundant in the turbidite beds but absent in underlying and overlying shoreline and marginal marine deposits which have a separate floral assemblage differences in grain sizes vertical stratification sequences and bed thicknesses between outcrops are interpreted to result from the spatial distribution of wave effects the time history of hyperpycnal flows and the interaction of these processes the latter varied both spatially and temporally and produced a wide range of bed types which are incorporated into a new conceptual model for storm influenced hyperpycnal flows

origin of minor limestone shale cycles climatically induced or diagenetic the opinion is widely held that each limestone bed in minor limestone shale cyclic sequences however modified subsequently by diagenesis reflects an environmental event on the assumption that this is true attempts have recently been made to relate limestone shale cycles to climatic variations affected by milankovitch orbital cycles a test is proposed and illustrated mainly with reference to the blue lias formation of dorset england that appears to indicate that many limestones can be produced solely by rhythmic unmixing of caco 3 during diagenesis unless such limestones can be clearly distinguished from those that record genuine environmental signals orbital cycle analysis based on such sequences will give meaningless results

variability in upwelling intensity and nutrient regime in the coastal upwelling system offshore namibia results from sediment archives the coastal upwelling system offshore namibia is ideally suited to address a focal question of the integrated marine biogeochemistry and ecosystem research programme what are the mechanisms that drive long term changes in ecosystems considerable interannual variability in climatic forcing is indicated by long time series of meteorological and remote sensing observation these accompany considerable interannual to interdecadal changes in the upwelling intensity over the last 100 years as well as a centennial trend on longer time scales the only archives available are sediment records spanning the late holocene to decipher the sediment record we mapped surface sediment patterns of proxies for physical sea surface temperature sst from alkenone unsaturation indexes and nutrient 15n on bulk sedimentary n variables their present day surface sediment patterns outline the coastal upwelling cells and filaments and associated high productivity area analysed in an array of dated sediment cores the spatial patterns of sst suggest long term 100 years variability in the location and intensity of individual upwelling cells the patterns of 15n outline an area of intense denitrification near the coast and advection of water with low oxygen concentrations in the undercurrent from the north 15n exhibits considerable downcore variability in particular over the last 50 years the variability appears to be governed by differences in extent of denitrification and thus of the shelf oxygen balance which appears to have deteriorated in the last 50 years together the data suggest that sst and denitrification conditions have remained in the narrow bounds outlined by today s patterns in surface sediments but that spatially small variability in upwelling intensity and make up of upwelling feed waters induced considerable changes in the lower trophic levels of the coastal upwelling ecosystem over the last 6 000 years attempts to correlate proxy records from sediments with observational time series and regional climate reconstructions were not successful possibly because annual to interannual environmental signals are erased in the process of sediment formation

meridional reorganizations of marine and terrestrial productivity during heinrich events 1 to study the response of the global carbon cycle to a weakening of the atlantic meridional overturning circulation amoc a series of freshwater perturbation experiments is conducted both under preindustrial and glacial conditions using the earth system model of intermediate complexity loveclim a shutdown of the amoc leads to substantial cooling of the north atlantic a weak warming of the southern hemisphere intensification of the northeasterly trade winds and a southward shift of the intertropical convergence zone itcz trade wind anomalies change upwelling in the tropical oceans and hence marine productivity furthermore hydrological changes associated with a southward displacement of the itcz lead to a reduction of terrestrial carbon stocks mainly in northern africa and northern south america in agreement with paleoproxy data in the freshwater perturbation experiments the ocean acts as a sink of co2 primarily through increased solubility the net atmospheric co2 anomaly induced by a shutdown of the amoc amounts to about 15 ppmv and 10 ppmv for preindustrial and glacial conditions respectively this background state dependence can be explained by the fact that the glacial climate is drier and the terrestrial vegetation therefore releases a smaller amount of carbon to the atmosphere this study demonstrates that the net co2 response to large scale ocean circulation changes has significant contributions both from the terrestrial and marine carbon cycle

effect of ocean gateways on the global ocean circulation in the late oligocene and early miocene 1 we investigate the effect of changes in the tectonic boundary conditions on global ocean circulation patterns using a fully coupled climate model in an idealized setup we compare situations corresponding to the late oligocene the early miocene and present day the model results show the existence of a flow reversal through the panama seaway between the oligocene and miocene this flow reversal is induced by global tectonic changes related to the widening of the southern ocean passages and the closing of the tethys seaway it mainly involves the wind driven ocean circulation in agreement with previous model studies the global thermohaline circulation in the oligocene and miocene simulations is significantly different from the present day conveyor circulation as there is deepwater formation in both the north atlantic and the north pacific oceans in particular in the oligocene simulation the salinity contrast between the atlantic and pacific oceans is reduced because of water mass exchange through the low latitude connections between the two oceans

a mechanism for freshening the caribbean sea in pre ice age time many believe that the central american seaway closed near 4 ma and that that closure led to increased salinity in the caribbean sea and stronger meridional overturning circulation in the atlantic which facilitated the waxing and waning of ice sheets in the northern hemisphere we offer an alternative explanation for caribbean salinification the atmosphere transports approximately 0 23 sv 1 sv 106 m3s 1 of fresh water moisture from the caribbean to the pacific today but that amount varies by 20 during el nino southern oscillation events regressions of moisture transport against the nino 3 index a measure of the sea surface temperature in the eastern tropical pacific show less moisture transport from the caribbean during el nino events than average abundant evidence indicates that at 3 4 ma the eastern tropical pacific was 3 5 4 c warmer than today and if so an extrapolation of such regressions suggests that smaller moisture transport across central america might account for paleoceanographic inferences of a smaller salinity difference between the caribbean and pacific at that time accordingly that decreased salinity difference at 3 4 ma would not require blockage of relatively fresh pacific water at 2 4 ma by the closure of the central american seaway but rather would be consistent with a transition from el nino to la nina like conditions in the eastern tropical pacific around that time

miocene to pliocene changes in south african hydrology and vegetation in relation to the expansion of c4 plants the miocene expansion of c4 plants mainly tropical grasses between 8 and 4 million years ma remains an enigma since regional differences in the timing of the expansion rules out decreased co2 pco2 as a dominant forcing e g tipple and pagani 2007 the early origins of terrestrial c4 photosynthesis annu rev earth planet sci 35 435 461 other environmental factors such as low latitude aridity and seasonality have been proposed to explain the low tree versus grass ratio found in savannahs and tropical grasslands of the world but conclusive evidence is missing here we use pollen and stable carbon 13c and hydrogen d isotope ratios of terrestrial plant wax from a south atlantic sediment core odp site 1085 to reconstruct miocene to pliocene changes of vegetation and rainfall regime of western southern africa our results reveal changes in the relative amount of precipitation and indicate a shift of the main moisture source from the atlantic to the indian ocean during the onset of a major aridification 8 ma ago we emphasize the importance of declining precipitation during the expansion of c4 and cam mainly succulent vegetation in south africa we suggest that the c4 plant expansion resulted from an increased equator pole temperature gradient caused by the initiation of strong atlantic meridional overturning circulation following the shoaling of the central american seaway during the late miocene

carbonate systems along nutrient and temperature gradients some sedimentological and geochemical constraints research over the past several decades has clearly demonstrated that changes in the ocean environment have had major impacts on carbonate systems changes in climate ocean circulation and seafloor spreading rates have influenced temperature and seawater chemistry including carbonate saturation state and nutrient availability and thereby have determined boundary conditions for the biota that form carbonate platforms in turn the biota determine accumulation rates and facies zonations thus controlling platform geometry and facies dynamics in the first section of this paper we examine how nutrient availability influences carbonate facies associations we first discuss the role of temperature and nutrient gradients in the modern ocean and their influence on biotic associations then we discuss how carbonate sedimentation can be characterized along nutrient gradients in the second section we review proxies currently used to reconstruct paleoproductivity in open ocean environments and discuss their applicability to neritic carbonate systems we highlight the variety of existing proxies and their limitations and suggest that multiple lines of evidence are needed for valid interpretations our short review discusses sedimentological biogenic and geochemical proxies that can be used to reconstruct past nutrient fluxes and to constrain paleoceanographic controls over the distribution of carbonate associations however it also reveals that more data and case studies are needed that integrate shallow and deep water carbonate sequences and elucidate the links between temperature vs nutrient supplies changes and facies in ancient carbonate sequences

marine and terrestrial biomarker records for the last 35 000 years at odp site 658c off nw africa several high resolution proxy environmental records have been obtained for the last 35 kyr from odp hole 658c a well studied site ca 200 km oa cap blanc nw africa the collective assessment based on the marine proxies u k 0 37 sst contents of toc and chlorins upwelling radiolarian index and the percentage of florisphaera profunda surprisingly indicates that the last glacial maximum lgm was characterized by warmer sea surface temperature sst weaker upwelling and lower marine productivity compared with the preceding older glacial and subsequent deglaciation periods of the terrigenous proxies the mean grain size of the non carbonate fraction and the terrigenous alkane content indicate that wind strength and aridity were high the weaker upwelling at the 658 site during the lgm may have resulted from changes in the strength and direction of the wind systems and or shifts in the position and geometry of the upwelling cell 2000 elsevier science ltd all rights reserved

colder subarctic pacific with larger sea ice caused by closure of the central american seaway and its influence on the east asian monsoon a climate model study

circulation through the central american seaway during the miocene carbonate crash changes in circulation associated with the shoaling of the isthmus of panama and the caribbean carbonate crash in the miocene were investigated using nd isotopes from fossil fish teeth and debris from two sites in the caribbean basin ocean drilling program sites 998 and 999 and two sites in the eastern equatorial pacific sites 846 and 1241 the total range for nd values measured from 18 to 4 5 ma in the caribbean is 7 3 to 0 these values are higher than atlantic water masses 11 and range up to values equivalent to contemporaneous pacific water masses confirming that flow into the caribbean basin was composed of a mixture of pacific and atlantic waters with an upper limit of almost pure pacific sourced waters throughout the caribbean record particularly during the carbonate crash 10 12 ma low carbonate mass accumulation rates mars correlate with more radiogenic nd values indicating increased flow of corrosive pacific intermediate water into the caribbean basin during intervals of dissolution this flow pattern agrees with results from general ocean circulation models designed to study the effect of the shoaling of the central american seaway low carbonate mars and high nd values also correlate with intervals of increased northern component water production and therefore enhanced conveyor circulation suggesting that the conveyor may respond to changes in circulation associated with shoaling of the central american seaway reduced pacific throughflow related to shoaling of the seaway led to a gradual increase in carbonate preservation and more atlantic like nd values following the carbonate crash

an epeiric ramp low energy cool water carbonate facies in a tertiary inland sea murray basin south australia the murray supergroup records temperate water carbonate deposition within a shallow mesotrophic oligo miocene inland sea protected from high energy waves and swells of the open ocean by a granitic archipelago at its southern margin rocks are very well preserved and exposed in nearly continuous outcrop along the river murray in south australia most facies are rich in carbonate silt contain a background assemblage of gastropods especially turritellids and infaunal bivalves and are packaged on a decimetre scale defined by firmground and hardground omission surfaces bioturbation is pervasive and overprinted resulting in rare preservation of physical sedimentary structures facies are grouped into four associations large foraminiferan bryozoan echinoid bryozoan mollusc and clay facies interpreted to represent shallow water 50 m deposition under progressively higher trophic resource levels from low mesotrophy to eutrophy and restricted marine conditions from relatively offshore to nearshore regions a large scale shift from high to low mesotrophic conditions within lower miocene strata reflects a change in climate from wet to seasonally dry conditions and highlights the influence terrestrially derived nutrients had upon this shallow land locked sea overall low trophic resource levels during periods of seasonally dry climate resulted in a deepening of the euphotic zone a widespread proliferation of foraminiferan photozoan fauna and a relatively high carbonate productivity inshore heterozoan facies became progressively muddier and restricted towards the shoreline in contrast periods of wet climate led to rising trophic resource levels resulting in a shallowing of the euphotic zone a decrease in epifaunal and seagrass cover and widespread development of a mostly heterozoan biota dominated by infaunal echinoids rates of carbonate production and accumulation were relatively low the murray basin is best described as an epeiric ramp wide facies belts developed in a shallow sea on a low angled slope reaching many hundreds of kilometres in length grainy shoal and back barrier facies were absent internally generated waves impinged the sea floor in offshore regions and because of friction along a wide and shallow sea floor created a low energy expanse of waters across the proximal ramp storms were the dominating depositional process capable of disrupting the entire sea floor

closure of the panama seaway during the pliocene implications for climate and northern hemisphere glaciation the panama hypothesis states that the gradual closure of the panama seaway between 13 million years ago 13 ma and 2 6 ma led to decreased mixing of atlantic and pacific water masses the formation of north atlantic deep water and strengthening of the atlantic thermohaline circulation increased temperatures and evaporation in the north atlantic increased precipitation in northern hemisphere nh high latitudes culminating in the intensification of northern hemisphere glaciation nhg during the pliocene 3 2 2 7 ma here we test this hypothesis using a fully coupled fully dynamic ocean atmosphere general circulation model gcm with boundary conditions specific to the pliocene and a high resolution dynamic ice sheet model we carry out two gcm simulations with closed and open panama seaways and use the simulated climatologies to force the ice sheet model we find that the models support the panama hypothesis in as much as the closure of the seaway results in a more intense atlantic thermohaline circulation enhanced precipitation over greenland and north america and ultimately larger ice sheets however the volume difference between the ice sheets in the closed and open configurations is small equivalent to about 5 cm of sea level we conclude that although the closure of the panama seaway may have slightly enhanced or advanced the onset of nhg it was not a major forcing mechanism future work must fully couple the ice sheet model and gcm and investigate the role of orbital and co2 effects in controlling nhg

closing of the central american seaway and the ice age a critical review 1 what role did the closing of the central american seaway play in enabling continental ice sheets to wax and wane over north america and fennoscandia a summary of relevant evidence presented here permits a causal relationship between them but can be interpreted to show none the common denominator of such evidence is the approximate simultaneity of that closing with global cooling and the first major ice advance at the resolution of paleoclimate however geologic evidence from central america places only weak constraints on when interchange between pacific and caribbean water ceased the strongest evidence for when north and south america became connected by a continuous land bridge the great american exchange of vertebrates assigns climate change the causal role for an arid central american climate typical of glacial times seems necessary for animals that inhabited savannas to pass through panama paleoceanographic and geochemical studies of environments and water masses in the eastern pacific western caribbean and atlantic in general call for continual change since 6 ma or earlier if evidence concurrent with global change near 3 5 3 ma seems more widespread than for other times hypothesized connections between a closed seaway and glaciation commonly call for profoundly different north atlantic ocean circulation but simulations using general circulation models provide a spectrum of differences in circulation for open and closed seaways might much not all of the evidence implicating the closing of the seaway in global climate change in fact be a consequence of that change and blind to central america s tectonic development

marine biota response to clastic sediment influx a quantitative approach abstract there have been few quantitative studies of the effects of siliciclastic and volcaniclastic sediment influx on carbonate producing organisms from the geological record this study presents the results of quantitative field petrographic and acid digestion analyses of samples from three cenozoic shallow water mixed carbonate volcani siliciclastic depositional environments the miocene wonosari formation of south java the miocene batu putih limestone of east borneo and the eocene collsuspina limestone of northeast spain carbonate production in these systems is dominated by larger benthic foraminifera coralline algae and corals analyses of the quantity grainsize and type of volcani siliciclastic sediment within these mixed successions have allowed assessment of the influence of these factors on shallow water carbonate producers larger benthic foraminifera and coralline algae exhibit the greatest tolerance to siliciclastic sediment influx abundant up to 55 and 38 wt respectively at a range of grain sizes to coarse sand platy corals are also present within sediments containing a large proportion of insoluble material geologically averaged rates of clastic accumulation and their potential influences on carbonate producers inferred herein may be difficult to compare directly with the daily and seasonal fluctuations experienced by modern clastic influenced systems however this study may aid our understanding of the likely effects of increased clastic influx today commonly anthropogenically related to marine carbonate environments over longer time periods

thin fine grained storm layers in a muddy shelf sequence an example from the lower jurassic in the stenlille 1 well denmark thin storm sand layers are a characteristic feature of many sandy shelf sequences while little evidence of storm layers is preserved in muddy shelf deposits where bioturbation is often total the present study shows that storm generated depositional processes governed sedimentation in a muddy shelf environment the studied sequence is dominated by blackish mudshale and silt streaked mudshale with thinner intervals of pale fine grained heterolithic sandstone it represents the lower jurassic fjerritslev formation in the eastern part of the danish basin and has been studied in four cores from the stenlille 1 well within the muddy outer shelf association four sedimentary facies and 14 types of laminae 0 5 10 mm thick may be distinguished most of the laminae share the characteristics of graded bedding erosive bases and occasional fading ripples with storm layers of sand grade the structures indicate that the mudshale laminae formed by settling of the suspension load carried by storm generated currents this kind of episodic deposition of fine grained storm layers may be common though rarely preserved in outer shelf environments

on the reconstruction of upwelling history namibia upwelling in context abstract eastern boundary upwelling is an important aspect of the modern ocean despite the relatively small area involved consequences of increased upwelling during the neogene as a result of intensifying trade winds include increased heat transport by the ocean increased mixing increased fractionation of phosphate from silicate and opportunities for evolution of new species coastal upwelling has been studied since the first decade of the 20th century the processes involved are complex and differ depending on geographic setting off namibia upwelling history has been studied by a number of drilling expeditions the last of these was odp leg 175 which occupied seven sites on the continental slope between walvis ridge and cape town during september and october of 1997 productivity proxies all along southwestern africa suggest the presence of increased upwelling and organic matter supply to the sea floor during glacial periods at the same time there is a decrease in the supply of diatoms and other siliceous plankton remains this is the walvis opal paradox established through the contrasting results of studies by diester haass 1985 and oberhansli 1991 we propose that the walvis opal paradox is fundamental to the understanding of glacial interglacial productivity fluctuations on a global scale furthermore a central feature of the history of late neogene upwelling off namibia is the matuyama opal maximum centered between gauss and olduvai magnetic chrons wefer et al proc odp 175 1998 it is due to the fact that diatom supply first increases during the gauss when the planet cools and then decreases again during additional cooling on entering the quaternary on a 400 000 year scale peak productions are coincident with or slightly lag maximum seasonal contrast potential in the high latitude insolation curve we suggest that this is further evidence that the nutrient content of thermocline waters was diminished during glacial periods the reasons why this should be so remain to be discovered

development of small carbonate banks on the south florida platform margin response to sea level and climate change abstract geophysical and coring data from the dry tortugas tortugas bank and riley s hump on the southwest florida margin reveal the stratigraphic framework and growth history of these carbonate banks the holocene reefs of the dry tortugas and tortugas bank are approximately 14 and 10 m thick respectively and are situated upon pleistocene reefal edifices tortugas bank consists of the oldest holocene corals in the florida keys with earliest coral recruitment occurring at 9 6 cal ka growth curves for the tortugas bank reveal slow growth

precipitation landsliding and erosion across the olympic mountains washington state usa abstract in the olympic mountains of washington state landsliding is the primary surface process by which bedrock and hillslope regolith are delivered to river networks however the relative importance of large earthquakes versus high magnitude precipitation events to the total volume of landslide material transported to valley bottoms remains unknown in part due to the absence of large historical earthquakes to test the hypothesis that erosion is linked to precipitation approximately 1000 landslides were mapped from google earth imagery between 1990 and 2015 along a 15 km wide 85 km long 1250 km2 swath across the range the volume of hillslope material moved by each slide was calculated using previously published area volume scaling relationships and the spatial distribution of landslide volume was compared to mean annual precipitation data acquired from the prism climate group for the period 1981 2010 statistical analysis reveals a significant correlation r 0 55 p

an assessment of the suitability of individual rhythmic carbonate successions for astrochronological application abstract a manual is presented to assist in assessing the suitability of individual calcareous rhythmites for high resolution stratigraphic applications and orbital dating while the astrochronological approach offers an unprecedented high temporal resolution for stratigraphy caution is required to carefully choose suitable rhythmites for the analysis diagenesis is known to distort or mimic primary signals in particular in carbonate rhythmites to keep orbital dating a reliable method the choice of sedimentary successions has to be undertaken with utmost scrutiny here we review and evaluate the methods that have been used for assessing the record of primary signals in calcareous rhythmites the goal is to provide the orbital stratigrapher with tools for a straightforward and systematic assessment of such successions

role of hurricanes in the holocene stratigraphy of estuaries examples from the gulf coast of florida abstract sarasota and little sarasota bays are shallow coastal bays located landward of a holocene barrier inlet complex on the west central microtidal gulf coast of florida sediments presently accumulating in the bay consist of 1 fine to very fine quartz sand contributed from the gulf shoreline during storm generated washover through tidal inlets and from reworking of older deposits 2 fine sand to pebble sized quartz and phosphatic sediment contributed by tertiary and pleistocene deposits 3 biogenic carbonate debris which is produced within the bays and or derived from the gulf of mexico 4 clay minerals derived from weathering of tertiary and pleistocene carbonates and clays and 5 particulate organic debris interpretations from 51 vibracores from throughout the bays have enable delineation of four major depositional sedimentary facies resulting from specific environmental conditions protected bay open bay tidal delta washover and storms bedrock in the area ranges from 0 to about 8 m msl and is largely responsible for the areal configuration of the bay and the location of the barrier islands the holocene stratigraphy of both bays has been greatly influenced by the passage of major hurricanes which carried large volumes of sand and shell gravel into the bays at least four of these storms are documented in these cores three storm units from sarasota bay have been radiocarbon dated at 2 270 1 320 and 240 years before present ybp historically documented severe hurricanes influenced this coast in 1848 and 1921 hurricanes interrupted the normal low energy slow deposition in the bays and caused inlets to open and close three storm generated sedimentary facies are identified 1 a graded storm facies which is composed of conspicuously fining upward units ranging from sandy shell gravel to slightly shelly quartz sand 2 a homogenous storm facies consisting of homogenous and relatively thin shelly sand and gravel units and 3 a fluvial storm facies which is muddy and generally free of shells representing terrigenous influx from runoff

changes in caribbean surface hydrography during the pliocene shoaling of the central american seaway pliocene d18o records of shallow and deep dwelling planktonic foraminifers from the caribbean ocean drilling program sites 999 and 1000 the tropical east pacific sites 1241 and 851 and the atlantic site 925 ceara rise and site 1006 western great bahama bank were used to examine atlantic caribbean pacific atmospheric and oceanic linkages associated with the progressive closure of the central american seaway 5 5 3 ma comparisons indicate the development of an inner caribbean salinity gradient in the mixed layer and salinity changes on precessional periodicities after 4 4 ma site 1000 when the pacific caribbean throughflow became significantly restricted the high amplitude variability in salinity is also observed at site 1006 monitoring the caribbean outflow into the atlantic comparisons of caribbean and atlantic planktonic d18o records suggest the north atlantic subtropical gyre as a major source for high salinity surface waters precession induced variations in the volume transport of pacific surface water masses through the panamanian seaway are considered a main factor to explain the caribbean salinity minima results from a coupled climate model point to changes in the el nin o southern oscillation state as a potential trigger for changes in the amount of pacific inflow into the caribbean

modelling tempo spatial signatures of heinrich events influence of the climatic background state abstract different sea surface temperature sst reconstructions for the last glacial maximum are applied to a hybrid coupled climate model the resulting oceanic states are perturbed by north atlantic meltwater inputs in order to simulate the effect of heinrich events on the atlantic thermohaline circulation thc and sst the experiments show that both the atlantic sst signature of the meltwater event and the time span of thc recovery strongly depend on the climatic background state data model comparison reveals that the overall spatial signature of sst anomalies is captured much better in the glacial meltwater experiments than in an analogous experiment under present day conditions in particular a breakdown of the modern thc would induce a much stronger temperature drop in high northern latitudes than did heinrich events during the ice age moreover our results suggest that the present day circulation can settle into a stable off mode whereas the glacial thc was mono stable mono stability may serve as an explanation for the recovery of the thc after heinrich event shutdowns during the last glaciation

reorganization of miocene deep water circulation in response to the shoaling of the central american seaway 1 the response of ocean circulation to the shoaling of the central american seaway cas is investigated using the massachusetts institute of technology mit ocean general circulation model ogcm in contrast to earlier model studies it is found that significant amounts of deep water are formed in the north atlantic prior to the closure of the cas however the circulation pattern is fundamentally different from the modern ocean in the upper layers of the cas there is a relatively strong geostrophic flow from the pacific to the atlantic controlled by the pressure difference across the seaway however when the cas is deeper than the level of north atlantic deep water nadw outflow a significant amount of nadw passes through the cas to the pacific ocean in the pacific the deep water traverses the basin from east to west in a relatively narrow zonal jet and becomes a southward flowing western boundary current before it joins with the antarctic circumpolar current acc to the south this implies that deep sea sediment records from the miocene pacific ocean could have been influenced by relatively young nadw and provides a new framework for the interpretation of geochemical tracer data diversification of benthic foraminifer fauna suggests that the cas shoaled to a depth of about 1000 m toward the end of the middle miocene this would have prevented the passage of nadw to the pacific and established the modern deep water circulation pattern at that time

evolution of the late miocene mediterranean atlantic gateways and their impact on regional and global environmental change marine gateways play a critical role in the exchange of water heat salt and nutrients between oceans and seas as a result changes in gateway geometry can significantly alter both the pattern of global ocean circulation and associated heat transport and climate as well as having a profound impact on local environmental conditions mediterranean atlantic marine corridors that pre date the modern gibraltar strait closed during the late miocene and are now exposed on land in northern morocco and southern spain the restriction and closure of these miocene connections resulted in extreme salinity fluctuations in the mediterranean leading to the precipitation of thick evaporites this event is known as the messinian salinity crisis msc the evolution and closure of the mediterranean atlantic gateways are a critical control on the msc but at present the location geometry and age of these gateways are still highly controversial as is the impact of changing mediterranean outflow on northern hemisphere circulation here we present a comprehensive overview of the evolution of the late miocene gateways and the nature of mediterranean atlantic exchange as deduced from published studies focussed both on the sediments preserved within the fossil corridors and inferences that can be derived from data in the adjacent basins we also consider the possible impact of evolving exchange on both the mediterranean and global climate and highlight the main enduring challenges for reconstructing past mediterranean atlantic exchange

4 ma coarsening of sediments from baikal chinese loess plateau and south china sea and implications for the onset of nh glaciation abstract intense tectonic movement occurred along the northeastern margin of the tibetan plateau 3 4 ma during the same time period global climate changed from a period of stability to a period of frequent and abrupt changes which prevented fluvial and glacial systems from establishing a state of equilibrium thus it is difficult to directly attribute the 3 4 ma coarse grained sediment accumulation in east asia to climate change or tectonic activity this study compares late cenozoic multi proxy records from the chinese loess plateau clp to the low latitude grain size sea level and benthic 18 o records from the south china sea scs and the high latitude grain size record from lake baikal all records suggest a steady and persistent cooling since 4 ma which correlates well with those cooling records from other regions around the world this coincided in timing with the modeling results that the closure of the central american seaway cas initiated strengthening of atlantic meridional overturning circulation between 4 8 and 4 0 ma which led to both warming of the northern hemisphere nh and cooling of the southern hemisphere sh cooling of the sh would induce a marked development of the antarctic ice sheets at 4 ma pushing the intertropical convergence zone itcz northward this was superimposed on warming of the nh and brought more precipitation to the middle latitudes of the nh resulting in increases in coarse grained sediments in the sikouzi section from the western clp since 4 2 ma notably the absence of coarse grained sedimentation in the sikouzi section during 3 0 2 1 ma and the gradual decrease in coarse grained peaks since 2 1 ma reflected by the sikouzi grain size record probably resulted from stepwise increase in the nh ice volume pushing the itcz southward on the other hand development of the antarctic ice sheets would induce global cooling and enhancement of physical weathering initiating increases in sedimentation rates as well as increases in grain size from lake baikal to the clp to the scs therefore the closure of the cas during 4 8 4 0 ma and its influence on ocean heat transport was possibly the major forcing factor for global cooling since 4 ma a persistent and steady cooling during 4 3 ma probably made a significant contribution to the establishment of the nh ice sheets at 2 75 ma ago

depositional events and their records an introduction abstract event deposits of some lateral extent as defined here occur almost in all types of sedimentary basins the reflect either local intra basinal processes or they are associated with regional or global mechanisms some types of event beds such as sediment gravity flows sandy and muddy turbidites are common and usually well preserved they make up large proportions of basin fills whereas others e g tsunami deposits and in situ earthquake structures appear to be less frequent or rare in the geological record volume frequency and facies of event deposits are controlled by several processes pre event sediment accumulation triggering and transport mechanisms and mode of final deposition

multiproxy approach to understanding the origin of cretaceous pelagic limestone marl alternations dsdp site 391 blake bahama basin limestone marl alternations are usually directly interpreted to reflect cyclic palaeoenvironmental signals however uncertainty in such interpretations stems from the differential diagenesis that most limestone marl alternations have undergone differential diagenesis results in markedly different alterations between limestones and marls and in the loss of comparability of many measurable parameters for an unequivocal interpretation of the origin of rhythmic alternations diagenetically robust parameters or parameters that clearly indicate the degree of diagenetic bias are required the present study uses a multiproxy approach independent biotic sedimentary and geochemical parameters in order to unravel the palaeoenvironmental signal recorded in valanginian early cretaceous limestone marl alternations from the blake bahama basin dsdp site 391 using this approach terrestrial and marine influences can be distinguished changes in nutrient levels estimated and prediagenetic differences in the non carbonate fraction constrained surprisingly no systematic variations in any of these parameters were observed between limestone and marl layers implying that none of these was directly responsible for the formation of the rhythmic alternation hence none of the current models of sedimentary formation of limestone marl rhythmites is applicable here calcareous nannofossils are equally well preserved in limestone and marl layers ruling out their dissolution in marl layers as a source of the calcite cement in the limestone beds sr values of 700 900 p p m indicate that aragonite may have been present in the original pelagic sediment the assumption of fine grained sedimentary aragonite imported from nearby carbonate platforms as the source of the cement would explain a number of otherwise enigmatic features in these rhythmites including the source of the calcite cement observed in the limestones the equally good preservation of calcareous nannofossils in limestones and marls and the higher concentration of calcareous nannofossils in marl layers the study demonstrates that examination of diagenetically inert parameters or parameters in which diagenetic effects can be filtered can yield unexpected results clearly careful analysis of such parameters needs to be undertaken in order to make reliable palaeoenvironmental interpretations from rhythmite successions

glacial interglacial variability in atlantic meridional overturning circulation and thermocline adjustments in the tropical north atlantic article i nfo k 37 tex86 atlantic meridional overturning circulation thermocline changes in the strength of atlantic meridional overturning circulation amoc are known to have profound impacts on global climate coupled modelling studies have suggested that on annual to multi decadal time scales a slowdown of amoc causes a deepening of the thermocline in the tropical atlantic however this process has been poorly constrained by sedimentary geochemical records here we reconstruct surface u k 37 index and thermocline tex h 86 water temperatures from the guinea plateau margin eastern tropical atlantic over the last two glacial interglacial cycles 192 kyr these paleotemperature records show that periods of reduced amoc as indicated by the 13 c benthic foraminiferal record from the same core coincide with a reduction in the near surface vertical temperature gradient demonstrating for the first time that amoc induced tropical atlantic thermocline adjustment exists on longer millennial time scales modelling results support the interpretation of the geochemical records and show that thermocline adjustment is particularly pronounced in the eastern tropical atlantic thus variations in amoc strength appear to be an important driver of the thermocline structure in the tropical atlantic from annual to multi millennial time scales

the early matuyama diatom maximum off sw africa a conceptual model abstract an important discovery during ocean drilling program leg 175 when investigating the record of upwelling off namibia was the finding of a distinct late pliocene diatom maximum spanning the lower half of the matuyama reversed polarity chron mdm matuyama diatom maximum and centered around 2 6 2 0 ma this maximum was observed at all sites off southwestern africa between 20 s and 30 s and is most strongly represented in sediments of site 1084 off luderitz namibia the mdm is characterized by high biogenic opal content high numbers of diatom valves and a diatom flora rich in southern ocean representatives with thalassiothrix antarctica forming diatom mats as well as coastal upwelling components before mdm time diatoms are rare until ca 3 6 ma after the mdm in the pleistocene the composition of the diatom flora points to increased importance of coastal upwelling toward the present but is accompanied by a general decrease in opal and diatom deposition here we present a simple conceptual model as a first step in formalizing a possible forcing mechanism responsible for the record of opal deposition in the upwelling system off namibia the model takes into account southern ocean oceanography and a link with deepwater circulation and deepwater nutrient chemistry which in turn are coupled to the evolution of north atlantic deep water nadw the model proposes that between the mdm and the mid pleistocene climate revolution opal deposition off namibia is not directly tied to glacial interglacial fluctuations as seen in the global 18o record but that instead a strong deepwater link exists with increased nadw production as seen in the deepwater 13c record accounting for higher supply of silicate to the thermocline waters that feed the upwelling process the opal record of site 1084 shows affinity to eccentricity on the 400 kyr scale but not for the 100 kyr scale this points toward long term geologic processes for delivery of silica to the ocean

impacts of andean uplift on the humboldt current system a climate model sensitivity study 1 using both an atmospheric global circulation model and a regional oceanic model we assess the impact of the andes uplift on the humboldt current system hcs with three topographic scenarios for the andes we show that the uplift played a crucial role in shaping the lower troposphere dynamics over the pacific ocean and in turn affected the physical characteristics of the system our results show that the uplift has triggered a marked southward shift of the hcs changing the latitudinal position of coastal upwelling areas as well as the strength of the south pacific gyre our oceanic modeling experiments show that the latitudinal pattern of sea surface temperatures is dramatically modified with reduced andes with anomalies reaching up to 3 c we discuss the feedbacks of these changes on south american climate as well as the potential onset of an altitudinal threshold that would have had triggered these changes

neogene stratigraphy paleoceanography and paleobiogeography in northwest south america and the evolution of the panama seaway abstract the atrato basin in the pacific coastal region of nw south america colombia exhibits stratigraphic and biostratigraphic characteristics similar to other neogene pacific coastal basins of southern central america and northern south america an evaluation of the neogene stratigraphy and foraminiferal biostratigraphy with emphasis on paleoceanographic and paleobiogeographic phenomena indicate the following 1 previous to the middle miocene well aerated deep and open oceanic conditions and free and active water circulation prevailed along the steep continental margins of nw south america 2 during the early middle miocene right after the neogene hiatus nh 2 15 2 ma tectonic disturbances triggered the initial uplift of the panama sill bottom water circulation foraminiferal and sedimentation changes are observed in both the pacific and caribbean coastal areas of nw south america 3 during the middle miocene coinciding with the neogene hiatus nh 3 12 9 11 8 ma an abrupt paleobathymetric change from lower to middle bathyal depths indicates a major uplift of the panama sill to about 1000 m reflecting the middle miocene tectonic disturbances in nw south america 4 during the late middle miocene immediately following this uplift a distinctive pacific benthic foraminiferal fauna abruptly appeared and extended from ecuador to california because these assemblages do not occur in the adjacent caribbean region a circulation barrier between the atlantic and pacific oceans as a result of intensification of the cool marginal california current is proposed anoxic middle to upper bathyal environments prevailed under well aerated cool surface waters in the pacific coastal areas of nw south america 5 during the latest miocene immediately following the neogene hiatus nh 6 7 0 6 3 ma surface water circulation between the caribbean and the pacific was re established and the previous influence of the california current disappeared a rapid filling and shallowing of the basin to mostly neritic depths is indicated along the pacific and caribbean coastal basins of colombia as well as a progressive increase in similarity between the coastal pacific and caribbean benthic biotas this shallowing in the early late miocene also appears to have been associated with the earliest terrestrial interchange of ground sloths to north america and raccoons and their allies to south america 6 immediately following early pliocene time the panamanian isthmus became completely emergent providing a terrestrial environment favourable for intermingling of terrestrial faunas and floras between north and south america i correlate this pliocene uplift with the neogene hiatus nh 8 3 7 3 1 ma four regional unconformities associated with tectonic disturbances in the nw corner of south america have also been recognized and correlated with neogene hiatuses recorded from the oceans early middle miocene middle miocene late miocene and early pliocene

a coastal upwelling seesaw in the atlantic ocean as a result of the closure of the central american seaway 1 based on coupled climate model experiments we suggest an antiphase relationship between the two major coastal upwelling zones in the atlantic ocean off northwestern and southwestern africa we describe this as an upwelling seesaw the state of the coastal upwelling seesaw is controlled by variations in the oceanic meridional heat transfer between south and north atlantic in particular we suggest that the pliocene closure of the central american seaway induced a large scale redistribution of heat in the atlantic ocean leading to an intensification weakening of upwelling off southwest northwest africa

testing the influence of the central american seaway on orbitally forced northern hemisphere glaciation 1 utilizing a climate model of intermediate complexity we test the hypothesis that the pliocene closure of the central american seaway was a necessary precondition for orbitally triggered northern hemisphere glaciation we conduct a series of sensitivity experiments in order to analyze the isolated and combined effects of panama closure and orbital forcing on northern hemisphere perennial snow cover while orbital forcing efficiently controls the extension of perennial snow cover the model results suggest that the closure of the central american seaway did not intensify orbitally forced glaciation in high northern latitudes

late pliocene vegetation and climate in namibia southern africa derived from palynology of odp site 1082 the present day condition of bipolar glaciation characterized by rapid and large climate fluctuations began at the end of the pliocene with the intensification of the northern hemisphere continental glaciations the global cooling steps of the late pliocene have been documented in numerous studies of ocean drilling program odp sites from the northern hemisphere however the interactions between oceans and between land and ocean during these cooling steps are poorly known in particular data from the southern hemisphere are lacking therefore i investigated the pollen of odp site 1082 in the southeast atlantic ocean in order to obtain a high resolution record of vegetation change in namibia between 3 4 and 1 8 ma four phases of vegetation development are inferred that are connected to global climate change 1 before 3 ma extensive rather open grass rich savannahs with mopane trees existed in namibia but the extension of desert and semidesert vegetation was still restricted 2 increase of winter rainfall dependent renosterveld like vegetation occurred between 3 1 and 2 2 ma connected to strong advection of polar waters along the namibian coast and a northward shift of the polar front zone in the southern ocean 3 climatically induced fluctuations became stronger between 2 7 and 2 2 ma and semiarid areas extended during glacial periods probably as the result of an increased pole equator thermal gradient and consequently globally enhanced atmospheric circulation 4 aridification and climatic variability further increased after 2 2 ma when the polar front zone migrated southward and the influence of atlantic moisture brought by the westerlies to southern africa declined it is concluded that the positions of the frontal systems in the southern ocean which determine the locations of the high pressure cells over the south atlantic and the southern indian ocean have a strong influence on the climate of southern africa in contrast to the climate of northwest and central africa which is dominated by the saharan low pressure cell

the hydrodynamic and sedimentary setting of nearshore coral reefs central great barrier reef shelf australia paluma shoals a case study the great barrier reef gbr shelf contains a range of coral reefs on the highly turbid shallow inner shelf where interaction occurs with terrigenous sediments the modern hydrodynamic and sedimentation regimes at paluma shoals a shore attached turbid zone coral reef and at phillips reef a fringing reef located 20 km offshore have been studied to document the mechanisms controlling turbidity at each reef waves currents and near bed turbidity were measured for a period of 1 month bed sediments were sampled at 135 sites on the inner shelf muddy sands are widespread with admixed terrigenous and carbonate gravel components close to the reefs and islands except on their relatively sheltered sw side where sandy silty clays occur at paluma shoals the coral assemblage is characteristic of inner shelf or sheltered habitats on the gbr shelf dominated by galaxea fascicularis up to 50 coral cover and is broadly similar to that at phillips reef further offshore and in deeper water the sediments of the paluma shoals reef flats consist of mixed terrigenous and calcareous gravels and sands with intermixed silts and clays whereas the reef slope is dominated by gravelly quartz sands the main turbidity generating process is wave driven resuspension and turbidity ranges up to 175 nephelometric turbidity units ntu in contrast at phillips reef turbidity is 40 ntu probably occurs for a total of 40 days each year and relatively little time is spent at intermediate turbidities 15 50 ntu the extended time spent at either low or high turbidities is consistent with the biological response of some species of corals to adopt two alternative mechanisms of functioning autotrophy and heterotrophy in response to different levels of turbidity sedimentation rates over periods of hours may reach the equivalent of 10 000 times the mean global background terrigenous flux btf of sediment to the sea floor i e 10 000 btf over three orders of magnitude greater than the holocene average for halifax bay of 3 btf as elsewhere along the nearshore zone of the central gbr dry season hydrodynamic conditions form a primary control upon turbidity and the distribution of bed sediments the location of modern nearshore coral reefs is controlled by the presence of suitable substrates which in halifax bay are pleistocene and early holocene coarse grained and relatively stable alluvial deposits

early pliocene change of deposition style in the cape basin southeastern atlantic many effects of climate change currents and tectonics on sedimentary deposition can be detected in seismic reflection patterns to unravel the influence of these different forces on sediments of the southwest african margin we established a seismostratigraphy tied to the drilling results from ocean drilling program leg 175 sites 1081 1087 three depositional regions are distinguished for the cape basin in the northern and middle cape basin continuous high amplitude reflectors characterize the upper acoustic sequences indicating a strong and probably climate related alternation in deposited matter as well as an undisturbed deposition accompanied by high sedimentation rates due to enhanced upwelling in the middle cape basin large reflection free zones indicate the presence of gas in contrast in the southern cape basin less upwelling related deposition restricted the thickness of sedimentary layers low amplitude reflectors and slump scarps indicate a permanent reallocation of accumulated matter by mass movements on the steeper slope of the southern margin a prominent feature in the sequences of the northern and middle cape basin is a marked increase in seismic reflection amplitudes since the early pliocene ca 4 4 ma in correspondence to a rise of cyclicity in reflector strength we suggest a reorganization in the depositional system associated with enhanced upwelling as a consequence of the closure of the central american sea way the upwelling system probably became more susceptible to short pulse variations 10 k y in climate and currents with it the composition of deposited matter severely changed which in turn is documented in the high amplitude reflectors

patterns and controls of surface sediment distribution west central florida inner shelf abstract the west central florida inner shelf represents a transition between the quartz dominated barrier island system and the carbonate dominated mid outer shelf surface sediments exhibit a complex distribution pattern that can be attributed to multiple sediment sources and the ineffectiveness of physical processes for large scale sediment redistribution the west florida shelf is the submerged extension of the florida carbonate platform consisting of a limestone karst surface veneered with a thin unconsolidated sediment cover a total of 498 surface sediment samples were collected on the inner shelf and analyzed for texture and composition results show that sediment consists of a combination of fine quartz sand and coarse biogenic carbonate sand and gravel with variable but subordinate amounts of black phosphorite rich sand the carbonate component consists primarily of molluskan fragments the distribution is patchy and discontinuous with no discernible pattern and the transition between sediment types is generally abrupt quartz rich sediment dominates the inner 15 km north of the entrance into tampa bay but south of the bay is common only along the inner 3 km elsewhere carbonate rich sediment is the predominate sediment type except where there is little sediment cover in which cases black phosphorite rich sand dominates sediment sources are likely within or around the periphery of the basin fine quartz sand is likely reworked from coastal units deposited during pleistocene sea level high stands carbonate sand and gravel is produced by marine organisms within the depositional basin the black phosphorite rich sand likely originates from the bioerosion and reworking of the underlying strata that irregularly crop out within the study area the distribution pattern contains elements of both storm and tide dominated siliciclastic shelves but it is dictated primarily by the sediment source similar to some carbonate systems other systems with similar sediment attributes include cool water carbonate sediment starved and mixed carbonate siliciclastic systems this study suggests a possible genetic link among the three systems

consequences of shoaling of the central american seaway determined from modeling nd isotopes the central american seaway played a pivotal role in shaping global climate throughout the late cenozoic recent geological surveys have provided new constraints on timing of the seaway shoaling while neodymium isotopic e nd data measured on fossil teeth debris and ferromanganese crusts have helped define the history of water masses in the region here we provide the first 3 d simulations of e nd responses to the shoaling seaway our model suggests that a narrow and shallow seaway is sufficient to affect interoceanic circulation that inflow outflow balance between the caribbean and the antilles responds nonlinearly to sill depth and that a seaway narrower than 400 km is consistent with an active atlantic meridional overturning circulation during the late miocene simulated e nd values in the caribbean confirm that inputs from radiogenic pacific waters in the caribbean decrease as the seaway shoals despite model limitations a comparison between our results and e nd values recorded in the caribbean helps constrain the depth of the central american seaway through time and we infer that a depth between 50 and 200 m could have been reached 10 ma ago

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

physical processes and fine grained sediment dynamics coast of surinam south america abstract the prograding holocene mud wedge between the amazon and orinoco rivers in the trade wind belt of northeastern south america provides a modern day example of muds accumulating under moderate wave energy conditions gigantic shore attached mudbanks 10 km 20 km composed partly of thixotropic fluid mud gel front this coast every 30 60 km to form a buffer to wave attack and a temporary storage for fine grained sediments this mesotidal coast tide range 2 0 m with gentle offshore slope 0 0006 allows the exposure twice a day of extensive tidal flat deposits which are backed by mangrove swamps on a well developed chenier plain complex field experiments were conducted in surinam du ing 1975 and 1977 to provide new information on process form relationships in this interesting but unusual muddy environment simultaneous measurements of waves currents tide elevation suspended sediment concentration and variations in mud density show that soft intertidal and subtidal muds are suspended at both tide and wave frequency suspended sediment concentrations typically exceed 1 000 mg l at the surface as incoming solitary like waves partially disperse fluid mud into overlying water on a falling or rising tide redeposition of mud may occur near time of high tide the strong attenuation of shallow water waves by these muds provides conditions that are favorable for further sedimentation high concentrations of suspended fluid mud together with solitary like waves from the northeast throughout the year can lead to extraordinarily high net sediment transport rates in the nearshore zone calculations based on solitary wave theory and on data obtained from this study indicate that 15 65 106 m3 of mud can move along shore each year without involving breaking waves the concept of radiation stress and a nearshore circulation cell or bedload transport farther offshore outside the zone of wave dominance wind driven currents and the guiana current combine to transport muds to the northwest consistent with the observed direction of mudflat migration

late miocene washhouse climate in europe abstract we present two eight million year long proxy records of precipitation for southwest and central europe covering the middle to late miocene 5 3 13 ma at a temporal resolution of about 60 kyr and 150 kyr respectively the estimates of precipitation are based on the ecophysiological structure of herpetological assemblages amphibians and reptiles from 13 0 ma until about 9 ma both records show a similar trend evolving from a long dry period 13 11 ma into a washhouse climate 10 2 9 8 ma characterized by global warm conditions and several times more precipitation than present the transition from washhouse to a dryer climate between 9 7 and 9 5 ma and the concomitant cooling episode appear to have triggered a severe biotic event known as the vallesian crisis which included the extinction of hominoids in western europe a second washhouse period 9 0 8 5 ma coeval with a global warm episode was unprecedentedly intense in southwest europe but less pronounced in central europe from 8 ma onward a divergence in the two precipitation records is observed with southwest europe staying wetter and central europe becoming dryer than present both precipitation records are combined into a common run off curve as a measure of the relative intensity of the hydrological cycle for moderate latitudes of continental europe the run off curve shows a remarkable positive correlation with atlantic deep water temperatures from ceara rise by lear et al 2003 which are significantly higher up to 3 c during the two washhouse periods and show no other positive excursion of comparable magnitude we discuss potential links and the role of the coeval temporary restriction of the central american seaway on ocean and atmosphere circulation

differential diagenesis of rhythmic limestone alternations supported by palynological evidence alternating cemented and uncemented fine grained layers from pliocene periplatform carbonates of the great bahama bank have fundamentally different diagenetic features the cemented layers consist predominantly of microspar interpreted as an early marine shallow burial cement the intercalated uncemented softer layers are devoid of microspar cement and exhibit signs of mechanical compac tion precursor sediments of both cemented and uncemented layers apparently consisted largely of aragonite needles the needles in the compacted layers show signs of dissolution suggesting that the calcium carbonate required for cementation of the uncompacted layers was provided by aragonite dissolution within the compacted layers the lack of compaction in the cemented layers shows that cementation and hence aragonite dissolution in the adjacent compacted layers took place in the shallow burial realm the dissolved carbonate was trans ported by diffusion to adjacent layers and reprecipitated as calcite cement thereby preventing significant compaction of these layers these processes are not yet complete in the material examined and some aragonite remains in the compacted layers the sedimentary composition of the two rock types compacted and uncompacted is similar indicating a similar precursor sediment for both between 1 5 and 7 times the concentration of palynomorphs oc curs in the compacted layers apparently the result of passive diage netic enrichment although the trigger for diagenetic differentiation has not been determined the pure limestone succession studied here appears to serve as a clay free analog to limestone marl alternations

severe storms during earth history although severe storms are recognized as important agents in sediment transport many questions remain to demonstrate genetic links between specific sedimentary structures and textures and proposed generative agents a detailed physically based consideration of climatic and geographic controls on the generation and distribution of severe storms is presented on the basis of modern observations physical processes deduced from these observations and climate model experiments using past geographies the results suggest that simple latitudinal distinctions for storm type severe winter storm or hurricane or inference from estimates of global warmth are not adequate to consider genetic links for storm related deposits climate and geography produce substantial variability in storm generation rates and in storm distributions for example geography may inhibit poleward penetration of hurricanes or enhance a broad latitudinal distribution the record of tropical sea surface temperatures indicates that values may have been below hurricane generation thresholds during the eocene and glacial stages land sea distribution and associated thermal contrasts strongly influence the distribution and intensity of winter storms the results from models can be compared favorably with observations of probable storm sedimentation

limestone marl alternations as environmental archives and the role of early diagenesis a critical review limestone marl alternations and other micritic calcareous rhythmites have long appealed to sedimentologists as they appeared to directly reflect high frequency environmental change in particular when orbital forcing gained popularity amongst sedimentologists and paleoclimatologists in 1980s such rhythmites seemed to offer an ideal tool for high resolution chronostratigraphy and environmental reconstruction however in spite of the fact that orbital forcing has become a routine interpretation of calcareous rhythmites and that the processes of formation of calcareous rhythmites are considered well understood research in the past 10 years again has questioned their primary origin and their direct interpretability detailed petrographic paleontological and geochemical data from numerous successions through geological time provided the basis for testing whether or not the regular alternation of limestone beds and marl or shale interlayers represents bimodally fluctuating environmental conditions in a direct way in particular these data supplemented by box model simulations imply that post depositional alteration diagenesis has the potential to not only seriously distort primary environmental signals but also to mimic primary signals this questions the use of micritic calcareous rhythmites for high resolution chronostratigraphy and for environmental interpretations where independent data of diagenetically inert parameters are not available diagenetic changes appear to have a yet widely underestimated influence on the appearance of limestone marl alternations and other calcareous rhythmites the aim of the present review is to summarize new approaches and give an overview of our research results in this field of the past decade this review also aims at pointing to still enigmatic aspects that need to be addressed before the interpretation of micritic calcareous rhythmites can be considered a reliable tool for high resolution chronostratigraphy and paleoenvironmental interpretation

estimating palaeo water depth from the physical rock record abstract palaeo water depth and its change with time is a key factor in environmental analysis and in sedimentology in general the literature published on this topic and on related subjects however is substantial albeit scattered within and between different communities geologists oceanographers engineers physicists etc here i focus on proxies for palaeo bathymetry as obtained from the physical rock record floral faunal and chemical evidence for palaeo water depth being equally important must be dealt with elsewhere an assessment of the main depth related parameters in actualistic settings is presented and their applicability to the fossil sedimentary record discussed the outcome reveals the full complexity of this topic there is for example no such thing as an average wave base depth applicable to a wide range of fossil case settings this because wave and current climates in general and particularly so in neritic and coastal settings are intriguingly complex furthermore observations from modern oceans are not a priori applicable to fossil ones acknowledging these problems and limitations critically evaluated quantitative proposals for depth indicative facies and features in the sedimentary record are presented and error bars discussed

changes in equatorial pacific thermocline depth in response to panamanian seaway closure insights from a multi model study article i nfo the early pliocene warm phase was characterized by high sea surface temperatures and a deep thermocline in the eastern equatorial pacific a new hypothesis suggests that the progressive closure of the panamanian seaway contributed substantially to the termination of this zonally symmetric state in the equatorial pacific according to this hypothesis intensification of the atlantic meridional overturning circulation amoc in duced by the closure of the gateway was the principal cause of equatorial pacific thermocline shoaling dur ing the pliocene in this study twelve panama seaway sensitivity experiments from eight ocean climate models of different complexity are analyzed to examine the effect of an open gateway on amoc strength and thermocline depth all models show an eastward panamanian net throughflow leading to a reduction in amoc strength compared to the corresponding closed panama case in those models that do not include a dynamic atmosphere deepening of the equatorial pacific thermocline appears to scale almost linearly with the throughflow induced reduction in amoc strength models with dynamic atmosphere do not follow this simple relation there are indications that in four out of five models equatorial wind stress anomalies amplify the tropical pacific thermocline deepening in summary the models provide strong support for the hypothesized relationship between panama closure and equatorial pacific thermocline shoaling

miocene pliocene vegetation change in south western africa odp site 1081 offshore namibia abstract aridification is an important component of late neogene climate change in south western africa probably caused by modifications in the atmospheric circulation in relation to the initiation and intensification of the benguela upwelling system due to globally steepening of the meridional pressure gradient intensification of the meridional pressure gradient influenced the climate intensively which had then an impact on the vegetation however vegetation changes of south western africa from the miocene to pliocene have not yet been reported and only indirectly investigated by sedimentological data here we present a pollen record of marine odp site 1081 retrieved 160 km offshore namibia covering the time between 9 and 2 7 ma using an endmember unmixing model we distinguished three vegetation phases as follows a relative wet phase during the tortonian showing higher representations of cyperaceae a transition phase during the messinian when especially grasses expanded and a dry one covering the pliocene with a strong representation of desert and semi desert plants the three phases indicate ongoing aridification probably caused by intensified meridional pressure gradients additionally aquatic vegetation indicators appear in our pollen record from around 5 ma on which we attribute to a relocation of the lower course of the cunene river to its modern outlet in the atlantic ocean redirection of the cunene river toward the atlantic would have deprived the palaeolake cunene of an important source of fresh water ultimately resulting in desiccation of the lake and the formation of the etosha pan

uplift of africa as a potential cause for neogene intensification of the benguela upwelling system the benguela current cooled over the past 12 million years numerical modelling suggests that uplift of parts of africa during this time enhanced coastal low level winds and promoted greater upwelling of cool subsurface waters

linking desert evolution and coastal upwelling pliocene climate change in namibia a late pliocene high resolution pollen record from the southeast atlantic ocean drilling program site 1082 registers vegetation development in southwest africa the marine record is continuous ranges from 3 5 to 1 7 ma and has a millennial resolution between 2 9 and 1 9 ma changes in climate and vegetation correspond to the matuyama diatom maximum of the namibian upwelling system and seem to be highly susceptible to latitudinal shifts in the polar front zone of the southern ocean a northward advance of the polar fronts is connected with an increase in winter rainfall in southwest africa rapid desiccation in namibia at 2 2 ma is associated with increasing upwelling and decreasing sea surface temperatures along the coast

palaeoceanography tectonically driven upwelling upwelling within the highly productive benguela current off the namibian coast began in and intensified throughout the neogene epoch model simulations indicate its development was intimately connected to evolving topography and mountain uplift in africa

shallow water marl limestone alternations in the late jurassic of western france cycles storm event deposits or both abstract the contribution of event deposits to various basin fills can be very significant higher than 90 in some cases events may lead to the formation of marl limestone alternations which can also result from cyclic changes in sea level or climate for example the marl limestone alternations of the late jurassic of western france contain abundant coarse grained accumulations that resemble storm deposits described in other western european successions the detailed analysis of facies evolution and hierarchical high frequency stacking pattern of depositional sequences of the phare de chassiron section ile d oleron western france allows the controls on marl limestone formation to be defined this section contains nearshore and shallow marine mud deposits that were exposed to high energy events elementary small and medium scale depositional sequences are defined the stacking pattern and the duration of these sequences suggest an orbital control on sedimentation precession 20 ka cycles notably controlled the formation of elementary sequences that correspond to marl limestone alternations the deposition of marly or carbonate mud occurred in this storm dominated system because of muddy sea beds the gentle slope of the shelf and the great amount of particles in suspension which reduced water energy resulting from storms sediment supply was also sufficient to limit bioturbation and favour the preservation of numerous storm deposits the production of carbonate mud was localised on positive structures and partly controlled by milankovitch scale sea level cycles transport by storms of carbonate mud to the adjacent marly depressions during high carbonate production periods led to the formation of calcareous beds marl limestone alternations in the late jurassic of western france therefore result from the combined effects of cyclic changes in carbonate production and high energy episodic events

palynological evidence for a southward shift of the north atlantic current at 2 6 ma during the intensification of late cenozoic northern hemisphere glaciation the position of the north atlantic current nac during the intensification of northern hemisphere glaciation inhg has been evaluated using dinoflagellate cyst assemblages and foraminiferal geochemistry from a 260 kyr interval straddling the base of the quaternary system from two sites eastern north atlantic deep sea drilling project site 610 in the path of the present nac and central north atlantic integrated ocean drilling program site u1313 in the subtropical gyre stable isotope and foraminiferal mg ca analyses confirm cooling near the marine isotope stage mis g7 g6 transition 2 74 ma however a continued dominance of the dinoflagellate cyst operculodinium centrocarpum sensu wall and dale 1966 indicates an active nac in the eastern north atlantic for a further 140 kyr at mis 104 2 60 ma a profound dinoflagellate cyst assemblage turnover indicates nac shutdown in the eastern north atlantic implying elevated atmospheric pressure over the arctic and a resulting shift in the westerlies that would have driven the nac these findings challenge recent suggestions that there was no significant southward shift of the nac or the arctic front during inhg and reveal a fundamental climatic reorganization near the base of the quaternary

upwelling intensification as part of the pliocene pleistocene climate transition a deep sea sediment core underlying the benguela upwelling system off southwest africa provides a continuous time series of sea surface temperature sst for the past 4 5 million years our results indicate that temperatures in the region have declined by about 10 c since 3 2 million years ago records of paleoproductivity suggest that this cooling was associated with an increase in wind driven upwelling tied to a shift from relatively stable global warmth during the mid pliocene to the high amplitude glacial interglacial cycles of the late quaternary these observations imply that atlantic ocean surface water circulation was radically different during the mid pliocene

early pliocene increase in thermohaline overturning a precondition for the development of the modern equatorial pacific cold tongue unraveling the processes responsible for earth s climate transition from an el nino like state during the warm early pliocene into a modern like la nina dominated state currently challenges the scientific community recently the pliocene climate switch has been linked to oceanic thermocline shoaling at 3 million years ago along with earth s final transition into a bipolar icehouse world here we present pliocene proxy data and climate model results which suggest an earlier timing of the pliocene climate switch and a different chain of forcing mechanisms we show that the increase in north atlantic meridional overturning circulation between 4 8 and 4 0 million years ago initiated by the progressive closure of the central american seaway triggered overall shoaling of the tropical thermocline this preconditioned the turnaround from a warm eastern equatorial pacific to the modern equatorial cold tongue state about 1 million years earlier than previously assumed since 3 6 3 5 million years ago the intensification of northern hemisphere glaciation resulted in a strengthening of the trade winds thereby amplifying upwelling and biogenic productivity at low latitudes

effects of glacial meltwater in the giss coupled atmosphereocean model 1 north atlantic deep water response varying magnitudes of freshwater input through the st lawrence are added to different versions of the giss coupled atmosphere ocean model the results show a generally linear response in percentage north atlantic deep water nadw reduction to the volume of water added without any obvious threshold effects when the estimated best guess freshwater input for the allerod younger dryas interval is added only small reductions in nadw production occur in less than a century with complete cessation requiring more than 300 years if the ocean circulation was weaker in the allerod preceding the younger dryas somewhat larger percentage reductions occur and it would take 150 200 years for a complete shutdown the freshening is aided by a positive feedback associated with the reduction in evaporation relative to precipitation in the north atlantic amounting to some 20 30 of the added freshwater runoff when the freshwater input is stopped after complete nadw shutdown nadw production does not resume with complete nadw cessation cooling eventually occurs throughout the northern hemisphere including regions where it is not thought to have arisen during the younger dryas and the globe cools by 1 c due primarily to the sea ice albedo feedback while in many regions the observed cooling is reproduced the cooling in greenland is much less than is estimated with only 50 nadw cessation cooling is primarily around the north atlantic no rapid warming is seen in response to freshwater cessation the sensitivity displayed here for nadw reduction due to freshwater input from the st lawrence is roughly similar to that associated with increased precipitation accompanying global warming in the next century so a similar sensitivity may arise with younger dryas freshwater inputs from other suspected locations greenland europe

miocene ocean circulation inferred from marine carbon cycle modeling combined with benthic isotope records 1 in a modeling sensitivity study we investigate the evolution of the ocean circulation and of marine carbon isotope 13c records during the miocene about 23 5 million years ago for this purpose we ran an ocean circulation carbon cycle model of intermediate complexity large scale geostrophic hamburg ocean carbon cycle model version 2s exploring various seaway configurations our investigations confirm that the central american seaway played a decisive role in the history of the miocene ocean circulation in simulations with a deep central american seaway depth range 1 3 km typical for the early to middle miocene deep water production in the north atlantic is absent or weak while the meridional overturning circulation is dominated by water mass formation in the southern ocean deep water formation in the north atlantic begins when the central american seaway shoals to a few hundreds of meters which is typical for the late miocene our results do not support ideas that the mid miocene closing of the eastern tethys contributed to antarctic glaciation on the other hand we find some water exchange between the indian ocean and the atlantic via the eastern tethys during the early miocene our model results for the atlantic meridional overturning circulation and for atlantic 13c during the late miocene are largely independent from depth variations of the greenland scotland ridge to a large extent the evolution of miocene deep sea 13c records can be explained with large scale ocean circulation changes our model data comparison for the middle and early miocene suggests that during the early neogene the seaway effect on benthic 13c may have been superimposed by further factors such as climate regime shifts and or terrestrial carbon cycle changes

sedimentary vs diagenetic control on rhythmic calcareous successions pliensbachian of asturias spain the diagenetic overprint in rhythmic hemipelagic successions can either enhance or change the original distribution of caco3 but it is difficult to evaluate its effect because in most field examples it is not possible to distinguish between sedimentary and diagenetic features the rhythmic succession of the pliensbachian of asturias spain shows alternation of bioclastic and laminated burrowed intervals the original content of carbonate brought by storms from shallow areas was larger in the bioclastic horizons however there is a widespread mismatch between facies alternations and the observed lithological rhythms the diagenetic redistribution of caco3 resulted in successive limestone marl clay couplets that do not match either in number or in the location of the boundaries the sedimentary cycles defined by facies alternations we conclude that interpreting the limestone marl rhythms as a direct response to primary changes is highly questionable unless there is unequivocal proof of a sedimentary origin of the alternation of the two lithologies

paleoclimate of the kimmeridgian tithonian late jurassic world i results using a general circulation model abstract the kimmeridgian tithonian 154 7 145 6 ma middle and late late jurassic was a time of expanded continental rifting increased sea floor spreading and a relatively high eustatic sea level stand these processes collectively caused the fragmentation and flooding of the megacontinent pangea as well as the alteration of the global paleoclimate using a version of the community climate model ccm from the national center for atmospheric research we report two kimmeridgian tithonian paleoclimate seasonal simulations with geologically inferred paleotopography one using a co 2 concentration of 280 ppm pre industrial level and the other 1120 ppm increasing the co 2 four fold warms virtually the entire planet the greatest warming occurs over the higher latitude oceans and the least over the equatorial and subtropical regions simulation of a warmer planet with an elevated greenhouse effect fits the distribution of paleoclimatically sensitive faunas floras and sedimentary rocks model results indicate that sea ice was restricted to the high latitudes of the boreal and austral seas making landfall only in restricted areas the trade winds bring heavy rainfall in december january february to eastern gondwana and in june july august to the tethys sea margins a strong june july august monsoon occurs over southeast asia the distribution of coals correlates to precipitation sufficient to maintain gymnosperm forests and coastal areas where water saturated sediments are a result of eustatic high stands of sea level evaporites are localized to areas of negative precipitation evaporation runoff is restricted to regions of intense precipitation overall the 1120 ppm co 2 simulation provides a reasonable paleoclimate for the kimmeridgian tithonian and provides a standard until a ccm with oceanic heat transport a coupled atmospheric oceanic model or one with a finer grid cell configuration is available the results need further scrutiny in areas with more detailed geologic information

persistent warmth across the benguela upwelling system during the pliocene epoch a feature of pliocene climate is the occurrence of permanent el nino like or el padre conditions in the pacific ocean from the analysis of sediment cores in the modern northern benguela upwelling we show that the mean oceanographic state off southwest africa during the warm pliocene epoch was also analogous to that of a persistent benguela el nino at present these events occur when massive southward flows of warm and nutrient poor waters extend along the coasts of angola and namibia with dramatic effects on regional marine ecosystems and rainfall we propose that the persistent warmth across the pliocene in the benguela upwelling ended synchronously with the narrowing of the indonesian seaway and the early intensification of the northern hemisphere glaciations around 3 0 3 5 ma the emergence of obliquity related cycles in the benguela sea surface temperatures sst after 3 ma highlights the development of strengthened links to high latitude orbital forcing the subsequent evolution of the benguela upwelling system was characterized by the progressive intensification of the meridional sst gradients and the emergence of the 100 ky cycle until the modern mean conditions were set at the end of the mid pleistocene transition around 0 6 ma these findings support the notion that the interplay of changes in the depth of the global thermocline atmospheric circulation and tectonics preconditioned the climate system for the end of the warm pliocene epoch and the subsequent intensification of the ice ages

simulating the impact of the panamanian seaway closure on ocean circulation marine productivity and nutrient cycling abstract the closure of the panama isthmus 14 3 ma caused large reorganizations of ocean circulation here we use a global climate ocean ecosystem model to investigate the effects of the closing gateway on ocean circulation marine productivity and nutrient distributions several sensitivity experiments with different sill depths and vertical diffusivities are performed consistent with previous model studies we find constricted throughflow due to shallowing of the sill leads to intensification of the north atlantic thermohaline overturning we demonstrate a strong coupling between the flows through the tropical gateways of panama and indonesia in a way that reduced outflow of upper ocean pacific waters via the panama gateway into the atlantic is compensated by increased flow through the indonesian archipelago the simulated rates of north atlantic deep water formation strongly depend on the vertical diffusion in the model particularly for a deep sill for the first time we document by model results shifts in nutrient distributions associated with reorganizations of ocean circulation with repercussions on marine productivity patterns reduced flow of nutrient rich sub surface waters from the pacific into the atlantic reduces biological productivity in the north atlantic in the eastern tropical pacific restriction of the nutrient rich outflow leads to nutrient accumulation which in turn maintains a strong increase in productivity these results seem to be largely consistent with the paleoproductivity proxy record a massive drop of opal accumulation rates however as found in north pacific sediments for the time of the final closure 2 75 ma cannot be simulated generally global marine net primary productivity npp is found to increase with proceeding gateway closure

influence of the amazon river development and constriction of the central american seaway on middle late miocene oceanic conditions at the ceara rise abstract sediment samples from odp 154 site 926a ceara rise western equatorial atlantic ocean spanning the neogene from 12 8 to 9 2 ma were investigated on their calcareous dinoflagellate contents to better understand the oceanographic changes in relation to the closure of the central american seaway and the development of the amazon river intervals with increased cyst accumulation rates and dissolution sensitive species occur from 12 4 ma onward they correspond to periods of enhanced north atlantic deep water production indicating the presence of this water mass at the research site during these intervals this suggests that pulses of north atlantic deep waters sporadically flew into the south atlantic related to uplift phases of the panama sill at about 11 2 ma the first appearance of leonella granifera indicates river influence at the sample site this first indication of river influence in the western equatorial atlantic can be linked to the developing amazon river the cyst association changes at about 11 2 ma from almost monospecific to highly diverse and the permanent presence and increased abundance of l granifera suggest that river waters were able to reach the study site by now probably as a result of the southward flowing north brazil current after 10 5 ma the cyst association indicates a decrease in amazon influence at site 926a this change can be correlated to a reducing inflow of pacific waters through the central american seaway leading to a reverse of the north brazil current to its modern northwards flow pattern

facies cycles and controls on the evolution of a keep up carbonate platform kimmeridgian swiss jura during the late jurassic accelerated ocean floor spreading and associated sealevel rise were responsible for a worldwide transgression which reached its maximum in the late kimmeridgian in many western european basins this major sea level rise led to the formation of marly and condensed sections in the swiss jura however a shallow carbonate platform kept growing and only subtle changes in the stratigraphic record suggest an increasingly open marine influence field observations and thin section analyses reveal that the central swiss jura was at that time occupied by tidal flats and by more or less open marine lagoons where shoals and bioherms developed the evolution through time of sedimentary facies and bed thicknesses permits the definition of small medium and large scale depositional sequences the diagnostic features of these sequences are independent of scale and seem largely controlled by the kimmeridgian second order transgression a high resolution sequencestratigraphic correlation with biostratigraphically well dated hemipelagic and pelagic sections in the vocontian basin in france reveals that i the most important increase in accommodation recorded in the kimmeridgian of the central swiss jura occurs in the eudoxus ammonite zone late kimmeridgian and corresponds to the second order maximum flooding recognized in many sedimentary basins ii the small and medium scale sequences have time durations corresponding to the first and second orbital eccentricity cycle i e 100 and 400 ka respectively suggesting that sedimentation on the platform and in the basin was at least partly controlled by cyclic environmental changes induced by insolation variations in the milankovitch frequency band the comparison of the high resolution temporal framework defined in the swiss jura and vocontian basin with the sequencestratigraphic interpretation realized in other western european basins shows that the large scale sequence boundaries defined in the kimmeridgian of the swiss jura appear in comparable biostratigraphic positions in most western european basins discrepancies that occur are probably because of local or regional tectonics

tectonic and eustatic control on a mixed siliciclastic carbonate platform during the late oxfordian kimmeridgian la rochelle platform western france boreal and tethyan realms of western europe present significant sedimentological paleontological and stratigraphic differences the purpose of this study is to constrain regional versus global controls on the dynamics of a sedimentary system located at the interface of these two realms in order to better understand the origin of their differences detailed sedimentological palynofacies and calcareous nannofossil analyses were performed on two sections from the la rochelle platform western france the pas section includes part of the late oxfordian and early kimmeridgian and the rocher d yves section is assigned to the late kimmeridgian they correspond to monotonous marl argillaceous limestone alternations limestones are essentially mudstones with echinoderms bivalves and foraminifera that suggest low energy open marine conditions highly bioclastic and or peloidal deposits occur commonly and show wackestones to wacke pack grainstones textures these deposits indicate frequent high energy events and are interpreted as storm deposits marls dominate in the most proximal depositional environments while calcareous deposits are more important in more distal environments the rocher d yves section is globally more marly than the pas section suggesting a more proximal setting palynofacies are dominated by woody particles suggesting shallow water proximal depositional environments calcareous nannofossils are ascidian spicules coccoliths and schizospheres watznaueria britannica dominate calcareous nannofossil assemblages in the pas section the rocher d yves assemblages are quasi exclusively composed of cyclagelosphaera margerelii and indicate more proximal paleoenvironments than those of the pas section different orders of depositional sequences are defined with sequence boundaries corresponding to the most rapid relative sea level falls they are hierarchically stacked and correlate on the basis of ammonite zones with the sequences of contemporaneous sections from tethyan and boreal realms the stacking pattern of these sequences suggests an orbital control on sedimentation small medium and large scale sequences correspond to precession 20 ky cycles and to 100 ky and 400 ky eccentricity cycles respectively the elementary sequences have durations shorter than 20 ky the kimmeridgian was a period of global sea level rise that ended in the late kimmeridgian more proximal depositional environments in the rocher d yves section late kimmeridgian than in the pas section early kimmeridgian imply a progradation of the la rochelle platform during the kimmeridgian this progradation resulted from a slowdown of the subsidence in the aquitaine basin during the kimmeridgian corresponding to the first steps of atlantic ocean opening high frequency cycles on the la rochelle platform formed in sync with milankovitch orbital cycles while tectonics controlled the formation of the low frequency cycles